The Gish Bar Times

Liquid Hot Magma Ocean Resolved in Galileo Magnetometer Data

2011-05-13T12:02:00.002-07:00

I'm sorry I haven't posted on this blog in eight months. I'm not sure why I stopped writing. I think it just became more difficult for me to push past writer's block when you have to be the one to generate new content for a blog as opposed to using a flow of new information to help me to populate new articles. But that's where I was last year, and it didn't help that I was interested in starting a new history blog which took up even more of my free time than I suspected. It didn't help that in the middle of trying to write about this news for you, Blogger went down for more than 20 hours. But as of five minutes ago, Blogger is back online and I can bring this exciting news to you.

Anyways, at least for today, that is over now as there is a fresh report out today in Science Express providing further evidence for a magma ocean beneath the surface of Io! I know! Big news! This is a paper I've been looking forward to seeing for more than year and half. Scientists have long suspected that Io has a mushy magma ocean based on tidal heating models where much of it is dissipated in the asthenosphere (otherwise known as the upper mantle) and on eruption temperature estimates from Galileo data. This new paper provides another method, electromagnetic induction sounding, to look for a magma ocean inside Io.

I reported on this new work in October 2009 when it was first presented at a Division of Planetary Sciences meeting and again in January 2010 when Richard Kerr wrote about that presentation, given by Krishan Khurana, in Science magazine. These results are now online as an article in press at the Science Express website (meaning the paper has been approved for publication but has not been published in the print version of Science magazine). For those outside the Science mag paywall, check out the JPL press release.

Khurana and his co-authors, Xianzhe Jia, Margaret Kivelson, Francis Nimmo, Gerald Schubert, and Christopher Russell re-examined Galileo Magnetometer data acquired during two of the spacecraft's encounters with Io in October 1999 and February 2000. Data was acquired on two other encounters, however they were polar passes and weren't nearly as useful for detecting a magma ocean. The magnetometer measured the absolute magnitude of the magnetic field surrounding the spacecraft and its magnitude in the three spatial components (B_x, B_y, and B_z). Near Io, the spacecraft mostly measured how Jupiter's magnetic field was perturbed by Io's atmosphere. In the atmosphere, plasma from Jupiter's magnetosphere is slowed as it takes on more mass and as charge is exchanged with these new particles. The magnetic field lines are also affected by interactions with Io's conducting ionosphere. Alfven wings which couple the Ionian and Jovian ionosphere in an electrical current called the Io flux tube further affect the local magnetic field vectors (this charged particle interaction produces the aurorae seen at Io as well as the auroral footprint in Jupiter's atmosphere). The big breakthrough in sorting out these interactions have been new magnetohydrodynamic (MHD) models developed in the last decade. With these interactions removed, Khurana and his colleagues were able to look at the residual magnetic field, with a field strength of greater than 500 nanoteslas. This residual magnetic field could either be intrinsic, generated by convection with Io's molten iron core, or from induction, within a conducive layer within Io.

The authors looked at an inducted magnetic field on Io first. So what could create an induced magnetic field at Io? Induced magnetic fields are created when a time-variable magnetic field sweeps through an electrically-conductive material, like the briny water oceans of Europa, Ganymede, and Callisto. Jupiter's magnetic field is tilted with respect to Io's orbital plane, so at times Io is above or below the normal plane of Jupiter's magnetic field. The time-variable magnetic field produces electrical currents within the conductive material, which produce a magnetic field through induction. The direction of this current changes twice each Jovian day (remember, the magnetosphere is co-rotational with Jupiter, even at the distance of Io), causing the poles of the induced field to switch twice each Jovian day. Additional induced responses are created using the greater rotational harmonics of Jupiter's internal dynamo.

In order to determine the best fit to the available Galileo data, Khurana and his group created a model of Io's interior using multiple shells, each layer with a conductivity based on its expected composition, temperature, and physical state, to measure the induced response to Jupiter's magnetic field. They were only able to test the three strongest rotational harmonics (13, 5.6, and 5 hours) given the limited data set that included two flybys, however these just by chance happened to fit these harmonics and were outside the densest part of the Jupiter plasma sheet. These harmonics are excited by the dipolar, quadrupolar, and octupolar terms of Jupiter's internal dynamo. The interior model for Io used a bulk chondritic composition divided into a solid, cold, silicate crust 50 kilometers thick and with zero conductivity, a molten iron core 900-1000 kilometers in radius, and a mantle in between consisting of 44% SiO₂, 32% MgO, and 14% FeO. The mantle composition is similar to lherzolite, a ultramafic igneous rock found in Spizbergen, Sweden and in the French Pyrenees. The research team used the conductivity of that rock at various temperatures to simulate Io's mantle.

Khurana's group found that using a solid mantle, even one with induction, didn't provide a good fit to the Galileo data. They then added a conducting asthenosphere shell between the cold lithosphere and the solid lower mantle in their model. They found that using a conductivity of 1 Siemen per meter for the asthenosphere, they were able to generate an induced field with a strength greater than 600 nanoteslas, closely matching the Galileo data. It turns out that like the salt water in the sub-surface oceans within Europa, Ganymede, Callisto, and Titan, ultramafic rock melts are also conductive with conductivities in the range of 1-5 S/m at 1200-1400°C or, according to the paper, partial molten rocks with conductivities ranging from "10^-4 to 5 S/m depending on factors such as temperature. composition, melt fraction, and melt connectivity." This approaches the conductivity of sea water, like the ocean found beneath Europa's crust.

The thickness of the conductive layer cannot be independently determined from the data other than it is thicker than 50 kilometers. Beyond 200 kilometers in thickness, the induction response saturates. They did find that the induced field strength is sensitive to melt fraction and the authors determined that Io's magma ocean would need to be at least 20% molten to replicate the Galileo data. So think of it as more a slurry rather than the ocean you might envision beneath Europa's surface, which would be much less viscous. Finally, the authors determined that in order to produce this induced magnetic field, the magma ocean would have to be global, rather than just a few patches near active volcanoes or just along the equator, though they don't rule out variations in asthenospheric thickness or melt fraction due to differences in tidal heating between the equator and the poles.

This discovery does help to put to rest the question of whether Io has a magma ocean beneath its surface. You would think it had one considering the wide-spread nature of its extreme volcanism. The idea was first proposed by M. H. Ross and Gerald Schubert in 1985 and it was revived in an Icarus note in 1999 by Laszlo Keszthelyi, Alfred McEwen, and G. Jeffrey Taylor [Taylor wrote an article for the University of Hawaii website on this model in case you are outside the Icarus paywall]. Keszthelyi et al. proposed that the then recent results from Galileo's Solid State Imager, suggesting eruption temperatures during 1997 eruption at Pillan reaching 2000 K, necessitated a large melt fraction within at least the upper portion of Io's mantle. Their article presented a model where the melt fraction was approaching 35% near the boundary between the crust and the asthenosphere and decreased the deeper you got into Io until you hit 6% near the core-mantle boundary. However, the high temperatures seen at volcanoes like Pillan would suggest melt fractions in some places as high as 70%. A re-evaluation of the Pillan data in 2007 by Keszthelyi et al. reduced the eruption temperatures required at Pillan and conversely the melt fraction needed in Io's upper mantle to 20-30% with interconnected magma reaching down as far as 600 km below Io's surface. The new results presented by Khurana confirm the presence of a magma ocean suggested by these authors and support Keszthelyi's current model of Io's interior.

The expected melt fraction, ≥ 20%, means that while this is a global ocean, it is more slushy hot magma, as opposed to liquid hot magma thanks to the 80% by volume suspended crystals in the ocean. In fact it would be physically impossible to the melt fraction to get to close to 100% as tidal heating would become much less efficient and the asthenosphere would cool down, decreasing the melt fraction. Conversely, the melt fraction can't be too low, or friction from tidal heating would become much more efficient that today, and the melt fraction would increase.

Finally, Khurana removed both the MHD and inductive response from the Galileo magnetometer data to look for evidence for an intrinsic magnetic field at Io, one that would be created by convection within Io's molten core. They determined an upper boundary of 110 nT, making it a very weak magnetic field if it exists.

In order to better determine the melt fraction within this magma ocean and to determine its thickness, more data is needed in order to resolve fainter rotational harmonics from Jupiter's magnetosphere. However, even just two flybys worth of data has been enough to provide a useful proof of concept for probing the interiors of bodies like Io using electromagnetic sounding. By timing future encounters with Io to conicide with times where Io is not within the densest part of the Jupiter's plasma sheet, researchers would have an easier time picking out induced fields produced by weaker harmonics in Jupiter's magnetic field which maybe lost in the noise of the moon/plasma interaction. If only there was a new spacecraft on its way to Io... another time perhaps.

Regardless, this is exciting news that Io's magma ocean has been independently confirmed by both eruption temperature data and models of Io's interior and by magnetic induction sounding. It is always nice to see Io get some press after all these years since the New Horizons flyby in 2007.

Link: Evidence of a Global Magma Ocean in Io's Interior [sciencemag.org]
Link: Galileo Data Reveal Magma Ocean Under Jupiter Moon [jpl.nasa.gov]
Link: PIA14116 - Io's "Sounding Signal" [photojournal.jpl.nasa.gov]

[I want to thank Emily Lakdawalla for providing a home for this post during Blogger's down time. She posted this article on her Planetary Society Blog this morning.]

Jupiter at its closest opposition in nearly 50 years

2010-09-22T00:14:00.000-07:00

Today at around 13:00 UTC, the Jupiter system will be in opposition, when the planet and its attendant moons are on the opposite side of the Earth's sky from the Sun. This means that the planet will be visible as a brilliant star in the sky all night long, rise in the east at sunset and setting in the west at sunrise. It also means that Earth is at its closest approach to Jupiter this year, with Jupiter only 591,560,000 kilometers (367,578,000 miles) away. From the perspective of any amateur astronomers out there, this makes it a great time to take a look at Jupiter as it is nearly 50 seconds of arc in diameter in the night sky (though let's kill any "Jupiter hoax" in the bud, Jupiter's apparent diameter is still 1/36 of the apparent diameter of the Moon). That is large enough to pick out Jupiter's cloud bands on even the most modest of telescopes. Despite its great distant, its large size also makes it bright enough to easily pick out in the sky.

In addition to being the closest Earth will be to Jupiter all year, this is also the closest Earth has been to the giant planet since 1963. That is because Jupiter is close to perihelion, its closest point in its orbit to the Sun. As you can see in the graphic above, Jupiter's orbit is slightly eccentric, and right now it is closer to the Sun (and the Earth) than it would be on the opposite side of its orbit (near apohelion), which is slightly off the graphic that is centered on the Sun. This makes this opposition a particularly good one to check out. Don't forget though that even if you are unable to check out Jupiter tonight, it will still be an excellent target to view for the next couple of months, though it will become more and more a planet to view in the evening.

Many planetary astronomers have been taking advantage of the current opposition to take some great images of the giant planet and its moons, like the one at left. I love the detail you can see in this image, taken on September 20 by astronomer Damian Peach. The southern equatorial belt is still faded and there is no indication that it will reappear anytime soon. A great place to check for more fresh images of Jupiter is the ALPO-Japan site, where astronomers from around the world post their latest and greatest shots. Another great site to check out is Cloudy Nights forum, which includes some great discussion of how these images are taken.

So please, definitely take this chance to look up at Jupiter!

Io Volcano of the Week: Isum

2010-09-21T21:42:00.000-07:00

I apologize for my absence the last couple of weeks. When you write a blog in your spare time, it exists at the pleasure of my other obligations, my health, and other demands on my spare time. So when a busy period with work, a nasty cold, and Halo: Reach all hit in the same week, well, unfortunately this blog takes a bit of a back seat. This week I am feeling much better, work is a bit quieter (wait, there is a Titan flyby on Friday, lalalalalalalalala, I can't hear you), and I have grown wary of Halo: Reach, so I can come back to my weekly series on Io's volcanoes. Today, we are discussing Isum Patera, one of Io's more active volcanic centers and the likely source of the largest lava flow field on Io, Lei-Kung Fluctus.

First off, let's get the basics out of the way. There really isn't a polite way to describe the shape of the volcanic depression that is Isum Patera; it looks like a sperm cell. Isum is located at 29.82° North Latitude, 208.46° West Longitude. The head of the Isum "spermatozoa" is 62 kilometers (39 miles) in length and 43 kilometers (27 miles) in width. The southern end of the patera appears to have a greater depth than the rest of the volcano, which can often be indicative of multiple collapses (if formed like terrestrial calderas) or sills embedded in different layers, however the low resolution of our best images of the region (1.3 kilometers or 0.8 miles per pixel), poor phase coverage, and the complex albedo patterns in the area precludes a clear analysis of the topography in this region. A small mountain may lie along the eastern margin of Isum Patera, though this is difficult to confirm from available imagery. The "tail" of Isum extends to the northeast from the northern end of Isum Patera. The tail measures 184 kilometers (114 miles) long and 11 kilometers (7 miles) wide. The floor of Isum Patera is generally dark green in color, similar to Chaac Patera, suggestive of chemically-altered basaltic lava, though a few spots along the tail of Isum Patera are much darker, more indicative of recent activity.

Isum lies at the center of a multi-colored region along the northern margin of Colchis Regio on Io's anti-Jupiter and trailing hemispheres. The background color of the area is reddish-brown, typical for Io's plains at this latitude, but it might be enhanced by sulfur deposits from activity at Isum. Green deposits dominate the terrain to the south and east of the head of Isum Patera, as well as on either side of its tail. The margins of these deposits are digitate, or finger-like, which is more suggestive of a pyroclasic deposit that a lava flow field, which typically have lobate margins (see Lei-Kung Fluctus to the north of Isum in the image at left, for example). The most intense of these dark pyroclastic deposits surround the tail of Isum Patera. Their lack of chemical alteration that results from the interaction between sulfur and the iron in the pyroclastic material suggests they were laid down most recently. More likely though, they are being covered by fresh material on a regular basis, as their dark albedo has been a constant since the Voyager encounters in 1979.

Volcanic activity has been detected at Isum Patera over a period of 31 years, since it was first observed in 1979 to as recently an adaptive optics observations at Keck Telescope on June 28, 2010. The first detection of a thermal hotspot at Isum, indicative of on-going volcanic activity, came from the IRIS (Infrared Radiometer, Interferometer, and Spectrometer) instrument on Voyager 1. It was detected again as a group of hotspots by Galileo's SSI camera when Io was in the shadow of Jupiter in June 1996, June 1997, and November 1997, every time the geometry was appropriate during one of the spacecraft's eclipse observations. In each case two or three hotspots were detected: at the head of Isum Patera, in the tail, and in the southern portion of Lei-Kung Fluctus. Galileo's Near-Infrared Mapping Spectrometer (NIMS) also detected a thermal hotspot at Isum Patera during every viewing opportunity during the Galileo Nominal Mission, between September 1996 and September 1997. It was also seen at high resolution by NIMS in August 2001 (show at right) during a flyby of Io. NIMS found a line of thermal emission within the middle portions of Isum's tail section. The intensity of the emission was so great that the NIMS detectors saturated at most of the wavelengths the instrument looked at except the shortest (1.313 and 1.593 μm). This suggests that both high-temperature volcanism and that large percentages of each pixel that covered the tail region were hot at the time of the observation.

Taken all together, what does the morphology of Isum Patera and its surrounding terrain and its history of persistent, high temperature volcanism with multiple hotspots tell us about the style of volcanic activity going on at Isum? The distribution of dark pyroclastic material external to Isum and bright and dark patera within the tail region are most similar to Pele, a persistently and vigorously active lava lake. The thermal emission history is also roughly similar. In this case, the tail of Isum is a large lava lake whose crust is continuously overturned due to fresh material being brought into the lake from below. This overturning, which can involve short lasting lava fountains, also permit the release of sulfurous gases and pyroclastic material. This latter material can then be laid down as dark deposits on either side of Isum's tail. The tail of Isum Patera may be a fissure that has opened up in Io's crust, allow magma to reach the surface and resupply the lava lake at Isum. This magma could have also formed a sill at one end of the fissure, which was then later unroofed to form Isum Patera proper. Another patera may also be located at the northeast end of the fissure, but it isn't clear.

However, some of the evidence can be deceiving. The global scale images from Galileo that are available of this volcano reveal a curved dark line connecting the northern end of Isum Patera to the southern end of the massive Lei-Kung Fluctus, a large compound lava flow field more than 125,000 sq. km (48,000 sq. mi.) in size. We know from SSI and PPR measurements from the Galileo spacecraft that at least the southern end of it was still active as of 2002. A similar relationship between an active patera and a nearby active lava flow field, with a curved dark line between the two, has been noted at other Ionian volcanoes, most importantly at Amirani. This suggests that Isum Patera may be the source of the largest lava flow field on Io. In this case, the dark curved line is a lava tube that channels lava from its source in the tail of Isum Patera north to active flow lobes across Lei-Kung Fluctus. I should point out that that given the huge extent of Lei-Kung, multiple sources can't be ruled out, and given the pattern of thermal emission seen by Galileo's Photopolarimeter-Radiometer (PPR), that's probably likely.

Today, we have looked at one of the most persistently active volcanoes on Io, Isum Patera. Isum has a rather unique shape for an Ionian volcano. Regardless, it is the site of rigorous but consistent activity that is suggestive of a large lava lake within the tail end of Isum Patera. That doesn't preclude the possibility that Isum is also the source (or one of the sources anyway) of Lei-Kung Fluctus, which may act as a kind of release valve for the lava lake, where the overflow from the lake is deposited.

This article is making up for the one I intended to write last Monday so I still need to catch up. Later this week we'll look at Maasaw Patera, a small volcano seen up close by Voyager 1.

References:
Radebaugh, J. (2005). "Formation and Evolution of Paterae on Jupiter's Moon Io". Ph.D. Dissertation. University of Arizona.
Lopes-Gautier, R.; et al. (1999). "Active Volcanism on Io: Global Distribution and Variations in Activity". Icarus 140: 243–264.

Paper: Detection of a "Superbolide" on Jupiter

2010-09-09T20:07:00.000-07:00

A paper was published today online in the Astrophysical Journal Letters on the June 3 fireball on Jupiter. The impact produced a bright flash that was seen all the way from Earth by two amateur astronomers: Christopher Go in Cebu, Philippines and Anthony Wesley in Murrumbateman, Australia. We discussed the impact at the time as not one but two detections of this impact were confirmed. This new paper is titled, "First Earth-based Detection of a Superbolide on Jupiter," by Ricardo Hueso, several co-authors include astronomers who observed the site using Hubble, Keck, and other large telescopes, and the two amateur astronomers who detected the impact. The paper discusses the circumstances of the observations of this impact, measurements of the energy released and consequently the size of the impactor, and observations by Hubble and other telescopes of the site in the days following the June 3, 2010 impact.

Prior to June 3, 2010, only a few extraterrestrial impacts or meteors had been directly observed. These included small flashes on the nightside of the Moon, a meteor streak across the Martian night sky by the rover Spirit, the faint flash that Voyager 1 saw in Jupiter's atmosphere, and the Shoemaker-Levy 9 impacts in 1994. Since June 3, two flashes have been seen in Jupiter's atmosphere, the impact on June 3 that is the subject of this paper and another on August 20 that was observed by several astronomers in Japan. These impacts produced a brief, bright 2-second flash in Jupiter's atmosphere. Subsequent observations failed to find the kind of visible scars that had resulted from the larger SL-9 impacts in 1994 and an asteroid impact in 2009. The discoveries this year by Wesley, Go, and the astronomers in Japan were aided by their use of webcam technology to record their observations of Jupiter. They sum multiple frames from the videos they record to produce spectacular color images of Jupiter and other planetary targets by reducing the signal-to-noise ratio of their data. These videos also allow for the detection of transient events like meteor fireballs that might otherwise go unnoticed or unconfirmed with additional observations.

In this new paper, Hueso et al. used the two videos taken by Wesley and Go to measure lightcurves of the June 3 fireball. By measuring how bright the bolide was compared to the brightness of the area before and after the impact, and by calibrating the photometric response of the filters and camera systems used, the authors were able to estimate the amount of energy released by the meteor. They estimated that the bolide released 1.0–4.0× 10¹⁵ Joules, or the equivalent of 0.25–1.0 megatons. This is about 5-50 times less energy than the June 30, 1908 Tunguska airburst, which flattened 2,150 square kilometers (830 sq mi) of forest in Siberia. Bolides with the energy of the June 3 event occur ever 6–15 years on Earth. Assuming an impact velocity of 60 kilometers (37 miles) per second and a density of 2000 kg per meter, Hueso estimated that the impactor had a mass of 500–2000 tons and was 8–13 meters (26–43 feet) across. This fits nicely within a gap in our knowledge of Jovian impactors, as the July 2009 asteroid had a mass that was 10⁵ times larger while the meteor that caused the flash seen by Voyager 1 was 10⁵ times smaller. According to the NASA press release, the August 20 impactor was on the same scale, though that event occurred a month after this paper was submitted.

Analysis of the bolide's optical flash reveals a number of characteristics that are similar to meteors here on Earth. The lightcurve of the event, which was visible for 1.5 seconds, is asymmetric as the event slowly brightened for one second, produced a bright central flash, then quickly faded. Analysis of both the blue filter data taken by Christopher Go and red filter data taken by Anthony Wesley also showed that the flash had three distinct peaks, again similar to bolides on Earth.

Anyway, the big result from this paper was the note that observations of Jovian bolides could help place constraints on the impactor (asteroids and comets) flux in the Jupiter system. Using similar systems as Go and Wesley, Jovian events five times less luminous than the June 3 impact should be detectable as well as events that involving slightly larger impactors on Saturn. Based on the impacts seen this year, it would appear that models predicting 30-100 collisions of this magnitude on Jupiter, like the dynamical model by Levison et al. 2000, maybe more accurate than those extrapolating from crater counts on the Galilean satellites. However, as always, more data is need. More than two data points will be needed to pin the impactor flux down.

For more details, definitely check out the original paper by Hueso et al. over on the European Southern Observatory website from their press release.

References:
R. Hueso, A. Wesley, C. Go, S. Perez-Hoyos, M. H. Wong, L. N. Fletcher, A. Sanchez-Lavega, M. B. E. Boslough, I. de Pater, G. S. Orton, A. A. Simon-Miller, S. G. Djorgovski, M. L. Edwards, H. B. Hammel, J. T. Clarke, K. S. Noll, and P. A. Yanamandra-Fisher (2010). First Earth-based Detection of a Superbolide on Jupiter The Astrophysical Journal Letters, 721 (2) : 10.1088/2041-8205/721/2/L129

Io Volcano of the Week: Shamshu

2010-09-06T22:23:00.000-07:00

Each week here on the Gish Bar Times, we profile one of Io's 400 active volcanoes as part of our volcano of the week series. This week, we take a look at fairly dormant Shamshu Patera, a large patera, or volcanic depression, on Io's leading hemisphere. If you haven't read it already, be sure to check out last week's volcano of the week, Tvashtar, which we covered in great depth over three articles (Part One - Part Two - Part Three).

As always, let's take care of the basics first about this volcano. Shamshu Patera is located at 10.1° South Latitude, 63.0° West Longitude, or about 500 kilometers (310 miles) ESE of Hi'iaka Patera, a volcano of the week back in August. The volcano measures 115 kilometers (72 miles) north-to-south and 107 kilometers (67 miles) east-to-west. The height of the patera wall which marks the outer edge of the volcano is variable as the surrounding terrain is not constant, the result of debris flows coming off a mountain that abuts the northeastern margin of Shamshu Patera. Shadow measurements along the northwestern wall of Shamshu show that it is 500 meters (1,640 feet) tall, however the lack of a shadow along portions of its western wall suggest that it maybe less than 50 meters (160 feet) tall in some areas. Shamshu Patera was named at the IAU General Assembly in August 1997 after a pre-Islamic Arabian sun goddess.

Galileo's best images of Shamshu were taken on February 22, 2000 during its I27 encounter with Io. I have reprojected the two images that covered Shamshu Patera into a two-frame mosaic, in a simple cylindrical projection with a scale of 350 meters (1,150 feet) per pixel. This mosaic covers all of Shamshu Patera, right of center, as well as Shamshu Mons to the west and portions of two other mountains: one abutting Shamshu Patera to its northeast and another a little farther away to the southeast. For a look of color in this region, see the map above as well as a global view from June 1997.

A few obvious features stand out about Shamshu Patera. The volcano's floor is dominated by dark lava flows of varying albedos. The different levels of brightness of its flows suggest that different eruptions produced new flow lobes that covered a different potion of its floor. As the lava flows age, they cool and all more sulfur dioxide and sulfur to condense on their surfaces. So as they age, the lava flows slowly brighten. Despite how dark many of these lava flow lobes look, I can't see any evidence for surface changes at Shamshu during the Galileo mission, so either very little SO₂ is deposited here, or they maybe persistently active. More on that in a bit. The shape of Shamshu's dark material, and the more central location in the patera, is more consistent with these resulting from lava flows rather than this volcano being a lava lake, like Pele or Loki. About half of the patera of covered in material that has the same brightness as the surrounding Ionian plains and has a brighter orange color. These areas likely haven't seen lava flows in recent times (> 100 years) or may have been coated in sulfur.

Just outside of the Shamshu's walls, a number of apparent bright flows are visible. To the west of Shamshu, these bright deposits are correlated with the margins of the debris flow that came off the mountain to the northeast of the volcano. That's right, I said margins, because layers are apparent within the edge of the debris flow. The visibility of layers combined with the presence of bright material correlated with the scarps that mark the edge of these layers suggest that they are eroded by sulfur dioxide sapping. The presence of layers in the debris flow (if that's what this is) would also mean that the landslide materials are remarkably well sorted with a mix of basalt/sulfur and sulfur dioxide layers. A very bright flow is visible along the southern edge of Shamshu Patera, either the result of a sulfur flow, or, more likely, a silicate lava flow whose surface has been chemically altered. Something odd is going on here because this flow is quite bright, so bright that from distant imaging, it almost looks like it should be the southern margin of Shamshu Patera.

As far as current volcanic activity, Shamshu was observed by the Galileo Near-Infrared Mapping Spectrometer (NIMS) as an active hotspot on only one occasion, during orbit C10 in September 1997. The region was observed on several occasions before and since by Galileo and by New Horizons in 2007 and no additional activity was detected.

Next week, we will shift our focus from this fairly quiescent (at least in the present epoch) volcano to the more active Isum Patera on the opposite side of Io.

References:
Bunte, M.; et al. (2010). "Geologic mapping of the Hi’iaka and Shamshu regions of Io". Icarus 207: 868–886.

The Remains of the Week

2010-09-03T15:40:00.001-07:00

Here is a wrap-up of outstanding issues from this week:

Ted Stryk requested that I include the C3 and C21 data covering Tvashtar to my comparison chart in order to provide a longer baseline from which to look at surface changes. Ted, ask and ye shall receive:


C3ISTOPMAP01 - 11/06/1996	21ISALBEDO01 - 07/02/1999	25ISGIANTS01 - 11/26/1999


27ISTVASHT01 - 02/22/2000	32ISTVASHT01 - 10/16/2001	Surface Changes at Tvashtar

The Huffington Post has an interview up on their site with Cynthia Phillips, who performed Io image processing during the Galileo mission. During the interview, she describes her work on reducing the effects of scattered light in Galileo images, which should improve color fidelity.
Coming soon, a video game set on Io.

Jupiter: Past and Present

2010-09-02T14:30:00.001-07:00

This week, two of the best images of Jupiter that I've seen in a long time were published online. One is a blast from the past, as Björn Jónsson released a mosaic he has been working on based on Voyager 1 data. The other is simply the best image of Jupiter I have ever seen taken from by an astrophotographer.

First up is a 12-frame mosaic covering the Great Red Spot and surrounding cloud features. The original data was taken by the Voyager 1 spacecraft the day before its famous encounter with the Jupiter system, and Io in particular. This mosaic was created by Björn Jónsson, who also created another 12-frame mosaic from Voyager 1 data a couple of weeks ago, covering a larger area of Jupiter's southern hemisphere. His processing techniques bring out small-scale details in Jupiter's cloud features, accounts for Jupiter's rapid rotation, and preserves color contrasts without over-saturating the colors (that often plagued Voyager image processing that was performed at the time of the encounter). This gentler approach to the data set allows Jónsson to bring out features such as shadows cast by high, convective clouds on the main cloud decks below. While the Great Red Spot has certainly changed in the 31 years since these images were taken, these remain some of the best color images ever acquired of this giant storm, as Cassini was too far away to acquire high-resolution data, Galileo lacked the bandwidth, and New Horizons' high-resolution camera has only a single bandpass.

According to Jónsson, "the images I used were obtained on March 4,1979 at a distance of about 1.85 million km. The first image (C1635314.IMQ) was obtained at 07:08:36 and the last one (C1635400.IMQ) at 07:45:24. The resolution is roughly 18 km/pixel." He used orange and violet filter data combined with a synthetic green filter.

Bringing us back to the present, Anthony Wesley, who is still out at Exmouth in Western Australia, was able to take a spectacular image of Jupiter on August 30. The extraordinary quality of his data were the result of excellent viewing conditions. What makes this image so remarkable is not so much its resolution, but its contrast. For example, you can clearly make out a pattern of waves created by turbulence between the faded South Equatorial Belt and the South Tropical Zone to the west of the Great Red Spot. Because both bands are bright, a high level of contrast is needed to pick out this kind of detail. At the time this image was taken, the Great and Little Red Spots were reaching their closest approach, with only limited signs of interaction between the two orange storms. Features are also visible within the Great Red Spot, again feat made possible by the incredible image contrast. Not shown are two bright ovals in the North Equatorial Belt that merged on August 28. Luckily, amateur astronomers were able to catch this merger as it happened, showing them get closer over the last few weeks before spinning around each other and merging.

Io Volcano of the Week: Tvashtar - Part Three

2010-09-01T22:06:00.003-07:00

Over the last few days, we have focused on the geology and volcanic history of Tvashtar Paterae, a string of four volcanoes located within Io's high northern latitudes. During the Galileo mission, Tvashtar was the site of several volcanic eruptions between November 1999 and October 2001, including a large, sulfur-rich plume that was seen by Cassini during its brief flyby in late December 2000. Since the end of the Galileo mission in 2003, monitoring of active volcanism on Io was limited to intermittent data taken at ground-based telescopes like the European Southern Observatory in Chile, Keck II, and IRTF in Hawaii. In addition, in late February 2007, the Pluto-bound New Horizons spacecraft flew by Io from a distance of 2.26 million kilometers (1.4 million miles), allowing the cameras on-board to search for surface changes on the moon since it was last seen five years earlier. Today, we will discuss the volcanic activity seen at Tvashtar since the end of the Galileo mission as what this volcanic history tells us about the variety of eruption styles exhibited by the volcanoes of Tvashtar and how their lavas are fed.

Don't forget to check out the previous two parts of our series on Tvashtar Paterae, if you haven't already done so! Part One - Part Two. This article is also part of our broader series where we examine one Ionian volcano each week.

The Tvashtari Reawakening

Throughout the 2000s, Io was imaged on numerous occasions using the adaptive optics system at the European Southern Observatory and Keck II telescopes. These two large telescopes use adaptive optics to partially correct for atmospheric effects on data acquired at these telescopes. Much of the data acquired of the Tvashtar region since of the end of the Galileo mission has been taken at the Keck II, 10-meter telescope at Mauna Kea in Hawaii. Other facilities have been used, of course, to monitor Io's active volcanism, but they often use techniques that only work for Io's Jupiter-facing hemisphere, on the other side of the moon from Tvashtar. After correcting for atmospheric effects, researchers using Keck II to observe Io can achieve a spatial resolution of 120 kilometers at near-infrared wavelengths. Unfortunately, because time on this one telescope is very limited, only a few nights each year were available to observe Io, and when you combine the possibility of inclement weather at Mauna Kea and the fact that Tvashtar wasn't always on the visible hemisphere, you can tell that there weren't many opportunities to observe Tvashtar. As far as I can tell from what has been published and what is online, between the [effective] end of the Galileo mission and the New Horizons flyby, the Keck group, which includes Franck Marchis, Imke de Pater, and Conor Laver, observed Tvashtar on: 12/22/2001, 12/26/2001, 03/08/2003, 05/29/2004, 04/17/2006, and 06/02/2006. Between at least December 2001 and May 2004, Tvashtar was not seen in Keck data. This suggests that it had become quiescent enough to not have any lava hot enough to produce detectable thermal emission.

However, the data from 2006 showed that Tvashtar had cut short its vacation and became more active. Intense thermal hotspots were observed at Tvashtar during observing runs on April 17 and June 2, 2006. The emitted power seen on June 2, the higher resolution of the two runs, was 7.7 ± 0.9 × 10¹² W, more than twice that seen at Pillan during its eruption in 1997, but still an order of magnitude less power than released at the most powerful volcanic eruption ever seen by humans, the Surt eruption in February 2001. Laver, de Pater, and Marchis published their data a year later, finding a blackbody temperature of 1240 K for June 2 data. This measurement was aided by the use of the then-new OSIRIS camera at Keck, which acquires high-spectral resolution data for each pixel, much like NIMS on Galileo, VIMS on Cassini, and CRISM on the Mars Reconnaissance Orbiter. Comparison between images like those at left with a Galileo/Voyager basemap revealed that the hotspot was centered at 59 ± 1 N, 121.5 ± 1W, placing Tvashtar C within the error box [see the map I posted yesterday for the letters used for the different volcanoes at Tvashtar]. The area covered by this basaltic lava was estimated at 57 km² (14,100 acres), consistent with the size of Tvashtar C. However, given the size of the hotspot and the resolution of the available data, it is not impossible that it was located a bit farther to the north, at Tvashtar B (the site of the November 1999 and December 2000 outbursts).

Operation: New Horizons

A few months before Tvashtar re-awakened, the New Horizons spacecraft was launched from Cape Canaveral, bound for the then-most distant planet in the Solar System, Pluto. To get all the way out to that distant world in a timeframe that wasn't ridiculously long, the spacecraft was launched on the fastest trajectory of any interplanetary spacecraft, and even then it required a gravity assist at Jupiter to fine-tune its path and to boost its velocity to get out to Pluto in 2015. This gravity assist at Jupiter took place on February 28, 2007, providing an opportunity to test the spacecraft's instruments on the many worlds of the Jupiter system. More than 190 images were acquired during this encounter of Io by LORRI, a high-spatial resolution camera system with a single, broadband band pass. Additional data was taken by MVIC, a lower-spatial resolution, 5-filter camera that covers visible and near-infrared wavelengths, and LEISA, a near-infrared mapping spectrometer.

A couple of weeks before New Horizons made it closest approach to Jupiter and Io, John Spencer and Kandis Lea Jessup observed Io using WFPC2 on the Hubble Space Telescope. Images taken on February 14 and a week later on February 21 revealed a large plume over Tvashtar Paterae, not unlike the one seen by Cassini in December 2000. While the plume was only seen in ultraviolet filter images, it was hoped that the plume would still be visible at high angles in New Horizons images.

When New Horizons started imaging on February 24, 2007, it turned out to be even better than that. A large volcanic plume, 350 kilometers (220 miles) in height, was visible in early, low-phase angle images of Io. The near-polar position of Tvashtar also meant that the plume was visible in nearly every image taken by New Horizons. This data also revealed a new red ring deposit surrounding Tvashtar Paterae as a result of this plume. In one case, repeated imaging over the course of eight minutes allowed John Spencer and his group to track clumps within the plume as they descended from the crest of the plume to the surface. Their motions are consistent with sulfur and sulfur dioxide gas condensing at a shock front at the top of the plume (rather than dust particles rising with expanding gases, like at Prometheus), then descending as it flows down along the shock flow front. The clumps likely form from electrostatic forces either generated by the interaction of plume particles (doubtful considering the spacing between individual grains) or from electrons brought in by Jupiter's magnetic field or the flux tube that connects Jupiter and Io. The fact that the plume was so easily visible in LORRI and MVIC images suggests that plume contained more dust than other giant plumes like Pele's or the Tvashtar plume seen in 2000.

A thermal hot spot was also seen at Tvashtar by all three cameras on New Horizons (neglecting ALICE since it barely resolved Io). The high-resolution data acquired by LORRI (~10-20 kilometers per pixel) allows the eruption site to be determined, corresponding with the southern half of Tvashtar B. This was also the site of the December 2000 outburst that also produced a large volcanic plume and high thermal emission. Using the LEISA spectrometer, temperatures around 1250 K were found, though the detection of a hotspot in daylight images with the visible light LORRI would suggest that higher temperature components are likely as part of an active lava fountain or curtain.

Eruption styles

The volcanic eruptions seen at Tvashtar since 1999 suggests that certain volcanoes experience specific eruption styles. For example, Tvashtar B was the site of three, maybe four, large outburst eruptions over a period of a little over seven years. Each of these eruptions involved lava fountains that generated intense thermal emission, even at visible wavelengths. The 2006 eruption by itself generated about 7% of the total power output of all of Io's volcanoes put together. That eruption may have occurred at the small Tvashtar C patera instead, but no prior eruption had been seen there except for some faint thermal emission seen by NIMS in August 2001. Based on observations of the 1999 eruption, these eruptions don't last very long, less than three months, or transition from one type of eruption to another, as the lava fountaining phase transitions to open lava channels flowing out across the surface, and later to one dominated by insulated flows where lava is transported through lava tubes, limiting the visibility of hot lava to remote sensing. Dark diffuse deposits surrounding Tvashtar B show these lava fountains also generated pyroclastic flows consisting of basaltic tephra. This eruption style is reminiscent of large, explosive volcanic eruptions on Earth, like Laki in 1783.

At Tvashtar A, a large, dark whale-shaped volcanic region dominates. It is uncertain based on the current data if this region is a large lava lake that is only intermittently active or is an insulated lava flow that is again, only occasionally active. Only one unambiguous eruption was been detected at Tvashtar A in February 2000, with a pair of hot vents and a broad area of hot lava. Same story for Tvashtar D, though there is no evidence it has been active in the recent past. A region of dark basaltic lava had brightened between February 2000 and October 2001 showing that it was cool enough to allow sulfur and sulfur dioxide from the eruption at Tvashtar B to condense on it.

The awakening of Tvashtar since 1999 is likely the result of magma from a deep source at a depth of 30 kilometers (19 miles) that has become active again. Its depth, in addition to help feed massive lava curtains, can also allow it to feed magma to multiple volcanoes. That's why you can see one eruption at Tvashtar B and a few months later an eruption can get started up at Tvashtar A. A similar situation was seen after the Thor eruption in August 2001. Small volcanoes nearby, which had also never been seen as active before 2001, came out of dormancy at about the same time. Kami-Nari experienced a phreato-magmatic eruption two years after the nearby Pillan eruption. Io's heavily fractured lithosphere can facilitate the movement of magma from these deep reservoirs to either shallow magma reservoirs (likely the case for Tvashtar A and D) or directly to the surface as dikes during intense, outburst eruptions (the case for Tvashtar B, maybe C too). The latter case can also transition to the former, as the dikes also feed sills below the volcano. These sills can later grow into shallow magma reservoirs. These provide a more consistent source of lava that can support persistent eruptions like Prometheus or Amirani.

Conclusion

Tvashtar is one of the most well imaged volcanoes on Io with three sequences at spatial scales between 183 and 315 meters (600-1,033 feet) per pixel. This imaging permitted Galileo researchers to study an outburst eruption up-close and changes in the distribution of pyroclastic deposits and lava flows as a result of these intense eruptions. The geology of this region is also intriguing, with a large plateau surrounding much of Tvashtar Paterae. This plateau is marked by evidence of sapping that has eroded the plateau back, in some cases forming canyons 40 kilometers (25 miles) long.

Next week's volcano of the week is Shamshu Patera, a volcano with not nearly as intense eruptions of Tvashtar. Over the next month, we will also look at Isum, Maasaw, and Loki. While Maasaw has been fairly quiet, both Isum and Loki have had very unique eruption styles that will be interesting to examine.

References:
Leone, G.; L. Wilson. (2001). "Density structure of Io and the migration of magma through its lithosphere". Journal of Geophysical Research 106 (E12): 32,983–32,995.
Spencer, J.; et al. (2007). "Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano". Science 318 (5848): 240–243.
Laver, C.; et al. (2007). "Tvashtar awakening detected in April 2006 with OSIRIS at the W.M. Keck Observatory". Icarus 191: 749–754.

Io Volcano of the Week: Tvashtar - Part Two

2010-08-31T22:05:00.002-07:00

This week, for our series covering one Ionian volcano each week, we are taking a look at a large volcanic region in Io's north polar region, Tvashtar Paterae. Yesterday for part one, we took a closer look at some of the images Galileo acquired and the geology of this region. In summary, Tvashtar Paterae is a string of four volcanoes that show various signs of recent or current volcanic activity. Three of these volcanic depressions, or paterae, are surrounded by a U-shaped mountain that has been modified by sulfur dioxide sapping, forming canyons that cut as deep as 40 kilometers (25 miles) into the plateau, and slumping. Today, we focus on the intense volcanic activity observed at Tvashtar during the Galileo mission, mainly between November 1999 and October 2001. Tomorrow, we will focus on more recent volcanism at Tvashtar, including the outburst that accompanied the New Horizons flyby in February 2007. We will also summarize what the eruption style at Tvashtar tells about how its lavas are fed.

Before I continue, I should point out the image at above right. I have labeled each of the four volcanoes at Tvashtar: A, B, C, and D. I hope this reduces confusion over the next two articles about which volcano I am referring to. So if I discuss an eruption at Tvashtar B, I am referring to the second volcano from the left. Tvashtar A is the large, heart-shaped volcano with the whale-shaped lava flow/lake on the northwest end of Tvashtar Paterae. Tvashtar C is the small patera with the faint lava flows that radiate out to its north and east. Tvashtar D is the steep-sided, kidney-shaped patera with a dark lava flow covering its southern half on the southeastern end of Tvashtar Paterae.

Million-to-one shot, Doc. Million-to-one.

It is hard to believe now after more than 10 years of observing various Tvashtar eruptions that prior to the outburst eruption of November 1999, a faint thermal hotspot, resulting from an excess of near-infrared energy being emitted by cooling lava flows, was seen at Tvashtar on only two occasions: by the Galileo SSI camera in April 1997 when the moon was in Jupiter's shadow and by Franck Marchis and his colleagues at the European Southern Observatory (ESO) on September 30, 1999. Over the period between the Voyager flybys in 1979 and the Galileo encounter with Io in November 1999, there doesn't appear to be any evidence for surface changes in and around Tvashtar Paterae. There were a few patches of dark material seen in color data acquired in July 1999 that represent lava flows or lakes that were active before then, including two dark regions in Tvashtar A, a 25-kilometer (16-mile) long L-shaped flow in Tvashtar B, and a dark region covering the southern half of Tvashtar D. So volcanism was a common geologic process at Tvashtar, but during the Galileo mission and perhaps during the 16 years leading up to it, the region had been largely dormant except for a pair of small, precursor eruptions.

Tvashtar's dormancy came to a crashing halt on November 26, 1999. In conjunction with the Galileo flyby of Io occurring that day, Bob Howell of the University of Wyoming used the NSFCAM at NASA's Infrared Telescope Facility (IRTF) in Hawaii to image Io a few hours after Galileo's encounter. His images revealed a bright hotspot at Tvashtar, one of a rare class of intense outburst eruptions. Images taken by Franck Marchis and his group at ESO and infrared photometry taken at the IRTF and the Wyoming Infrared Observatory (WIRO) revealing only a faint hotspot at Tvashtar provide time constraints for the start of the eruption to sometime after November 24. Howell estimates that such eruptions are occurring 2–4.5% of the time somewhere on Io. Assuming that the most intense period of a volcanic eruption on Io last approximately two days before calming down, this suggests that approximately six such eruptions occur each year somewhere on Io. At least two others were observed by ground-based observers in 1999: at Grian Patera in June and at either Tawhaki or Gish Bar in August. So the chances that Galileo would be able to image an eruption such as these at high resolution without knowing where an eruption might occur, given the amount of coverage it acquired during an individual flyby (~2–4% of the surface), were pretty slim.

However, it just so happened that the Galileo imaging team had targeted Tvashtar for a two-frame mosaic during Galileo's I25 encounter on the day of the most intense period of the eruption. More luck came when Galileo engineers were able to send up commands quickly enough to bring the spacecraft out of safe mode before the Tvashtar observation was to be taken. The safing event also ensured that there would be enough downlink time to return all of the images in this mosaic, as the higher-priority, high-resolution observations were not taken. These images were returned in early December and revealed a pair of over-exposed streaks along the northern margin of Tvashtar B. These streaks resulted from bleeding in the CCD detector array in Galileo's SSI camera. Based on their knowledge of how this type of camera overexposure occurs, Galileo imaging scientists were able to reconstruct the geometry of the exposed, hot lava that caused it. Assuming that the fissure it originated from was roughly linear, they estimated that the bleeding area was caused by the intense thermal emission from a curtain of lava that reached 1.5 kilometers (5,000 feet) into the cold, Ionian sky. This lava curtain, or line of lava fountains, was split into two main sections along the western and eastern halves of a 25-kilometer (16-mile) long fissure vent that runs along the northern edge of the floor of Tvashtar B. Considering the global rate of outburst-class eruptions (~3-6 eruptions with a VEI > 4–5), it was incredibly fortuitous to observe at high resolution such a rare class of Ionian eruption.

To better understand how this bleeding occurs and how they were able to estimate the height of the lava curtain, let's use some *shock* horrible, over-used analogies. Imagine that each of the 640,000 pixels of the Charged-coupled detector (CCD) on Galileo's camera is like a bucket that you fill with photons, which are converted to electrical charge in the bucket. The number of photons it takes to fill the bucket (pixel, or DN, value of 255 in an 8-bit camera) is defined by the observation's gain state (lower gain states mean fewer photons can fill the bucket, higher gain states require more), and the time you leave the bucket out to be filled is defined by the exposure time. Filter selection and the sensitivity of the silicon the CCD was made out of constrains the types of photons (wavelengths) you allow in the bucket. When the bucket is filled with so many photons that it overflows, or saturates, photons pour out into the buckets, or pixels, above and below it in the detector array. How this overflow or bleeding develops was determine through calibration of the camera system. For every bucket or pixel above the original over-exposed pixel that is itself overflowing, nine pixels are filled below it. The longest column of bleeding consisted of 94 pixels, and taking into account the lava flow below the fissure, Milazzo et al. 2005 found that 11–16 of those pixels covered the lava curtain and flow. Milazzo et al. estimated that the lava fountains on the western end of the fissure, least contaminated by the presence of a lava flow on the ground, were 360–900 meters (1,180–2,950 feet) tall.

Adding up all the electric charge caused by the intensity of photons from this eruptions and the exposure time of the image, they found an electron flux, corresponding to the rate at which photons filled those pixels, of between 0.94 and 1.8 × 10⁸ e^- pixel^-1 s^-1, which corresponds with lower limit on the brightness temperature of 1300–1350 K. Similar brightness temperatures were detected at the western end of the fissure. Spectra from the Near-Infrared Mapping Spectrometer (NIMS), taken as the instrument rode along with the SSI mosaic observation, also covered a portion of the lava curtain, though much of their data over the eruption was also saturated. Using a non-saturated pixel in Tvashtar B, Lopes et al. 2001 found a color temperature for the hot component of 1060 ± 60 K. This was considered a lower limit since the unsaturated pixel used did not cover the hottest areas seen by SSI, other NIMS pixels that were saturated likely covered areas that were hotter, and the influence of reflected sunlight all cause measured temperatures to be underestimates.

Tvashtar: Master of its (thermal) domain

To follow up on the November 1999 observation of an outburst at Tvashtar, an eruption they could pin down to a specific date, Galileo observed Tvashtar during an encounter on February 22, 2000 (also known as I27) using images with a scale of 315 meters (1,033 feet) per pixel. This observation used five images at different filters (violet, clear, 756 nm, 889 nm, and 968 nm) to measure lava temperatures more accurately than was possible in the I25 data and to determine the silicate composition of the lava by looking for an absorption band around 900 nm that is thought to be formed by orthopyroxene, a mineral found in basalt and other mafic igneous rocks. A composite of these images is shown at right. As you can see, the activity at Tvashtar B had largely shut down, leaving behind a dark lava flow that matches the appearance of an older flow seen before the eruption. This is confirmed by color temperatures measured by NIMS of between 500 and 600 K, indicative of cooling silicate lava and/or very small exposures of fresh lava.

While B had quieted down, Tvashtar A had heated up. Images acquired using filters that were sensitive to near-infrared light revealed glowing lava within much of a whale-shaped lava flow that runs along the southern and eastern end of Tvashtar A. These thermal hotspots were not seen in the clear-filter data in November 1999. Small glowing hotspots were also visible at the end of each of the flukes of the "whale" (note to John Spencer: I think it looks like a whale, and since this is my blog, I can say it looks like a whale, so there :-p ). These hotspots are possibly the source of the lava that is seen glowing at a cooler temperature farther south and west. Milazzo et al. 2005 measured a color temperature of at least 1220 K for these small hotspots. Farther south and west, the dark region within Tvashtar A glowed to the point that it saturated the camera's detector in places in the 889 nm and 968 nm filter images. This shows up as the red and orange area within the flows in the image above. Milazzo found a mean temperature of the unsaturated pixels in this region to be approximately 1300 K, though higher temperatures or greater fractional areas (the more likely of the two) is possible in the saturated pixels. Given the high temperature of this large region, a cooling lava flow is unlikely. Milazzo suggests that this area maybe a lava flow that is fed by lava tubes and flows mostly below a cooled lava crust, in which case the thermal emission observes comes from sub-pixel skylights, or the area is one big lava lake that is only intermittently active. I know Moses favored the lava lake hypothesis, but in reality, a mixture of eruption styles was likely.

Tvashtar remained active into late 2000 when Galileo and the passing Cassini spacecraft made joint observations Io and the Jovian system. While Cassini observed Io from ten times farther away than Galileo, the increased wavelength coverage its camera provided allowed researchers to observe more gas-rich plumes, like Pele's, as well as detect a thermal hotspot at that volcano. Using such images taken at ultraviolet wavelengths, a large volcanic plume with a height of 385 kilometers (240 miles) was observed over Tvashtar Paterae by the Cassini ISS. This corresponds with a red oval plume deposit, seen in Galileo images taken on December 30, 2000, that encircles Tvashtar. This deposit is quite similar to the one that surrounds the volcano Pele, and suggests that the plume was enriched with elemental sulfur enough to form an optically thick layer of S₄ on the surface. This plume accompanied yet another intense, outburst eruption, observed from Earth by Marchis et al. using the adaptive optics system at the 10-meter Keck II telescope in Hawaii. Tvashtar remained active in observations from Keck taken on February 19, 2001, but it was less energetic than it was in December 2000. The area and temperature reported by Marchis et al. 2002 was consistent with a cooling lava flow with only a moderate amount of new activity, relative to the intense, lava fountain-enhanced eruptions seen in November 1999 and December 2000.

During the next two Io flybys on August 6 and October 16, 2001, Galileo took advantage of several opportunities to image Tvashtar up-close in the wake of these intense eruptions. The best of these two flybys for observing Tvashtar was the first (I31) as the spacecraft flew nearly directly over the volcano at an altitude of 300 kilometers (186 miles), within the plume seen by Cassini. Two very-high-resolution observations were planned by Galileo SSI during the encounter: a 6-frame mosaic at 3-5 meters (10-16 feet) per pixel across the I25 vent region in Tvashtar B and its northern patera wall and a 6-frame, context mosaic at 50 meters (164 feet) per pixel that covered Tvashtar B and portions of Tvashtar Mensae to its north. Unfortunately, due to a camera anomaly, these observations were lost. Low-resolution global images at 19.6 kilometers (12.2 miles) per pixel showed that the plume deposit encircling Tvashtar that Galileo observed in December 2000 had faded quite a bit due a mix of relative inactivity and deposits from a fresh eruption at Thor to the southwest of Tvashtar. NIMS was not affected by the SSI anomaly and took high quality data over Tvashtar during this encounter. This observation revealed the complex distribution of warm lava and pyroclasts across the Tvashtar portion. The main red hotspot in the data shown at above left corresponds with the likely source for the December 2000 plume and lava fountains along the southwestern wall of Tvashtar B. Two additional hotspots are visible at the lava flow that formed during the November 1999 eruption and along the northeastern margin of Tvashtar A. Fainter flows were also detected from the whale-shaped flow in Tvashtar A and from Tvashtar C.

Galileo's final opportunity to image Tvashtar came on October 16, 2001 during its I32 flyby of Io. This time Galileo flew over Io's south polar region, leaving only an opportunity to image Tvashtar at a more oblique angle later in the flyby. The resulting two frame mosaic, at 200 meters (656 feet) per pixel, does reveal changes that occurred at Tvashtar as a result of the December 2000 volcanic eruption. These include a fresh lava flow across portions of Tvashtar B, likely forming during the eruption seen by Cassini and fresh pyroclastic deposits to the east and southwest of that patera. The extent of these dark deposits is consistent with the type of volatile-rich volcanic eruption that would also spawn a 400-kilometer-high, sulfur-rich, volcanic gas plume. New dark material was also seen along the northeastern edge of Tvashtar A, the site of a NIMS hot spot in August 2001. This suggest that volcanic activity had gotten going in the interval between the I27 and I32 SSI observations there as well. The distribution of this fresh dark material with a new pyroclastic deposit as it appears to cover both the edge of the patera floor and the terraced wall above it, though the NIMS observations suggests that at least some lava flowed out from this vent. Finally, the dark lava flows that covered the southern half of Tvashtar D had pretty much faded by October 2001.

The Tvashtar region endured a series of violent volcanic eruptions between November 1999 through at least December 2000, and Galileo and Cassini had front row seats to the action. Galileo observations from the SSI camera and NIMS spectrometer indicated that volcanic activity in the region was waning throughout 2001, with much of the thermal emission coming from older, cooling lava flows, though some fresh activity was likely at Tvashtar B and in parts of Tvashtar A and C as late as August 2001. However, by December 2001, ground-based observations from Keck showed that Tvashtar had quieted down to the point that it was no longer detectable using their instruments. However, that was not all she wrote for Tvashtar. As we will see tomorrow, there are third and fourth acts for Io's volcanoes as we examine observations of Tvashtar from ground-based telescopes and the New Horizons spacecraft after Galileo's mission ended. Tomorrow, we will also discuss what the various eruption styles at Tvashtar tells us about how its lavas are fed.

References:
McEwen, A. S.; et al. (2000). "Galileo at Io: Results from High-Resolution Imaging". Science 288 (5469): 1,193–1,198.
Keszthelyi, L.; et al. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". Journal of Geophysical Research 106 (E12): 33,025–33,052.
Lopes, R. M. C.; et al. (2001). "Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000". Journal of Geophysical Research 106 (E12): 33,053–33,078.
Howell, R. R.; et al. (2001). "Ground-based observations of volcanism on Io in 1999 and early 2000". Journal of Geophysical Research 106 (E12): 33,129–33,139.
Marchis, F.; et al. (2001). "A survey of Io's volcanism by adaptive optics observations in the 3.8-μm thermal band (1996-1999)". Journal of Geophysical Research 106 (E12): 33,141–33,159.
Marchis, F.; et al. (2002). "High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io". Icarus 160: 124–131.
Porco, C.; et al. (2003). "Cassini Imaging of Jupiter’s Atmosphere, Satellites, and Rings". Science 299 (5612): 1,541–1,547.
Turtle, E.; et al. (2004). "The final Galileo SSI observations of Io: orbits G28-I33". Icarus 169: 3–28.
Milazzo, M.; et al. (2005). "Volcanic activity at Tvashtar Catena, Io". Icarus 179: 235–251.
Lopes, R.; et al. (2004). "Lava lakes on Io: observations of Io’s volcanic activity from Galileo NIMS during the 2001 fly-bys". Icarus 169: 140–174.

Io Volcano of the Week: Tvashtar - Part One

2010-08-31T01:14:00.002-07:00

It is four for the price of one for this week's Io Volcano of the Week: Tvashtar Paterae. During the month of August, we have examined the five volcanoes that were imaged up-close by the Galileo spacecraft during its encounter with Io on November 26, 1999. During that flyby, Galileo acquired five observations with a scale of 160-280 meters or 525-920 feet per pixel (higher resolution observations were lost due to a spacecraft safing event). Thus far we have profiled Zal, Emakong, Hi'iaka, and Culann, active volcanoes seen across various parts of Io's leading hemisphere. This week, we travel to the great red north to Io's camera-loving volcano, Tvashtar Paterae. Due to the sheer amount of data acquired of and papers written about just this one volcanic region, I am going to split this discussion up in to (at least) three parts. Today, we will examine the geology of Tvashtar Paterae and the surrounding region, as well as the imagery Galileo returned of this volcano. Tomorrow, August 31, we will focus on the eruptions that occurred at Tvashtar during the Galileo mission. Finally, on Wednesday, September 1, we will examine the volcanic eruptions that have occurred there since the end of the mission, including the massive one that happened during the New Horizons flyby, and what these various eruptions tell us about how Tvashtar's lavas are supplied.

First, let's stick to the basics. Tvashtar Paterae is a chain of volcanic depressions located at 62.76° North Latitude, 123.53° West Longitude, placing it in the high northern latitudes of Io's anti-Jovian (i.e. the "far" side) and leading hemispheres. All together, Tvashtar measures 306 kilometers (190 miles) from its northwest to southeast ends. The volcanic region is named after Tvastar, a solar deity and blacksmith to the gods of the Vedic religion. Tvastar crafted, like Hephaestus for Zeus in Greek mythology, the thunderbolts of Indra and other magical implements. There are a number of Ionian volcanoes named after characters from Vedic and Hindu religious texts, such as Savitr (a large volcanic depression 300 kilometers, or 186 miles, south of Tvashtar), Surya, Vivasvant, Arusha, and Agni. Originally, the region was named Tvashtar Catena in 2000, using the IAU term for a string of craters, but that feature type was deprecated for Io in 2006, so the descriptor term was changed to the plural of patera (technically, an irregular depression, but used for Io as a geologic term for a volcanic depression).

Images

Before we get to the geology of Tvashtar Paterae, let's take a look at the available imagery, which you will see a lot of over the next few days on this blog. While it was clearly visible in global-scale images taken by the Voyagers and Galileo before (including the C21 global color mosaic, a portion of which is shown at the top of this article), Tvashtar was first imaged up-close on November 26, 1999 during Galileo's I25 flyby. This two-frame mosaic, with a scale of 183 meters (600 feet) per pixel, was originally designed as a 2 frame-by-4 frame mosaic that covered both Tvashtar and Savitr Patera to the south. The goal was to better understand the geology of giant paterae (i.e. volcanic depressions) that appeared to be common across Io's polar regions. Based on measurements made by Jani Radebaugh and her colleagues and published in 2001, the paterae at high latitudes are larger but less numerous, and these two large depressions were thought to be typical of this. Their eruption style also seemed to differ from volcanoes observed at lower latitudes. 25ISGIANTS01 was trimmed down to a 2-frame mosaic covering only Tvashtar Paterae in late October 1999 after the I24 flyby when the malfunction of the camera's summation mode meant that fewer frames could be taken given the available downlink. Frames also had to be cut because of the decision to use pre-downlink compression off the tape recorder, rather than the camera's on-board compression, for some of the images taken. The observation was returned slowly during December 1999, revealing a violent new eruption in the Tvashtar region.

As a result of the eruption there, Tvashtar was targeted for imaging on three of Galileo's four remaining Io flybys, in order to look for new activity and to monitor the region for surface changes. The two datasets that made it back it to Earth include a five-color observation from the I27 encounter (February 22, 2000) and a two-frame, clear-filter mosaic from the I32 flyby (October 16, 2001). The first observation, 27ISTVASHT01, has a scale of 315 meters (1,033 feet) per pixel, while the second, 32ISTVASHT01, has a pixel scale of 200 meters (656 feet). Higher resolution imaging was planned for an encounter in August 2001, however they were lost due to a camera anomaly. These included a very high resolution mosaic that would have covered the I25 eruption site and nearby patera wall at 3-5 meters (10-16 feet) per pixel.


25ISGIANTS01 - 11/26/1999	27ISTVASHT01 - 02/22/2000	32ISTVASHT01 - 10/16/2001

The three returned data sets are shown above. Each has been reprojected to an orthographic map projection centered on Tvashtar. The scale is 200 meters (656 feet) per pixel. Finally, the resulting mosaics were cropped so that they cover the same area to make comparisons easier.

Geology

Tvashtar Paterae is not a single volcano, but a chain of four separate volcanic centers. A rough schematic based on the three data sets is shown at right. Orange lines mark the margins of volcanic depressions. Blue lines mark the edges of plateaus, while green lines mark the visible edges of landslide debris deposits. From this image, we can see that Tvashtar can be broken up into roughly two parts. The northern end of Tvashtar consists of a large, 145×105-kilometer (90×65-mile), heart-shaped depression and is located at 64.7° North Latitude, 127.0° West Longitude. This low-depression is host to a large area of dark material, with the darkest of this distributed in a whale-shaped region along the southern and eastern margin of the patera, though an additional very dark region was also seen in the western portion of the patera in July 1999, but had brightened by October 2001. The green color of the rest of the dark terrain of this patera suggest that it consists of older lava or pyroclasts that have been modified chemically by infalling sulfur and sulfur dioxide, creating a film of iron sulfide. High-resolution color observations from I27 revealed a bright deposit along the northeastern wall of this patera, possibly resulting from sulfur dioxide sapping, a geologic process we will encounter often as we explore the geology of Tvashtar, or fumaroles.

The southern end of Tvashtar consists of a 196×70-kilometer (121×44-mile), footprint-shaped region that is almost entirely enclosed by a low, U-shaped plateau named Tvashtar Mensae. This area may be closed off by a patera wall to the northwest as well, but it is not clear if this region is lower than the local Ionian plains. I do not consider this in-and-of-itself a separate volcano. Nested within this region are three smaller paterae, each with signs of recent volcanic activity. Going from west to east, the first nested patera is located at 62.5° North Latitude, 123.2° West Longitude and is 49 by 32 kilometers (30 by 20 miles) in size. A fissure along the northern margin of this volcano was the site of violent volcanic eruptions in November 1999 and February 2007. Lava flows associated with the 1999 eruption, earlier eruptions over the same place (if you look at the July 1999 images) and later eruptions are visible on the patera floor. This patera is surrounded by a dark (occasionally dark green) pyroclastic deposit that reaches out at least 30 kilometers (20 miles) from the edge of the volcano. The area covered by this deposit grows and shrinks in places over the period from 1999-2001, suggesting that volcanic activity leaves behind these deposits in the present epoch and sulfur dioxide released by sapping from the base of Tvashtar Mensae obscures some of it over time.

The second volcano is located at 60.6° North Latitude, 120.4° West Longitude and is 14 by 8 kilometers (9 by 5 miles) in size. This patera is surrounded by digitate lava flows, suggesting that in the past it filled with lava and overflowed onto the surrounding landscape, mostly to its north and east. Finally, the third patera is located at 59.6° North Latitude, 117.9° West Longitude and is 49 by 27 kilometers (9 by 5 miles) in size. Unlike the rest of Tvashtar's constituent paterae, this volcano is bounded by steep cliffs, with a shelf forming a low ledge at its base. This shelf may have formed when some of the lava that once filled the patera to a bit higher up the patera wall than it does today drained back down into the shallow magma reservoir. Through Galileo's February 2000 observations, the southern half of this patera was covered with dark lava, while the northern half was covered with brighter green material, again thought to be chemically-altered basalt.

On-going volcanic activity was detected at all four volcanoes of Tvashtar Paterae at one time or another by Galileo, ground-based telescopes, and New Horizons. This activity will be discussed in later posts this week.

I should also discuss the plateau that surrounds most of Tvashtar Patera. This U-shaped mountain, Tvashtar Mensae, is named after the chain of volcanoes it nearly surrounds. It can be roughly split into eastern and western halves. The eastern half is a smooth, flat plateau that rises approximately 2 kilometers (6,600 feet) above the surrounding plains. The cliffs that run along the edge of this plateau are marked by large alcoves that give it a spur-and-gully pattern. These alcoves grow into large canyons in several spots that penetrate as deep as 40 kilometers (25 miles) into the plateau. These canyons also include small mesas. Running outward from the outer margins of the mesa is a low debris field that is a few hundred meters above the surrounding plains. These morphological characteristics suggest that the mesa has been heavily modified by sulfur dioxide sapping. As I discussed a few days ago, sapping occurs when frozen or liquid sulfur dioxide escapes from the base of a slope on Io and is deposited as a layer of sulfur dioxide frost as much as 70 kilometers (45 miles) away from the cliff. This process can undermine the slope above where the sapping occurred, causing it to collapse and form a gully along the cliff face. Repeated sapping events can cause the slope to retreat. Uneven slope retreat caused by excess sapping in one area, perhaps due to heating from below, can result in the formation of the wide-mouth canyons visible at Tvashtar Mensae and create small mesas, remnant portions of the plateau that have been cut off by sapping and slope retreat, not unlike the one discussed the other day. Sapping and slope retreat can be sped up by heat from sub-surface magma or an interbedded sill. This may be responsible for the U-shape of Tvashtar Mensae, as increased heat flow in the area promotes the removal of material from plateau, eating away at it and forming the eastern half of Tvashtar Paterae. Material that didn't become vaporized during the sapping events and mass wasting events form the hummocky debris deposits to the east and north of Tvashtar Mensae.

The western half of Tvashtar Mensae has a very different morphology. Rather than being flat and smooth, the western half is rougher and rises nearly 6 kilometers (19,700 feet) above the surrounding plains in places. The rough texture of its surface suggests that it has slumped outward since it formed, creating a lobate landslide deposit off the eastern side of the mountain, on the floor of the southern "patera" of Tvashtar Paterae. The 2 kilometer (6,600 feet)-tall cliff that bounds the western edge of the mountain has a regular arcuate margin that is explained more easily by simple mass wasting through slumping and small landslide events, rather than sapping like the eastern half of Tvashtar Mensae. This close connection between a tall mountain, perhaps created by thrust faulting, and a lower smooth mesa has been seen at several other locations across Io, including Zal Montes, where the two components may have broken apart by strike-slip faulting.

The geology of the Tvashtar region is strongly affected by the volcanic activity that occurs there. Galileo, New Horizons, and ground-based telescopes have observed major volcanic eruptions at Tvashtar on several occasions since 1999. Over the next two posts, we will examine the volcanic history of Tvashtar Paterae. Tomorrow, we will focus on the volcanic activity observed at Tvashtar between 1999 and 2001. On Wednesday, we will look at more recent activity in 2006 and 2007, including the incredible volcanic plume seen by New Horizons. I hope you enjoy!

References:
Keszthelyi, L.; et al. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". Journal of Geophysical Research 106 (E12): 33,025–33,052.
Radebaugh, J.; et al. (2001). "Paterae on Io: A new type of volcanic caldera?". Journal of Geophysical Research 106 (E12): 33,005–33,020.
Turtle, E.; et al. (2004). "The final Galileo SSI observations of Io: orbits G28-I33". Icarus 169: 3–28.
Milazzo, M.; et al. (2005). "Volcanic activity at Tvashtar Catena, Io". Icarus 179: 235–251.
Moore, J.; et al. (2001). "Landform degradation and slope processes on Io: The Galileo view". Journal of Geophysical Research 106 (E12): 33,223–33,240.
Schenk, P.; et al. (2001). "The mountains of Io: Global and geological perspectives from Voyager and Galileo". Journal of Geophysical Research 106 (E12): 33,201–33,222.

High-Resolution Mystery in Bulicame Regio on Io

2010-08-27T19:27:00.002-07:00

On February 22, 2000, Galileo flew close to Io for its 4th targeted flyby of the satellite with a altitude of 198 kilometers (a flyby known as I27). Right at closest approach, Galileo turned its suite of remote sensing instruments at the southeastern edge of a low plateau east of the active volcano Isum Patera, on the southern end of Bulicame Regio. The camera was pointed significantly off nadir and was instead looking much closer to the local horizon at an emission angle (an angular measure of how far a point on the surface is from nadir: looking straight down is 0° and looking at the horizon is 90°) was The observation, 25ISSAPPNG01, was designed to look for evidence of sapping and other slope degradation processes on the margin of this plateau. Sapping, a geologic process by which sulfur dioxide in gas or liquid form escapes from a mountain slope, undermining the slopes strength and causing it to downslope movement. Four images were planned for this observation and three were eventually played back, though only 2/3rds of the lines of each frame (525 out of 800) were played back due to the limited amount of downlink available since the camera and spacecraft worked perfectly during this encounter, in contrast to the camera and spectrometer anomalies experienced during I24 and the spacecraft safing event on I25 that led to loss of data. However, in the 10 years since these images were sent back from Galileo, they have become a bit of a mystery for planetary scientists, as interpreting the geology of the terrain seen in the images of 25ISSAPPNG01 has been difficult.

I, for the bazillionth time I swear, have processed the data into a mosaic that you can see above, though you can check out a full-resolution version here. Like my version of the Chaac Patera mosaic taken a few minutes later, the images have been reprojected into a point-perspective map projection. This approximates the view Galileo had at the time. North is just to the left of up in this mosaic, as it has been rotated so north is up. The perspective is from the point when the middle image was acquired (though I had to adjust the center latitude and longitude to eliminate distortions in the middle image, compared to the sub-spacecraft point reported in the PDS). Since Galileo was moving rapidly over Io's surface during the observation, the first and third images are a bit distorted compared to the original data in order to match the perspective of the second, with the first images becoming stretched and the third becoming squished. However, the distortion is not so bad that we can't examine the features seen in the images.

The first problem we face however in interpreting the surface features seen in this mosaic is that we lack proper context. The images in this mosaic have an effective resolution of approximately 11.6 meters per pixel when you take into account that we are looking toward the horizon rather than straight down at Io. This more than two orders of a magnitude better than the next best data over this area, shown at left. This is a clear filter mosaic from October 1999, blown up about three times its original size. With such a disparity in resolution between the two observations, it becomes difficult to correlate structures seen in both since each of the three I27 frames are 6 pixels across in the I24 data. The other complication this disparity presents is that we can't tell exactly where the I27 images are in the I24 data. The pointing was more than a entire SSI field-of-view off from the planned pointing. From there we can only correct the pointing relative to each other, rather than to any basemap since the next best data doesn't even show the scale of features we seen in the I27 images.

Basically, we don't know how the features relate to the regional geologic context. We know some basics. We know that the region we are looking at is a few kilometers southeast of the base of a scarp that bounds a low mesa. In lower resolution images, this area appears to be a patch work of bright and dark material on the edge of the scarp, perhaps the result of sulfur dioxide-modified lava flows. At that scale, this imaged area was mapped as bright flows by Williams et al. 2010. Finally, we also know that there has been recent volcanic activity in this area.

Let's take a look at the features we can observe in this mosaic. The most prominent feature is a 400-meter-tall mesa at the bottom of the middle frame. There doesn't appear to be much evidence for layering along the scarp face of this mesa, except for a 50-meter-thick cap rock. Surrounding the mesa is a low plain marked by swirls of bright and dark terrain. Beyond that is region of smoother dark terrain that is marked by large boulders and narrow channels. Finally, beyond that in the third frame, there is a region of bright and dark layered terrain.

This is certainly quite an enigmatic landscape. One interpretation provided by Moore et al. 2001 suggests that the mesa was once much larger, but over time sapping has undermined the edge of the mesa, causing it to shrink. The mottled terrain that surrounds the mesa, filled with bright and dark swirls maybe the area the mesa retreated from. However, note that this is not the main plateau, which is to the north west of the area covered in these images, and that the mesa seen in the high-res data was not seen in the lower resolution images, suggesting that it isn't much bigger than the what we see here. I don't think this necessarily changes the story. On Earth, headward sapping erosion of plateaus have been known to cause portions to be cut off, forming small mesas. The bright and dark swirls may result from sulfur dioxide filling low areas as it flowed out from the base of the cliff and from fumaroles with the adjacent dark terrain (seen in the middle frame). Like the water in the gullies on Mars, the sulfur dioxide is boiling as it flows across Io's surface, either escaping to the atmosphere as a gas or freezing as a bright frost in low regions. The bright/dark mottled terrain appears to follow the shape of the mesa, further suggesting that it is the result of scarp retreat. As for where the material that made up this scarp went, it is thought that the upper 1-2 kilometers of Io's crust is made up largely of sulfur and sulfur dioxide with cooled silicate existing as layers in the near-surface. This scarp, and others like it, may then retreat more akin to the polar ice caps on Mars than mesas on Earth, leaving behind only a trace amount of talus near the slope.

Alternatively, the features seen here could be explained by volcanic processes. After all, this area was mapped as a bright flow from global-scale images. One of the reasons I wanted to write up this article is that the bright/dark patchwork landscape bears a resemblance to other sulfur-modified lava flow fields seen on the floor of Chaac Patera and to the southeast of Emakong Patera, the latter of which I discussed a couple of weeks ago. This mottled terrain result again from a similar process as I discussed above: fumaroles on the surface of a cooling lava flow bring up liquid sulfur dioxide where it then freezes in low areas of the flow surface. However, there don't seem to be any indications of flow lobes in this areas, which you would expect to see if this were once a compound flow field. This terrain also hugs the base of the mesa, which would you would not normally see since the base of the cliff should be higher topographically that the surrounding plains. However, the irregular pattern of the mottled terrain at the base of the mesa and the fracture that runs across the middle frame could be the result of the interaction between hot silicate lava and volatile sulfur dioxide.

In February 2000, Galileo imaged a very enigmatic landscape at high resolution. The terrain seen in these images has been difficult to interpret due to the lack of proper context imaging of this region. The difficulty in interpreting this data forced a change in the imaging strategy for later flybys as super high-resolution observations were dropped and other sequences with resolutions similar to this mosaic were accompanied by lower-resolution images taken at later in the encounters to help interpret small-scale features. However, this lack of regional context doesn't make the features seen here any more interesting or make us less capable to at least speculate as to what caused this mess, whether it was erosion from sulfur dioxide sapping, volcanic lava flows, or fumaroles leaving behind frozen sulfur dioxide.

References:
Turtle, E. P.; et al. (2001). "Mountains on Io: High-resolution Galileo observations, initial interpretations, and formation models". Journal of Geophysical Research 106 (E12): 33175–33199.
Moore, J.; et al. (2001). "Landform degradation and slope processes on Io: The Galileo view". Journal of Geophysical Research 106 (E12): 33223–33240.

The Errata of the Day

2010-08-26T16:34:00.000-07:00

Happy Thursday everyone! Only one more day till Friday... Just thought I would post a few quick notes:

Jupiter and the Moon are particularly close tonight in the late evening and night sky. The simulated view to the right shows the sky to the southeast at around 11:06pm local time (since the moon is moving slowly across the field of background stars, YMMV). And nothing special happens right at 11:06pm, that's just when I pressed stop in Celestia ;-) The close proximity of the Moon to Jupiter (and Uranus) makes it a bit easier to locate those planets, so it is worth taking a look, even for those who don't have telescopes (like myself). Jupiter is approaching opposition, which occurs on September 22. On that date, Jupiter will be opposite the Sun from the Earth, meaning the planet (and its attendant moons) will rise over the horizon at sunset, reaches its highest point in the sky at local midnight, and set below the horizon at sunrise. This is also the best opportunity to observe Jupiter because it will reaches its largest size in the night sky. In fact, this will be Jupiter's closest approach to Earth than at any other time between 1963 and 2022.
The Carnival of Space #168 is now up over at Weird Sciences. Read up on Trojan kuiper belt objects, bistatic experiments on the Moon, and the Laser Interferometer Space Antenna (LISA). I submitted my write up on a method for creating true color images of Io for this week's Carnival of Space.
Emily Lakdawalla of the Planetary Society Blog is this week's featured Woman in Planetary Science. I'll admit, I never read that blog much because, well, I'm not a woman in Planetary Science. But after reading a few of the profiles, I see that many of the experiences are pretty universal, and can be useful for both men and women who are considering a career in this field.

"Io" Episode of Wonders of the Solar System on TV Tonight

2010-08-25T13:05:00.002-07:00

Just a quick reminder to my readers in the US that the "Io" episode of the Science Channel mini-series, Wonders of the Solar System, is on tonight at 9pm EDT/6pm PDT. An encore presentation will air at 12am EDT/9pm PDT in case you missed it the first go around. The five-part series, a production of the BBC, is hosted by Brian Cox, a particle physicist from the Univ. of Manchester. The show aired back in its home country earlier this year. The episode is actually titled, "Dead or Alive", and the topic up for discussion is planetary geology. In addition to the extensive discussion of Io's volcanism, the episode also focuses on volcanism on Mars, Venus, and the Earth as well as impact cratering on the Earth. If you get the Science Channel (preferably in HD) from your cable, fiber optic, or satellite TV provider, then I would definitely recommend checking it out tonight. The last episode, which airs next Wednesday, will focus on the potential habitability of Europa and Mars and the influence water has had in shaping many of the worlds of our solar system.

To get a feel for what to expect, one of the people on the BBC Two film crew who produced the series posted some behind the scenes videos on Youtube from the shoot at Erta'ale in Ethiopia:

All I will say is that people who put on gas masks for just a wee bit of sulfur dioxide are silly! Come on! That sulfuric acid that is created in your lungs will put hair on your chest! You wouldn't see Morgan Freeman using a gas mask if he did one of his Wormhole shows from Erta'ale. And in the second video, I have that book! Not the Ethiopia one, the other one! I even wrote a review of it a couple of years back.

One other reminder is that the series will be coming out on Blu-ray and DVD on September 7. Not sure if these are the BBC Two versions that will be released in the US, as they are about 15 minutes longer due to the lack of commercials breaking up the fun.

Io Volcano of the Week: Culann

2010-08-25T03:05:00.004-07:00

This month for my Io Volcano of the Week series, we are looking at volcanoes that were observed at moderate resolution (160-280 meters or 525-920 feet per pixel) during Galileo's I25 flyby of Io on November 26, 1999. Over the last three weeks we have looked at Zal, Emakong, and Hi'iaka, three large paterae - volcanic depressions - on Io's leading hemisphere. This week we take a look at Culann Patera. Like Emakong, the green-colored patera is surrounded by a number of distinct lava flows of varying surface colors and ages. The most recent of these flow lobes formed between 1979 and 1996, along with a number of other changes at Culann between how it appeared during the Voyager flybys and the Galileo mission. Unlike Emakong, lava temperatures measured at Culann by the Galileo spacecraft are clearly in the silicate range, suggesting that the colored lava flows are a surface coating after they had cooled, rather than the result of sulfur volcanism. It is also the site of a faint, dust-poor plume, observed by Galileo in 1998, though a fairly distinctive plume deposit was seen by the spacecraft.

Culann Patera was first observed in Voyager 1 images. It is located 600 kilometers (375 miles) south-southwest of the active volcano Prometheus at 20.2° South, 160.2° West. The spacecraft observed dark lava flows surrounding the small patera (not seen by Voyager due to the low resolution) with a narrow flow lobe extending out to the northwest of the main flow field before turning west. The darkest of the flows seen by the Voyagers was to the southwest of the patera that was later seen by Galileo. The volcano was named by the IAU in 1979 after a blacksmith from the Ulster cycle of Irish mythology.

Culann was next observed by Galileo 17 years later. Its first images of the region were taken in September 1996, revealing a number of changes as a result of volcanic activity in years between observations including fresh silicate lava flows and a reddish plume deposit. Many of the flows that were dark in 1979, suggesting they were active at the time, had brightened by September 1996, meaning they had cooled enough to permit sulfur and sulfur dioxide to condense on them. NIMS detected a thermal hotspot at Culann during this encounter and through 75% of the observations of this region during the nominal mission, showing that the volcano remained active throughout the Galileo mission. A small, faint plume may have been detected in September 1996 (which could have been the source of the new reddish deposits at Culann), along with a gas plume seen while Io was in Jupiter's shadow in May 1998.

Details on the style of volcanic activity at Culann would come during Galileo's flybys of Io in late 1999 and 2001. Lower resolution color data from throughout the Galileo mission showed that Culann Patera was host to a variety of colored terrains, with green, red, and yellow colored lava flows and plains in close proximity. As a result, the imaging team targeted the volcano for a medium resolution, two-by-two frame, color mosaic during Galileo I25 encounter on November 26, 1999. I've processed the images that were returned into the three mosaics below. All three use orthographic projection with a scale of 206.3 meters (676.8 feet) per pixel. The first is a mosaic of the red, green, and violet frames returned from the 25ISCULANN01 observation [high-resolution version available]. Only those areas covered by all three colors are included. The second mosaic covers the same area as the first, but the blue channel uses a synthetic blue filter image created using the method I described last week [high-resolution version available]. The third mosaic includes the area from the first mosaic, but includes the rest of the green filter data from the 25ISCULANN01 observation [high-resolution version available]. This additional data was colorized using data acquired in July 1999.

These images revealed that Culann Patera was a shallow, green floored depression, seven kilometers (4.3 miles) by 23 kilometers (14.3 miles) in size. Both the patera and the region surrounding it (particularly to the south and east) are coated with green material, and this greenish terrain is in turn surrounded by a 150- to-170-kilometer-diameter (93 to 106 mile) ring of red deposits (that was a bit more distinctive to the east of Culann Patera earlier in the Galileo mission). Many of these deposits coat an overlapping set of lava flows that radiate out from the patera. A curved, dark red line runs for 42 kilometers (26 miles) from the southwest end of the patera toward the northwest, growing fainter the farther northwest it goes. It terminates in the proximal end of Culann's darkest and youngest lava flow fields, which run another 75 kilometers (47 miles) to the north west. Multi-colored, older lava flows are seen to the southeast of Culann Patera. These correspond to the darkest flows seen by the Voyager spacecraft in 1979. These lava had flowed into a shallow depression that is surrounded by low, etched mesas.

Comparing these images to a regional mapping observation taken in October 2001 reveal a number of changes within this dark flow field, suggesting that like Prometheus, it is a compound flow field that is built up by fresh, darker breakouts of lava. One suggestion provided by Keszthelyi et al. 2001 is that the curved, red line seen running northwest of the patera is a lava tube that brings lava from the source at the patera to the flow field. The reddish material that marks the location of this lava tube may result from sulfurous gases escape through skylights on the roof of the lava tube that then condense on the colder plains nearby. Lava flows through this lava tube from its source at Culann Patera to the lava flow field, where it spills out into first small breakouts in the medial portion of the dark flow field, then larger areas of warm, dark lava toward the distal end of the field. This idea is supported by high resolution near-infrared spectrometer data, which revealed two hot spots within the Culann volcanic system. The brightest hotspot is located over the dark flow field due its large exposures of recent lava. A smaller thermal hotspot is located at Culann Patera, the source of the lava flows in this region. The long length of the flows radiating out from it suggest that the lavas they are composed of had a very low viscosity, as there is very little difference in altitude between the margins of Culann Patera and the surrounding plains, based on stereo images by Galileo.

The colors seen at Culann are thought to be result of the interaction between sulfur and silicate lava. When sulfur, emitted from the source vent at the patera or from the main active lava tube, condenses and lands on the surface, it normally creates a reddish deposit on the surface. The size of the deposit is the result of the temperature of the escaping gases and the vent pressure. Given the size of the red deposit at Culann, both factors are likely to be less than at Pele, which has a red ring plume deposit more than a 1,000 kilometers (600 miles) across. When this sulfur condenses on Io's cold plains (~120 K/-244° F), this produces a red deposit. When it lands on warm lava, the sulfur is thought to interact chemically with the iron in Io's lavas to produce greenish iron sulfide, or pyrite as it is known in its mineral form. A few bright white spots are seen along portions of the current flow field to the northwest of Culann Patera. These result from the interaction between cold sulfur dioxide frost on Io's surface and hot, silicate lava that flows over it. The lava vaporizes the frost, which then condenses on the cold ground outside the flow field, or on older, colder areas of the flow field (gradually brightening the flow as it ages as more SO₂ frost and sulfur is deposited on it).

The color observation of Culann Patera in November 1999 allowed researchers to examine the relationship between different color units seen all across Io's surface. They revealed a complex interaction between sulfurous gases, emitted either from the primary vent or from re-volatilization by flowing lavas, and the terrain these gases land on. They also provided further evidence for how large lava flow fields develop, an idea that seems to be back up at other sites like Prometheus and Amirani.

Next week, we will take a look at the target of Galileo's 25ISGIANTS01 observation, Tvashtar Paterae.

References:
Williams, D.; et al. (2004). "Mapping of the Culann–Tohil region of Io from Galileo imaging data". Icarus 169: 80–97.
Geissler, P. E., M.T. McMillan. (2008). "Galileo observations of volcanic plumes on Io". Icarus 197: 505–518.
Lopes-Gautier, R.; et al. (1999). "Active Volcanism on Io: Global Distribution and Variations in Activity". Icarus 140: 243–264.
Keszthelyi, L.; et al. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". Journal of Geophysical Research 106 (E12): 33,025–33,052.

Follow-up on Friday's Impact on Jupiter

2010-08-23T18:10:00.002-07:00

As I reported yesterday, Japanese astronomer Masayuki Tachikawa recorded on video the impact of a small asteroid or comet on Jupiter's northern hemisphere on August 20 (early August 21 in Japan). The optical flash of the meteor streaking across the Jovian sky was also seen by two other Japanese astronomers, Kazuo Aoki and Masayuki Ichimaru. Both astronomers imaged Jupiter, again using webcams connected to their telescopes, during the impact event and recorded the optical flash of the fireball. Kazuo Aoki's recording allows a more precise estimate of the timing of flash to within a second of 18:21:56 UTC on August 20. Having more than observation of the event provides a confirmation of the observation and eliminates other potential sources of the flash, such as nearby artificial satellite.

Isshi Tabe has a webpage up where he is collecting the observations from various astronomers across the western Pacific of this impact event. More information on the methodology of these observations as well as links to images and videos can be found on his website.

Kelly Beatty reports over at the Sky and Telescope website that Imke de Pater and Heidi Hammel were also observing Jupiter using the 10-meter Keck II telescope in Hawaii. They did image the impact region during their two-day run, but in their initial look at the data, they didn't see anything new such as an impact scar. The impact occurred when the Sun was already up in Hawaii, so they most likely didn't observe the actual impact.

As I pointed out yesterday, this is the second fireball to be seen in Jupiter atmosphere this year, after only two previous impact event seen from Earth, in 1994 and 2009 (though the latter was only seen after the impact). This doesn't mean that impact events are somehow occurring more frequently. The discoveries this year are helped by the method many amateur astronomers use to record their observations and built up high-quality images. Because they tend to use smaller telescopes than most professionals, they are limited by the amount of light their telescopes can collect. This lowers the signal-to-noise of their observations. Increasing the exposure times is fine for faint targets, but for bright targets like Jupiter, that will just over-exposing the data, limiting their usefulness. Instead, they use USB cameras (webcams) to record video of their target of interest at 20-60 frames per second. Software is then used to stack the numerous frames they record to build up images with a much higher signal-to-noise ratio than each separate frame. This is akin to how we take images of Titan's surface, where we acquire 3 images of Titan's surface at 938 nanometers and then sum them on the ground. These webcam videos, taken by numerous amateur astronomers from around the world also have the side benefit of allowing them to detect transient events like meteor fireballs in Jupiter's atmosphere, which would have been difficult to detect otherwise. With so many amateur astronomers taking long videos of Jupiter, the likelihood that an impact event is detected is improved.

On that note, I want to repost something the head of the Jupiter section of the Association of Lunar and Planetary Observers, John Rogers, sent around today on the potential of this data set:

It would really be worth determining the frequency of these events. Some ideas:
1) Regular observers: Please can you tell me: On a typical night, how many minutes of video do you record and look through?
2) From now on, could regular observers record the start and end times of video they view each night, and could anyone volunteer to collect this information? Perhaps regional associations (ALPO, ALPO-Japan, etc.) might be able to collect this info? I hope this would not be a burden for observers -- Obviously it is wonderful when you process and send the best of your images as soon as possible, and I certainly would not ask you to delay until you have completed the paperwork! A list of video times sent once a month would be fine. But it would be worthwhile, so the amateur observers' network could make an important measurement of the frequency of these events
3) Maybe someone could devise software for scanning the webcam videos and identifying these fireballs automatically??

With these measures in place, perhaps more impact events will be recorded and detected going forward. This information will scientists to estimate the impactor flux in the Jupiter system. With some modeling, this can further improve our age estimates for the surfaces of Io and Europa.

On Isshi Tabe's website, he also points out that Christopher Go found a reference to a Voyager 1 observation of a fireball. The paper is by A. Cook and T. Duxbury and is titled, "A Fireball in Jupiter's Atmosphere". It was found while the spacecraft was eclipsed by the Sun, shortly after the Io encounter on March 5, 1979. The Voyager narrow-angle-camera was using this opportunity to image Jupiter's night-side with limited contamination from sunlight to search for lightning and meteor fireballs. They were able to find one fireball, in Jupiter's high-northern latitudes, with an absolute magnitude of -12.5 and a path length of 75 km (image c1639630 from 03/05/1979 17:45:24 UTC). Cook and Duxbury mass of the impactor was 11 kg. Assuming an impact velocity of 64 km/sec., the flash occurred over a period of 1.17 seconds, which is on the same order as the meteors seen this year from Earth. From their observations, they suggest that the number density for objects larger than 3 kg is a factor of 6 less than the estimate obtained from terrestrial meteors. Additional modeling and observations from ground-based telescopes should help to pin down this estimate.

Link: Isshi Tabe - Fireball on Jupiter in 2010 20th August UT [yokohama.cool.ne.jp]
Link: Paper - A Fireball in Jupiter’s Atmosphere [dx.doi.org]

Meteor fireball spotted in Jupiter's Atmosphere - Again

2010-08-22T14:37:00.009-07:00

Remember a time when Jupiter only had 16 satellites? Or when the only extra-solar planets know where a trio around a pulsar and a recently discovered handful? Or when the only known impacts on Jupiter seen by astronomers (or its after effects) were the Shoemaker-Levy 9 impacts in 1994? I do. Sometimes in science, once a discovery is made or observation recorded, it unleashes a torrent. First, one new moon is discovered out beyond Callisto (Themisto) in 1999, then another, then another at Saturn, and the next thing you know, both Jupiter and Saturn have 60+ known moons. Same for extra-solar planets, we've found so many, they are merely just part of a statistic for the Kepler team. Last July, amateur astronomers found an impact site on Jupiter, formed after an asteroid struck Jupiter's southern hemisphere. Then this year, on June 3, astronomers Anthony Wesley and Christopher Go spotted a meteor impact the planet's upper atmosphere, burning up before it could leave its own mark on the solar system's largest planet. Well, it has happened again.

On Friday, August 20 at 18:22 UTC (Saturday morning in Japan), Japanese astronomer Masayuki Tachikawa detected an optical flash, likely from a meteor striking Jupiter's upper atmosphere using a Philips Toucam Pro2 USB camera attached to his telescope. The impact is the first to be seen over Jupiter's northern hemisphere, occurring on the northern edge of the North Equatorial Belt (south is up in the image above) just to the east of the Great Red Spot's longitude. A detailed report on the discovery can be found over at the ALPO-Japan website (scroll down to the bottom for an English description of the observation). A video of the optical flash can be found on that site as well, though I found it best to save the file first before playing it. A French description (the first link I saw with this news, posted by Eric Soucy, tip o' the hat to him for this news) can be found at Ciel et Espace.

The ALPO-Japan website also has images from after the impact, taken yesterday. So far there doesn't appear to be any evidence for an impact scar, like the June 3 fireball.

Anyways, soon, observations of meteors in Jupiter's atmosphere will be just statistics used to determine the current impact flux in the Jupiter system...

UPDATE: Nick Previsich found a great English language link describing how this discovery was made from the Sky and Telescope's website.
Another Update: Two more excellent blog posts from Daniel Fischer and Emily Lakdawalla (who has a Youtube version of the optical flash video up) I would remiss to point out.

UPDATE 08/23 2:38 am: Thanks to Kelly Beatty and Emily Lakdawalla for pointing out some independent confirmation that this optical flash is in fact an impact on Jupiter.

Link: Optical flash on the surface of the Jupiter by M.Tachikaw [alpo-j.asahikawa-med.ac.jp]
Link: Another Flash on Jupiter? [www.skyandtelescope.com]
Link: Offenbar schon wieder ein Impaktblitz auf Jupiter von Amateur gefilmt [skyweek.wordpress.com]
Link: Yet another Jupiter impact!? August 20, seen from Japan [www.planetary.org]

The Remains of the Week

2010-08-20T15:54:00.002-07:00

I just wanted to fire off a quick post with other Jupiter/Io-related news that is hitting the intertubes this week:

The Carnival of Space, hosted here last week, is hosted this week for the 167th Edition over at Space Tweeps, a group of Tweeters who post quick space news on Twitter. A couple of posts that caught my eye include one from Paul Schenk showing off some new videos he created using topography and color data from Cassini and another from the IAG Planetary Geomorphology Working Group discussing hematite-rich regions on Mars (like the area Opportunity is sprinting across this week).
We are approaching next month's once-every-13-months alignment with Jupiter known as opposition. As we get closer to the opposition on September 22, many amateur planetary astronomers are pointing their scopes up at Jupiter to catch the best views of the planet on offer this year. One of my favorites showing Jupiter with Io is this one at right from Anthony Wesley, an astrophotographer out of Australia (who is best known for discovery not one but TWO impacts on Jupiter). It was taken on Wednesday from Exmouth, Western Australia. You can find more images of Jupiter by astronomers from around the world over at the ALPO-Japan and from the Cloudy Nights forum.
I want to thank Emily Lakdawalla for re-publishing my post on Io's true-color processing. She even dug up a nice chart showing the spectral curves of the cones in your retinas :)
Finally, Björn Jónsson created a nice, 12-frame color mosaic from Voyager 1 data of Jupiter. Very nice! I hope he posts more.

Link: Carnival of Space #167: The Space Tweeps Edition [spacetweepsociety.org]
Link: ALPO-Japan's Jupiter Section [alpo-j.asahikawa-med.ac.jp]

Honey, I Shrunk the Moon!

2010-08-19T23:18:00.004-07:00

Today at a press conference at NASA HQ in Washington, DC, scientists from the Lunar Reconnaissance Orbiter Camera (LROC) team announced their discovery of lobate thrust fault scarps on the surface of the Moon that indicate that not only has the Moon's radius shrunk by 100 meters, but it has done so in geologically recent times. High-resolution images of these scarps show that they cross-cut small impact craters, which would normally be removed over hundred-million-year timescales by thermal cycling of the soil by the moon's month-long day, solid body tides from its gravitational interaction with the Earth, and micrometeorite impact gardening.

What would cause such shrinking? When the moon was formed out of the impact of a large planetoid into the infant Earth, it initially was molten. Over time the crust and core solidified, but its mantle still contained a large amount of molten silicate magma. Magma from this "magma ocean" on occasion reached the Moon's surface, filling many of its large impact basins, creating the dark mare basalt regions. Over time, the Moon's mantle cooled and solidified as a result of reduced heating from radioactive elements, conduction through the Moon's crust, and the removal of hot magma through volcanism. Now as you may remember, for most materials, the phase change from a liquid to a solid causes a change in the density of the material. For silicate magma, the density increases, so for a given amount amount of magma, the volume will decrease when it solidifies. When this occurs on a planetary scale, such as a the Moon or Mercury, as the interior of the planet cools and solidifies, the planet or moon shrinks.

When this shrinking occurs, the crust of the planet must accommodate it. After all, when the volume decreases, the surface area also decreases. This causes compression within the crust which can be taken up by compacting loose surface material or reducing pore space, or through thrust faulting. Thrust faults are low-angle faults that allow the surface above the fault to be pushed up on top of the rock beneath it. The surface expression of thrust fault on the Moon or Mercury is a arcuate scarp. Examples of these scarps are shown above from the Moon (shown on the left side of the side-by-side image above from LROC data) and from Mercury (shown on the right). The lunar scarps are smaller than their Mercurian cousins, rising only 100 meters above the surrounding terrain at most, with the majority only rising a few tens of meters. On Mercury, like Beagle Rupes shown above cutting across the oblong Sveinsdóttir crater, these scarps are much than those found on the Moon, often rising a kilometer or above the surrounding terrain. Their height, length, and estimated horizontal displacement suggest that Mercury may have contracted by several kilometers. MESSENGER images will be used to date these scarps to determine when this shrinkage occurred just as the LROC group has done.

The situation is a bit different for icy bodies. Unlike silicates, when water solidifies its density decreases and volume increases when it freezes. So the radius of icy bodies increases rather than shrinks like rocky worlds like the Moon and Mercury. The equivalent feature to the lobate scarps of the rocky bodies formed from this expansion are the narrow extensional fractures that criss-cross ancient worlds like Tethys, Dione, and Rhea. The image at left, taken last Friday by Cassini, shows a pair of narrow fractures that cut across Penelope, an ancient impact basin on Saturn's moon Tethys. These fractures suggest that these worlds have expanded as a result of the solidification of water in their interior more recently than the heavy bombardment that produced the plethora of impact crater visible in scenes like the one at right. Like the work being performed at the Moon and Mercury, finding and mapping these fractures and measuring the amount of displacement they accommodated will tells us how much these moons have expanded (and thus how much of their interiors remained molten until comparatively recently) and approximately when, based on superposition relationships with impact craters.

Now this wouldn't be an Io-centric blog without pointing out that the lobate thrust fault scarps seen on the Moon and Mercury are puny when compared to equivalent features on Io. Now, obviously, unlike those two geologically dead(ish) worlds, Io is much more active and its mantle still has a large melt fraction. Its volcanoes dredge up enough material every year that it could cover the entire surface of Io in a layer one centimeter thick (though the amount of resurface varies greatly from place-to-place). These centimeters add up, and after a million years, the surface of Io is covered in approximately 10 kilometers (6.2 miles) of cooled lava, sulfur, and sulfur dioxide (though some of this material is locally recycled, so 10 km maybe an upper limit). This has the effect, for the present-day surface, of shrinking the radius of Io by 10 kilometers in one million years. As a result of global subsidence, thrust faults drive up massive "lobate scarps", also recognized as the mountains of Io. Two clear examples of this are shown at left, Gish Bar Mons and Euboea Montes. Both mountains reach 8-10 kilometers above Io's surrounding plains. Of course, this may be an over-simplified model for how Io's mountains form, so I would recommend reading my article on various formation models for more information on the details.

This study of lunar lobate scarps highlights the need for global high-resolution studies, whether we are talking about the Moon, Mars, Mercury, or Io, as LROC images allowed researchers to assess the global population of these cliffs. Once their true distribution was determined, they could then come up with a proper model of how they formed. This provided an important glimpse to the Moon's recent geologic activity, after the formation of the mare basalts.

Link: NASA's LRO Reveals 'Incredible Shrinking Moon' [www.nasa.gov]

Exposing Io's True Colors

2010-08-19T03:54:00.004-07:00

Thanks to its active volcanic activity and sulfur-rich surface, Io is one of the most colorful worlds yet seen in the Solar System, save the Earth of course. Publicly released images of Io from the Voyager and Galileo missions show a variety of colors on Io from reds surrounding Pele and Tvashtar, to yellow cyclo-sulfur and gray-white sulfur dioxide frost. Greens and red-browns crop up across Io's mid-latitudes and polar regions, respectively, either from sulfur impurities or radiation damage. However, the colors seen on most images of Io available on the internet use exaggerated colors, either from the use of filters that pick up light at wavelengths that are invisible to the human eye or by stretching the image to emphasize color differences from region-to-region. Both are quite useful for science as they reveal regional compositional differences that might not be apparent to the naked eye.

On this blog and on my Io image website, I have processed Galileo images with limited amounts of color exaggeration added, creating true color images of Io. I have done this by ensuring that the three images that make up each color composite use filters that allow photons in the visible spectrum to reach the camera's CCD (red, green, and violet) and by stretching each image in the color composite the same way. For the latter, this means that each of the three images (per frame if it is a color mosaic) was calibrated so that the pixel values were equal to the intensity of detected photons divided by the incoming flux from the Sun (I/F) at each effective wavelength. When I converted the data to a tiff format image (for editing in Photoshop), I linearly stretched all three color filters images so that the a pixel value of 0 was equal to the I/F of the black sky and 255 was equal to the maximum I/F seen in the RED filter (the brightest of the three color filters, at least for Io). The image of Io against the clouds of Jupiter at right was processed in this manner. As long as the images were properly photometrically calibrated, the resulting color images should be as close to true color as I can get without further manipulation of the images.

Now you may be asking yourself, "What do you mean 'without further manipulation of the images'? Didn't you just say that you were 'creating true color images of Io'?"

As Björn Jónsson points out on his webpage, Io's color, Galileo and its predecessors, the Voyager space probes, did not carry the necessary filters that would correspond to the Red-Green-Blue color space most commonly used either as channels in Photoshop, as the color space of modern LCD monitors, or the three types of cones in the human eye. Voyager's color filter set included ultraviolet (effective wavelength of 332 nanometers), violet (402 nm), blue (475 nm), green (564 nm), and orange (589 nm) filters. Galileo's color filter set included violet (404 nm), green (557 nm), and red (663 nm) filters as well as four narrow-band filters that were sensitive to light at near-infrared wavelengths (734, 756, 887, and 986 nm).

Now for most bodies that are relatively bland in color at visible wavelengths, like most of Saturn's satellites, the differences between an object's appearance in blue and violet filters would be pretty minor, and so it would make little difference if a violet filter image was used in place of a blue filter one for the blue channel. However, Io's albedo changes dramatically from a Galileo violet filter mean of 0.2771 to a green filter mean of 0.7443. Thus, the filter used for the blue channel is important as Io's average brightness is strongly dependent on the effective wavelength of the filter. Green and red filters for Galileo, Voyager, and Cassini are centered on either side of another ramp in Io's spectrum, and so they are less dependent on effective wavelength. Voyager's lack of a red filter prevented it from detecting the red color of the Pele plume deposits (instead they appear dark or with a dark, red-brown hue in Voyager color composites). With Galileo color composites, having to use a violet filter instead of a blue one for the blue RGB channel gives Io a strong, yellow hue, when it would likely be more muted.

You can see this in the two graphs below. These compare Io's geometric albedo (charted in black in these two graphs) to the spectral responses of color filters used by the narrow-angle camera on Voyager 1 and SSI on Galileo. Each colored curve represents the sensitivity of that filter to different wavelengths of light. Both factor in the additional complication of the camera sensor's (CCD for Galileo SSI, vidicon for Voyager ISS-NA) sensitivities. These data were obtained from the calibration reports for the Galileo and Voyager cameras. In the case of the Voyager NAC, both the violet and blue filters are located in the steep spectral slope in Io's geometric albedo. This slope is caused by the absorption of violet and ultraviolet sunlight by sulfur deposits on Io's surface.

In Voyager images, this slope means that there is a steep progression in the average brightness of Io from ultraviolet filter images to green filter ones. For Galileo, the violet filter is located near the bottom of this ramp while the green one is located at the base of another slope that results from reddish material in Io's polar regions and near active volcanic centers like Pele.

So how do we go about generating true color images of Io from Galileo images? First, I should point out that Galileo images are a better choice for synthesizing true color images since while the SSI camera may not have had an optimal filter set, at least it covered the full range of the visible spectrum, unlike the Voyager camera's vidicon sensor prevented it from detecting photons beyond 650 nm, cutting off a sizable portion of the red part of the spectrum. As I said earlier, Björn Jónsson worked on this problem earlier, and created a method of synthesizing a blue filter by mixing the violet and green filter images. He focused on recreating the overall appearance of Io as seen in blue filter Voyager images. When combining this synthesized blue filter images with the normal red and green filter images, he hoped to create a data set that approximated Io's appearance as it would appear to the human eye. He found that the best results came when blue channel pixel values could approximated as:

Blue = 0.61*Green + 0.39*Violet.

While this method seems to produce appropriate results, I am a bit concerned about using the Voyager blue filter for the blue channel. Voyager used a blue filter that I feel was a bit to green and was closer to being a cyan filter. Remember just because it is called a "blue" filter doesn't make it so :-) So, what I've done instead is try to synthesize blue (BL1) filter images from Cassini, which is part of the true-color set of filters on the ISS camera system (BL1-GRN-RED). The BL1 filter has an effective wavelength of 455 nm, a bit shorter than Voyager's blue filter. This would better approximate the blue end of the CIE color chart. I used following equation to mix a percentage of the green and violet filter pixel values to create a synthetic BL1 filter image:

Blue = 0.55*Green + 0.45*Violet.

These factors were calculated by finding Io's average geometric albedo at 455 nm (effective wavelength of the Cassini ISS BL1 filter), 404 nm (wavelength for the Galileo violet filter), and 557 nm (wavelength for the Galileo green filter) using ground-based spectral data published by Spencer et al. in 1995 (their data was also used for the Io spectrum in the graphs above). Then, I determined the weighted average between Io's albedo at 404 nm and 557 nm that would equal its albedo at 455 nm. The image at right shows the same global color image from March 29, 1998: the version on the left is the original, unstretched version, and the one on the right has had this correction factor applied to its blue channel. The version on the left should approximate a true-color image. Instead of a vivid yellow world, Io appears more subdued, and many white-gray regions on Io appear more pinkish than they did before.

I know I have go on long enough with this article, but I want to point out a few caveats, just as Björn did. Keep in mind that Io's surface materials have different spectra at visible wavelengths. While most have steep spectral slopes between 350 and 500 nm resulting from sulfur before shallowing through the rest of the spectrum detectable by Galileo's camera, for red materials this steep slope continues on to around 750 nm before shallowing. So the appearance of Io's reddish material would depend on the effective wavelength of the filter used for the red channel. Both the Cassini and Galileo red filters are on the long side of the effective wavelength used for the red channel on most computer LCD's and the red cone in the human retina. This makes reddish materials appear more vivid than they would in real-life, though not quite as muted as seen in Voyager color composites since the human eye is sensitive to longer wavelengths than the Voyager Orange filter even if its effective wavelength is closer to that of your red cones than the red filter on Cassini or Galileo.

Creating color composite images from spacecraft data is almost as much an art as science, and how you create them can depend on the image processor's preferences and the purpose of the processing. I tend to prefer not to enhance the data anymore than necessary, and try to stay true to the original data, even if it isn't exactly true color. That said, it has been an interesting journey into trying to create a "true" color Io image. I may have to post a few more examples using the above formula. I also realized that creating "true" color images may be affected by the conversions of the original DN values of the Galileo raw data to I/F (intensity over flux), so stay tuned on this.

References:
Spencer, J.; et al. (1995). "Charge-coupled device spectra of the Galilean satellites: Molecular oxygen on Ganymede". Journal of Geophysical Research 100 (E9): 19,049–19,056.
Spencer, J.; et al. (1996). "Io on the Eve of the Galileo Mission". Annu. Rev. Earth Planet. Sci. 24: 125–190.
Geissler, P.; et al. (1999). "Global Color Variations on Io". Icarus 140: 265–282.

Io Volcano of the Week: Hi'iaka

2010-08-17T16:52:00.001-07:00

This month for my Io Volcano of the Week series, we are looking at volcanoes that were observed at moderate resolution (160-280 meters or 525-920 feet per pixel) during Galileo's I25 flyby of Io on November 26, 1999. In the last couple of weeks we have looked at Zal and Emakong, two large paterae - volcanic depressions - on Io's leading hemisphere. This week we turn our focus a few hundred kilometers to the east of Emakong at the strangely shaped lava flow field, Hi'iaka Patera. It maybe pretty quiet as far as Ionian volcanoes go, but it may have had a wild past with a violent formation from the breakup of two massive mountains and lava flows that formed in the 17 years between Voyager and Galileo.

Let's get some of the basics out of the way first. Hi'iaka Patera is located near Io's equator on the moon's leading hemisphere (3.64° South, 79.47° West) and is 128 kilometers (80 miles) wide. The oddly shaped volcano is bounded by a set of low faults that confine a lava flow field on the eastern side of the volcanic depression. The western edge of Hi'iaka is bounded by a 3.5-kilometer (2.2-mile) tall massif named North Hi'iaka Montes. The southern margin of the volcano is less than 10 kilometers (6.2 miles) from the northern tip of another mountain, South Hi'iaka Montes.

Images of Hi'iaka taken during that Thanksgiving 1999 flyby were particularly useful for determining how the volcano may have formed. Observations of the spatial relationships between these two mountains and the volcano, the comparable heights for the two mountains (save the tall peak on the northeast tip of North Hi'iaka Montes) led to the suggestion that the two mountains were once joined. Subsequent extension and strike-slip tectonism then rifted these two mountains. In this scenario, Hi'iaka Patera may be a pull-apart basin, which forms when there is a bend or gap in a strike-slip fault system, creating extension at the bend. The image at left shows a schematic of these basins are formed. Depending on the speed of the rifting, lava can exploit the faults along the margin of this basin to reach the surface and cover portions of the depression that is formed between the two transform faults [this scenario was further explained back in December 2009 and February 2009 when I discussed work performed by Melissa Bunte and her co-authors]. Lava using extensional faults on Earth certainly isn't unheard of. The East African Rift Valley has a number of prominent active or dormant volcanoes (*cough* Kilimanjaro), and fissure eruption are not unheard of in the last few million years. The active lava lake, Erta'ale, is located in the Afar triangle and the triple junction between the Red Sea, the Gulf of Aden, and the East African Rift Valley. The motions of Ionian micro-plates like that suggested for Hi'iaka Patera may have resulted in the formation of several volcanoes including Zal, Monan, and Shamshu, though additional information on Io's sub-surface will be needed to pin down this theory, since later deposition of volcanic ash and sulfur has obscured most surface expressions of tectonic faults on Io's surface.

In term of recent activity, Hi'iaka is a relatively quiescent. While there are dark lava flows with different albedos (and presumably ages) covering much of the eastern half of the depression, no high-temperature eruptions have been observed at the site. Galileo's Near-Infrared Spectrometer observed thermal emission from the volcano on six occasions during the Galileo Nominal Mission, but it was never seen by camera system during an eclipse, even though the nearby volcano, Tawhaki Patera, was. This suggested that low levels of effusive activity were present at Hi'iaka during the Galileo mission, which is consistent with the morphology of the dark lavas seen in the high-resolution Galileo images. Bunte et al. 2010 suggested that Hi'iaka Patera might be a lava lake, or in the process of forming one. At present, however, the morphology of the dark lava at Hi'iaka, with its multi-lobed structure and suggestions of a series of overlapping flows, is more consistent with an inflated flow field that is built up by a series of thin lava breakouts. Similar eruptions, though more vigorous, are seen across Io at volcanoes like Zamama, Prometheus, Marduk, and Amirani.

That's not to say that more vigorous eruptions are not possible. A faint, reddish plume deposit was seen surrounding Hi'iaka when Galileo first started imaging Io in June 1996 that slowly faded as the mission progressed. Even more curiously, Hi'iaka is barely visible in Voyager images of the region taken in 1979. This suggestions that the lava flows Galileo and later New Horizons saw at Hi'iaka formed between 1979 and 1996, in same time frame as Zamama and the new flow fields at Prometheus and Culann. Using the Pillan eruption as a possible template, perhaps it is possible that these flow fields built up quickly, on the scale of only a few months. As the eruptions died down, activity at these volcanoes may have transitioned to from a violent, fire-fountaining, outburst eruption to a more quiescent insulated lava flow. However, given the number of years between Voyager and Galileo, there is no reason to require an initial outburst at these flow fields, and they may have built up gradually over the 16 year time span. However, I will point out that only minor expansions of the extent of these flow fields were ever seen during 4 years of the Galileo mission at Io. Several outbursts were known to have occurred in Hi'iaka's region (or at least its hemisphere) during this interval that may have resulted in the initial creation of the Hi'iaka flow field, including a bright outburst seen in 1986 that provided evidence for silicate volcanism on Io.

That's it for this week's star volcano, Hi'iaka Patera. Come back next week when we will profile the colorful Culann.

References:
Bunte, M.; et al. (2010). "Geologic mapping of the Hi’iaka and Shamshu regions of Io". Icarus 207: 868–886.
Turtle, E. P.; et al. (2001). "Mountains on Io: High-resolution Galileo observations, initial interpretations, and formation models". Journal of Geophysical Research 106 (E12): 33175–33199.
Johnson, T. V.; et al. (1988). "Io: Evidence for Silicate Volcanism in 1986". Science 242 (4883): 1280–1283.

DPS 2010 Meeting Abstracts Posted Online

2010-08-11T21:43:00.001-07:00

The abstracts for the 42nd Meeting of the Division for Planetary Sciences of the American Astronomical Society (DPS) were posted online earlier today. The annual DPS meeting focuses on a variety of planetary science topics. This year's meeting is scheduled for October 4-8 and will be held at the Pasadena Convention Center in Pasadena, California.

Two Galilean satellite oral sessions planned during the mornings of Wednesday, October 6 and Friday, October 8. A poster session is also planned for the afternoon of October 6. In addition to these Galilean sessions, there is a town hall meeting covering the Europa/Jupiter System Mission during lunch on Tuesday, October 5.

Five talks and three posters are planned during Galilean Satellite sessions scheduled for October 6. I have listed them below.

Talks

32.01. DSMC Simulations of Irregular Source Geometries for Io's Pele Plume
William McDoniel1, D. B. Goldstein, P. L. Varghese, L. M. Trafton, D. A. Buchta, J. Freund, S. W. Kieffer
32.02. Morphology and Temperatures at Pele
Robert R. Howell, R. M. C. Lopes
32.03. Io’s Active Volcanoes from New Horizons MVIC and LORRI Data
Julie A. Rathbun, K. B. McMillian, L. W. Kamp, R. M. Lopes, J. R. Spencer
32.04. Impact of Plasma Chemistry on Io’s Atmosphere
Chris H. Moore, H. Deng, D. B. Goldstein, D. Levin, P. L. Varghese, L. M. Trafton, A. C. Walker, B. D. Stewart
32.05. Effect of Orbital Phase on the Structure of Io's Atmosphere
William H. Smyth, M. L. Marconi

Posters

26.01. Density and Temperature of Io's Atmosphere from Mid-Infrared Observations, 2001 to 2010: Continued Evidence for Inflation as Perihelion Approaches
Constantine C.c. Tsang, J. R. Spencer, E. Lellouch, M. J. Richter, M. A. Lopez-Valverde, T. K. Greathouse
26.11. Simulated Ionian Column Densities
Andrew C. Walker, D. B. Goldstein, P. L. Varghese, L. M. Trafton, C. H. Moore
26.12. Io Volcanism: Modeling Vapor And Heat Transport
Daniel R. Allen, R. R. Howell

Link: DPS 2010 [dps.aas.org]

Blogger Static Pages at The Gish Bar Times

2010-08-10T22:29:00.000-07:00

As some of you may have noticed, over the last couple of weeks, I have been taking advantage of Blogger's new Pages system, which allow me to add static pages to the blog and link them using tabs below the heading banner. If you haven't noticed them, or read my blog using an RSS Reader and therefore haven't seen them, I encourage you to check them out or use them for reference if need be. I have converted the Io Basics posts I wrote a few months into a static page for easier reference, and will use that to present a more extended introduction to Io and my blog. The Io Information page is basically a copy of the three main Wikipedia articles on Io (Io, Exploration of Io, and Volcanism on Io) and provides a more extended introduction to Ionian science than what is provided in the About Gish Bar Times page. I have also added an index of the Io Volcano of the Week articles and an informative table on spacecraft encounters with Io, both past and future.

Perhaps the most significant static page I have added is a zoomable global map of Io. The map is based on the USGS global basemap and includes labels for named surface features. The Zoomify map, based on Adobe Flash, allows you to zoom in on the global map as well as pan around using the buttons at the bottom of the player, the navigation inset at top left, or clicking and dragging across the image. You can even expand the image to display full-screen on your computer monitor. In the future I hope to add more functionality to this page, including separate layers for the labels and another indicating the location of plumes and hotspots. Not sure I can do that with Zoomify, so I am open to other suggestions for perhaps using other platforms.

Link: Zoomable Global Map of Io [www.gishbartimes.org]

Io Volcano of the Week: Emakong

2010-08-09T23:24:00.000-07:00

This month for my Io Volcano of the Week series, we are looking at volcanoes that were observed at moderate resolution (160-280 meters or 525-920 feet per pixel) during Galileo's I25 flyby of Io on November 26, 1999. Last week, we examined Zal Patera, a large volcano on Io's northern hemisphere that has been the site of large lava flows and a small volcanic plume. This week we take a look at Emakong Patera, a large lava lake smack dab in the middle of Bosphorus Regio. While a largely inactive lava lake during the Galileo mission, high-resolution observation of this volcano by the camera and near-infrared spectrometer on Galileo have relaunched the debate over the predominance of sulfur and silicate volcanism on Io.

Emakong Patera, like Zal Patera described last week, is a larger than average, roughly heart-shaped patera, or volcanic depression, being 79 kilometers (49 miles) long north to south and 72 kilometers (45 miles) wide west to west. The name of the volcano is derived from the mythology of the Sulka people of the southeast coast of the island of New Britain in Papua New Guinea. In the myth, Emakong dives into a stream to retrieve an ornament he dropped. Upon reaching the bottom, he found that he was in the yard in front of a house. The people from this house allowed him to stay for the night around the hearth fire, both alien concepts to Emakong. The next morning, Emakong was given night and fire as gifts to bring back to his own people. Unlike another volcano named for a mythical fire bringer, Prometheus, Emakong had never been seen as a very active volcano, despite its dark surface and numerous surrounding lava flows.

The surrounding lava flows however, are not dark but are bright instead. Bright white to yellow flows radiate for up to 370 kilometers (230 miles) from the edge of Emakong across much of Bosphorus Regio. Based on medium-resolution imagery acquired in November 1999, researchers discovered that some of these flows are fed by narrow channels that formed when lava overflowed the walls of the Emakong basin. Other bright flows, particularly along the western margin of Emakong, appear to be broader over-flows from the patera. Galileo scientists suggests that rather than being composed of silicate basalt like most of the lavas seen on Io, instead Emakong's lava flows are composed of sulfur. When quenched at different temperatures, cooled sulfur flows can have different colors. The higher the quenching temperatures, the darker and red the cooled flows appear. So it would follow that the cooled sulfur lava in the channels that feed the bright flows and on the floor of Emakong Patera is darker than the bright flows themselves, as they were quenched at higher temperatures.

Alternatively, the flow may have originated as a silicate flow that was over time covered more and more by sulfur. Such as process is seen at Chaac Patera, where brighter sulfur ponds in the valleys within the silicate lava that covers the patera. Over time, this slowly brightens the lava flow. Like Chaac, Emakong also has a greenish color, thought to form from the interaction between cooling, iron-rich silicate lavas and sulfur. Personally, I prefer this theory over one that suggests that this is a sulfur flow. During the Galileo and New Horizons, several examples were observed where bright yellow flows were covered over by basaltic lava during more recent high-temperature eruptions. This provides a potential connection between current activity on Io and earlier activity, though there are examples on Earth where the same volcanic vent has been known to release both sulfur and silicate lava. While Emakong has been generally inactive in the current epoch, it is always possible the volcano may later re-activate these flows, forming broad silicate lava flows to cover the older sulfur-coated flows.

Emakong and the dark channel that flows to the east from its margin were imaged at high spatial resolution by both the SSI and NIMS instruments on Galileo in October 2001. The mosaic at right shows the southwest margin of Emakong Patera with its dark band of hot lava along its outer margin. The dark channel that is the focus of this observation starts out as a crusted over, multi-braided channel near the edge of Emakong before turning into an open channel after flowing a few kilometers to the east. The morphology of the channel in this region is similar in appearance to lava lake overflow channels that have formed at Kilauea when the overflow occurs over an entire sector of the lake with hot, low-viscosity lava that had a variable flow-rate. The flanks of the channel are formed by relatively dark material, which is itself surrounded by bright material. Further downstream, the channel becomes broader and progressively more crusted over, suggestive of the transition from an open channel lava flow to a lava tube.

The mix of bright and dark material in the plains southwest of Emakong Patera is difficult to assess. The lack of clear topographic shading makes it difficult to even determine whether bright material lies on top of dark, or vice versa, which is needed to determine stratigraphy of these lavas. The shading nearer the lava channel does suggest that lava over flowing the channel is initially dark before becoming bright, which could occur if hot sulfur is quenched near the channel, while cooler sulfur is quenched more distally. Strangely, the interplay between bright and dark does appear similar to another Galileo observation, a 5.5-meter (18-foot) per pixel mosaic east of Isum Patera taken in February 2000. In both cases, the complexity of the scene makes it difficult to assess the relationship between bright and dark material. In both cases, a bright lava flow is covered at high-resolution. Discussion of that observation may have to wait for another day.

What of Emakong Patera itself, what type of volcanic activity occurs there? The patera was not seen as a hotspot until the Galileo flybys in November 1999 and February 2000. These low-resolution observations indicated the presence of either cooled silicate or warm sulfur flows on the floor of the volcano. A high-resolution (2 kilometers or 1.25 miles per pixel) NIMS observation was obtained over Emakong Patera during a Galileo flyby in October 2001. This observation confirmed the presence of warm material on the floor of the patera, at least compared to the surrounding terrain. While much of the patera floor had a consistent temperature, the margins were much warmer, reaching a peak near 270 ± 90 K (26±194°F). This observation is consistent with a quiescent lava lake, as the warmer temperatures along the margin correlate with a ring of dark material that lines the outer margins of the patera. The temperatures detected by NIMS are still too cool for even molten sulfur (unless it is highly impure and forms a low-temperature eutectic), suggesting that regardless of whether Emakong is a site of sulfur or silicate volcanism, it was relatively inactive during the Galileo mission (and there is nothing to suggest that this has changed since the mission ended). The temperatures within Emakong are also low enough for sulfur dioxide frost to be detected in the same pixel as a thermal hotspot, the only place on Io this has been detected.

While the debate between the predominance of sulfur or silicate volcanism has been settled for much of Io in favor of silicate volcanism, Emakong is one of several locations where uncertainty remains. Regardless of whether sulfur or silicate volcanism dominates at Emakong, it has been years since it was last active beyond thermal emission from warm lava leaking out through cracks in the crust of the Emakong lava lake, unless the sulfur is highly impure and forms a low-temperature eutectic. The large bright lava flows surrounding Emakong tell the tale of past glory for the volcano, which may one day erupt again to flood hundreds of square kilometers in hot silicate or sulfur lava.

References:
Williams, D.; et al. (2001). "Evaluation of sulfur flow emplacement on Io from Galileo data and numerical modeling". Journal of Geophysical Research 106 (E12): 33,161–33,174.
Keszthelyi, L.; et al. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". Journal of Geophysical Research 106 (E12): 33,025–33,052.
Turtle, E.; et al. (2004). "The final Galileo SSI observations of Io: orbits G28-I33". Icarus 169: 3–28.
Lopes, R.; et al. (2004). "Lava lakes on Io: observations of Io’s volcanic activity from Galileo NIMS during the 2001 fly-bys". Icarus 169: 140–174.

The Carnival of Space: Issue #166

2010-08-09T14:11:00.009-07:00

Welcome one and all to Io and the Gish Bar Times for this week's Carnival of Space - a weekly series of articles highlighting the best space exploration blogs have to offer. Each week a different blog hosts the festivities, and this week the Gish Bar Times was selected as the host. The Carnival is organized by Universe Today, and you can check out their website for information on how to participate in future editions of the Carnival of Space and for back issues.

Now when Fraser selected my blog to host this week's Carnival of Space, I was in a quandary over how to organize the submitted articles. Last time I hosted back in October, I faced a similar, perplexing problem. How do I make it unique? For that edition, I settled on ordering the subject planetary/stellar bodies by the amount of energy output, placing fairly inert bodies like the Moon and the asteroids first, then more energetic moons and planets next, then stars, and finally the universe in general. So for this issue of the Carnival of Space, that just wouldn't do. For this Carnival, let's try out the old tried and true, Chronological order.

So welcome, stay awhile, explore the Gish Bar Times and the great space articles discussed below. And as always, don't feed the space bears! Yes, they are part of the carnival, but they have this special diet, I really don't want to get into it... Anyways, enjoy!

We begin this journey through time, space, and space blogs by visiting the Rundetaarn, a nearly 400-year-old Danish observatory in Copenhagen, built during the waning years of the Thirty Years' War. Ian Musgrave has posted a series of photos from his visit to the observatory on his blog, Astroblog. The Rundetaarn was built in the early days of telescopic astronomy, only three decades after Galileo's discovery of Io and the other Galilean satellites. One important task that the early astronomers at Rundetaarn and backyard astronomers today needed to perform was determining true north. The Urban Astronomer has a treatise on a simple method for determining true north by using the Sun at local noon, a method simple enough to be possible on almost any solid surface in the Solar System.

Over the next few hundred years, telescopes were used to make a number of momentous discoveries, far too many to recount here. One such revelation in the 19th Century was the connection drawn between short-period comets and meteor showers. The story of the Perseids and their parent comet, 55P/Swift-Tuttle, is explored at Simostronomy. Alan Boyle over at the Cosmic Log provides an observer's guide to this year's incarnation of the Perseid meteor shower, which peaks this week.

Also in the 19th Century, German astronomer Samuel Heinrich Schwabe and Swiss astronomer Rudolf Wolf discovered that solar activity waxed and waned with a period of 10.7 years. This periodicity is known as the Solar cycle. At solar minimum, the number of sunspots is low, approaching zero at times. At solar maximum, as we are expected to reach in the next few years, solar activity is high with more than 100 sunspots at a time observed as well as severe coronal mass ejections (CMEs), which create concentrations of charged particles that seem like gusts and gales in the solar wind. The Sun's activity at solar maximum can wreck havoc on the electrical systems of orbiting satellites. The Chandra Blog explores the effects of the upcoming solar maximum on the X-ray space observatory, and how this Solar cycle maybe weaker than expected. Of course, tell that to people in the mid-northern and mid-southern latitudes of our planet last week as a powerful CME touched off spectacular auroral displays. PlanetBye presented a series of pictures from Scandinavia of the incredible aurora borealis that were visible last week. Unfortunately, I live too far south in the United States to have seen them in person, so thanks to Bente Lilja Bye for hosting these great pictures. As he and other aurora watchers waited for last week's display, Stuart Atkinson over at Cumbrian Sky recounted his experiences of amazing set of aurora borealis over the UK in April 2000.

We move on now to the 20th Century where a major development in our exploration of space was the commencement of manned spaceflight. In 1961, US President John F. Kennedy committed his country to the goal of landing men on the Moon before the end of the decade. The effort succeeded despite its champion's tragic death, but when it was initially proposed, there were several competing schemes for accomplishing this goal. David Portree over at Beyond Apollo took at look at one such idea called Lunar-Surface Rendezvous (LSR) and presented by the Jet Propulsion Laboratory that would have included a number of unmanned probes for each manned landing site. In the end, only Apollo 12 landed near an earlier unmanned landing site, visiting Surveyor 3. The most recent manned spaceflight development, new equipment and capabilities continue to be added to the International Space Station, even as the Space Shuttle winds down. Kentucky Space has a video showing the installation of the NanoRacks system on the station, a new platform that NASA says went from "concept to on-orbit capability in less than 10 months." collectSPACE discussed a humanoid robot called Robonaut 2 that will be going to the space station with the Shuttle Discovery on STS-133 in November. Perhaps the coming of even testing models of humanoid robots should go in the paragraph about the end of the world at the end of this post ;-)

Of course, humans are not the only ones exploring space, as our robotic emissaries beam back images of the planets and moons of our solar system. This week, on my own blog, I discussed what we can learn at Jupiter's moon Io while the satellite is in eclipse, how Io is not quite as smelly as this summer's news reports suggest, and the volcanic history and geology of one of Io's volcanoes, Zal Patera.

Despite the doomsayers, the end of manned spaceflight is not nigh, as engineers and scientists are looking to the future of human exploration of the solar system and beyond. One group is the Space Studies Institute, which is getting ready for the Space Manufacturing 14 conference on the technology and policy needed for space settlement. Habitation Intention has an interview up with Dr. Lee Valentine, the executive vice-president of the Space Studies Institute. Of course, getting to other places in the solar system is half the battle, and Next Big Future has a pair of articles on a study by high school student Erika DeBenedictis on low-energy trajectories and a video by Reaction Engines demonstrating their designs on a proposed Mars mission.

We may even find that we are not alone in this universe as we begin our exploration of our own little corner of it. Centauri Dreams examines Project Argus, which would use quite a number of radio telescope to provide more continuous temporal coverage of the entire sky in order to look for short radio bursts from extraterrestrial civilizations. WeirdWarp this week discussed the possibility of finding artifacts from aliens on nearby worlds in the solar system, particularly on the Moon (with shades of 2001: A Space Odyssey). Finally, Weird Sciences has a pair of posts suggesting that extraterrestrial contact is not a crazy idea, even given the vast distances to even the closest star systems. They also discuss the possible evidence we have on hand for life elsewhere in the solar system.

All good things must eventually come to an end. A pair of blogs this week looked at our end in the distant future. StarryCritters examined a composite image of the Antennae Galaxies (NGC 4038 and 4039), created from data taken by the Hubble, Chandra, and Spitzer Space Telescopes. These galaxies are in the process of colliding, creating spectacular displays at visible, X-ray, and infrared wavelengths, much as the Milky Way and Andromeda Galaxies will in a few billion years time. It also doesn't help that our universe isn't getting any younger and that we are all headed toward a very dimly lit distant future, all thanks to entropy as discussed in a podcast [MP3 file] over at Cheap Astronomy.

Thanks everyone for coming out here to the best moon in the solar system for this Carnival of Space! I hope everyone enjoys these wonderful articles.

In the Shadow of a Giant: Observing Io in eclipse

2010-08-09T00:01:00.000-07:00

One technique scientists use to monitor Io's active volcanism and atmospheric processes is by observing the moon while it is in eclipse. Every Io day, the satellite passes into the shadow of Jupiter for a period of 2 hours and 20 minutes, plunging the entire moon into darkness. But as first faintly seen by the Voyager 1 spacecraft when it flew by in 1979, Io glows in the dark. These glows come from a variety of sources on Io's surface and atmosphere, such as thermal emission from lava flows and lava lakes, and aurorae from the interaction between gases in the atmosphere and Jupiter's magnetic field. In this article, we will explore what scientists can learn from observing Io while in the darkness of Jupiter's giant shadow.

A few hours after its flyby of Io on March 5, 1979, Voyager 1 observed the moon while it was in eclipse. It was a very long exposure, wide-angle-camera image designed to capture the night-side of Io against the background of stars at the end of the encounter. The image contained multiple exposures of the night-side with a camera slew between each exposure. It (FDS 16395.39, PICNO 0376J1 + 000) revealed a number of faint glows across Io. Cook et al. in 1981 examined this image, and found that most of the faint glows, smeared as they were by the multiple exposures, were related to the plumes detected in Voyager 1's daylight images of the satellite, including faint flows from Pele, Marduk, Amirani-Maui, Prometheus, and Volund. The poles of the moon were also faintly illuminated, with the north pole being a bit brighter than the southern one. No thermal emission from Io's volcanic hot spots were observed on this occasion because the vidicon sensors used by the Voyager camera system could not detect photons with wavelengths longer than the orange portion of the visible spectrum. This is much too short to detect thermal emission from all but the hottest of volcanic eruptions. Cook et al. concluded that the glowing gases were not the result of extremely hot plasmas, but were glowing due to being excited by electrons from Jupiter's magnetosphere. They suggest, however, that the polar glows are the result of gases emitted from numerous small vents, as opposed to plumes, which as we shall see from Galileo's results, may not be the correct interpretation.

The Galileo spacecraft arrived at Jupiter 16 years after the Voyager encounters, and stayed to observe Io every few months until its eventual destruction in Jupiter's atmosphere in September 2003. On Galileo's first orbit, it imaged Io in eclipse on June 29, 1996. These first images were taken in the Solid State Imager's clear filter, just as Voyager 1's eclipse observation was, but unlike Voyager's camera, the SSI was sensitive to photons from the near-infrared portion of the spectrum (0.7-1.0 microns). This allowed the SSI camera to detect volcanic hot spots with temperatures greater than 700 K. Scientists found several visible hotspots in this first observation (Pele, Reiden, Marduk, Isum, Mulungu, Fo, and Zamama), providing a proof of concept for this method, which supplemented the spectral observations of the Near-Infrared Mapping Spectrometer (NIMS). The greater sensitivity to high-temperature volcanism, and finding it all across Io's surface also provided further proof that basaltic volcanism was wide-spread. In addition to these more discreet glows, Galileo also observed fainter glows from volcanic plumes (such as Ra Patera), similar to the Voyager observation. The limb of Io also glowed, not just the poles Voyager had seen, though the south pole was brighter in this case (the north pole was brighter when Voyager observed Io in eclipse).

Galileo observed Io while the moon was in eclipse during most of its nominal mission orbits and on several occasions during the extended missions. These images allowed scientists to monitor high-temperature volcanic activity on the surface. A particularly brilliant hot spot was seen at Pillan in June 1997, providing evidence for a massive volcanic eruption there, which would be confirmed by the presence of a volcanic plume in daylight images from that orbit and a massive surface change around the hotspot seen three months later. Eclipse images even showed how Pillan's hot spot had split by September and November 1997 due to lava flowing down into Pillan Patera from the plains above, thus creating a new source of thermal emission in the near-infrared. Such small-scale details were not visible to the NIMS instrument until Galileo's encounters with Io later in the mission. Images taken in the clear filter as well as SSI's near-infrared filters, such as the one at 1.0 micron, provided a way to crudely measure the blackbody temperature of Io's high-temperature volcanoes. This was accomplished by calculating the ratio between the clear filter radiance of a hot spot and its radiance in the one micron filter.

Galileo's observations of Io in eclipse also allowed scientists to better understand the faint glows from the plumes and atmosphere. These glows are the result of interactions between Jupiter's magnetosphere and the atmosphere of Io. On two of Io's orbits, G8 and E15, Galileo scientists imaged Io in color to better understand the types of emissions that make up the aurorae on Io. The best of this data set, from May 1998, is shown at left. They reveal glowing aurorae of various color across different parts of Io's surface. These can be categorized as bright blue, equatorial glows, red polar limb glows, and fainter green, anti-Jupiter hemisphere glows. When an excited atom or molecule returns to the ground state, it sends out a photon with a specific energy. This energy depends on the type of atom and on the level of excitement, and we perceive the energy of a photon as color as the energy relates to a specific wavelength on the visible spectrum. Just as red aurora on Earth are caused by atomic oxygen, the red limb auroral glows on Io are thought to be due to emission from neutral atomic oxygen at 630 and 636 nm. The oxygen in Io does not derive from organisms on the surface, but from the break-up by solar ultraviolet light of sulfur dioxide in a process called photolysis:

SO₂ + hν → SO + O.

The green glows are thought to caused by excitation of neutral atomic sodium, which releases photons in the sodium D lines at 589 and 590 nm, within the bandwidth of the green filter. Atomic sodium is another photolysis product, this time deriving from sodium chloride belched from Io's volcanoes. The bright blue glows along the limb near Io's equator are thought to be due to the excitation of molecular sulfur dioxide. Most of these glows are related to various plumes visible near Io's limb at the time of the observation, including Amirani, Acala, Prometheus, Zamama, and Culann. The identification of these chemical species are supported by ground-based and Hubble observations of Io's aurorae, which revealed the presence and absence of various atomic and molecular emission lines, including sulfur and oxygen lines in the ultraviolet at 190 and 135.6 nm, respectively.

Geissler et al. (2001) explored much of the available Galileo and Cassini eclipse image dataset and reported on the how the intensity and position of these auroral emissions change with time. For example, across the Galileo dataset, the red limb emissions were brightest over either Io's north or south pole. The bright equatorial glows, visible as blue glows in the E15 dataset, tend to be located either at plumes or near the sub- and anti-Jovian points on Io's surface. In the latter case, these glows at various times either appear north of the sub-Jovian point and south of the anti-Jovian point, or vice versa. Both temporal variations in the red limb glows and the blue equatorial glows are related to Io's position in the Jovian magnetosphere. Jupiter's magnetic field is tilted with respect to the orbital plane of its main satellites by 9 degrees, so at various times as Io's orbits the planet, the moon is either above or below the equatorial plane of Jupiter's magnetic field. At a system III longitude (λ_III) of 0°, Io is below the normal plane of the magnetic field (this occurs with λ_III between 290° and 110°). The ion flux from the plasma torus is greatest over the north polar region of Io, causing greater excitation and increasing the brightness of Io's red auroral glows over the north pole. Jupiter's magnetic field lines are also tilted with respect to Io's equatorial plane, so they are tangent to surface north of the equator near Io's sub-Jovian point, and south of the equator near the moon's anti-Jovian point. When the λ_III longitude is between 110° and 290°, Io's situation is reversed as it is now above the plasma torus. This time, the south pole limb emissions are brighter than the north's, and the equatorial glows (that aren't related to volcanic plumes) have switched hemisphere as they are now bright south of the sub-Jovian point and north of the anti-Jovian point. This cycle recurs every 12.95 hours, the synodic period between Io's orbital period and the rotational period of Jupiter's magnetic field.

A word should be said about one of the mysteries regarding Galileo and New Horizons eclipse observations of Io: scattered around the sub-Jovian and anti-Jovian points on the satellite are a great number of glowing spots. These spots appear similar to hot spots in size and intensity, but their concentration is much greater than the rest of the satellite. Each of these spots can be correlated with a volcanic feature in Voyager images of this region. Other infrared observations of the satellite suggest that thermal emission is not higher in these regions compared to the rest of Io, so the spots are probably not thermal emission from high-temperature lava. As you can see in the image at left, the glows seem to be concentrated north of the equator near the sub-Jovian point (the white lines in the image are the equator [horizontal line] and the prime meridian [curved vertical line]), consistent with the tangent point of the Jovian magnetic field lines at the time of this observation. From this one could surmise that these spots are the result of glowing gases rather than thermal emission. But what would concentrate these gases into discreet spots that "look" like hot spots? One theory that I favor suggests that these spots are related to gases being slowly leaked from Io's interior at cooler volcanoes. Not enough gas is released at these volcanoes to form plumes, but there is enough of sulfur dioxide that their auroral emissions are brighter than the surrounding regions. They show up near the sub- and anti-Jovian points and not in other regions, not because there are more volcanoes there, but because Jupiter's magnetic field lines, and the electrons that are transported along them, connect to Io in these two regions. This increased electron flux increases the brightness of the auroral emissions.

Future missions to Io will certainly use this technique to monitor changes in Io's volcanic activity and atmospheric emissions. Narrow-band filters on future cameras would allow researchers to focus on specific line emissions from various chemical species such as atomic oxygen, sulfur, and sodium. They would also allow for improved measurements of hot spot temperatures. With increased bandwidth, more images using different filters can be taken during each eclipse, allowing for temporal variability of auroral and volcanic thermal emission to be monitored. Finally, these measurements will allow researchers to determine the contribution outgassing from smaller volcanoes provides to Io's atmosphere. Previously modeling focused on the contribution a few volcanic plumes provide, but not weaker outgassing from cooler, but more numerous, volcanoes.

References:
Cook, A. F.; et al. (1981). "Volcanic Origin of the Eruptive Plumes on Io". Science 211 (4489): 1,419–1,422.
McEwen, A. S.; et al. (1997). "High-temperature hot spots on Io as seen by the Galileo solid state imaging (SSI) experiment". Geophysical Research Letters 24 (20): 2443–2446.
McEwen, A. S.; et al. (1998). "High-Temperature Silicate Volcanism on Jupiter's Moon Io". Science 280 (5373): 87–90.
Geissler, P.; et al. (1999). "Galileo Imaging of Atmospheric Emissions from Io". Science 285 (5429): 870–874.
Geissler, P.; et al. (2001). "Morphology and time variability of Io's visible aurora". Journal of Geophysical Research 106 (A11): 26,137–26,146.

The Mud Volcano from the Carnival of Wonders of the Solar System

2010-08-04T17:56:00.003-07:00

I just wanted to throw up a few quick notes from the news and other blogs this Wednesday evening:

The Cumbrian Sky, a great astronomy blog by Stuart Atkinson, is this week's host of the Carnival of Space. Check it out for some great photos taken over the last week from the Opportunity rover, still trucking along through Barsoom on its way to Endurance crater. The Carnival of Space is a series of posts that is hosted each week by a different space or astronomy blog. These posts highlight the best articles in the space and astronomy blogosphere that week. This week, the Carnival of Space looks at the near-future of space exploration, the anniversary of the Voyager encounters with Saturn, and the distinction between Earth-like planets and Earth-sized planets (Stuart's own post on this latter topic has the funniest scientific graphic involving a kitten ever made).

Speaking of blogs, Emily Lakdawalla's Planetary Society blog has a new post up in reporting on the possibility of mud volcanoes in northern plains of Mars. I personally assumed they were pingos, but Emily's article, covering a paper by Oehler and Allen, makes mud volcanoes as the origin of the many flat, rounded mounds that dot the lands seem pretty reasonable. I am not a Martian geologist so I can provide any useful commentary, however. Mud volcanoes are cool though.

Finally, the documentary series, Wonders of the Solar System, is premiering tonight here in the United States on the Science Channel. I brought this up a couple of week ago on this blog, and you can see that original post to view a trailer for the five part series. The show airs at 9pm EDT/6pm PDT, and re-airs for an encore at 12am EDT/9pm PDT. The first episode is called, "The Empire of the Sun," and covers solar science and the long reach of the sun in the solar system through the solar wind and the Sun's magnetic field. The other four episodes of the series air over the next four Wednesday at the same times.

Wednesday, August 11: "Order out of Chaos" - the formation of the Solar System; Saturn's rings
Wednesday, August 18: "The Thin Blue Line" - Planetary atmospheres
Wednesday, August 25: "Dead or Alive" - Planetary geology (impact cratering, volcanism, long segment on Io)
Wednesday, September 1: "Aliens" - Astrobiology, Water, the Mars/Europa episode

Link: Carnival of Space #165 [cumbriansky.wordpress.com]

The University of Arizona Goes to Mars...Again

2010-08-04T11:03:00.003-07:00

I don't usually talk about Mars, but I think for this occasion I think we can excuse it. It is a big week here at the Lunar and Planetary Laboratory (I would literally mean "here" if it weren't for the stupid Tucson transit strike). My boss, Alfred McEwen, has been selected to be the principal investigator of the High-resolution Stereo Color Imager, or HiSCI camera on the joint NASA-ESA Mars Trace Gas Orbiter mission. This will be his second camera that will make it to Mars, after the highly successful HiRISE. HiSCI will beam back images with a resolution of 2 meters per pixel. While this is not as crisp as the HiRISE camera, HiSCI will allow the team to image a wider swath of the Martian surface. Not only that, but the entire swath will be in color, rather than a narrow band at the center of the HiRISE swaths currently. The four colors to be used by the camera will help scientists distinguish various surface components such as water ice and dust.

Another exciting feature of the camera is its Yaw Rotation Drive. This drive will allow the camera to scan across a target before it passes over it, then rotate so it can scan the target again when the spacecraft is directly over the surface feature. This allows the camera to obtain near-simultaneous 3D stereo coverage, rather than using two images that are taken months apart, as is currently done with other Mars cameras, like HiRISE or CTX, or with Io stereo images like this one of Zal. This will mean that every target HiSCI images should have stereo, allowing detailed digital terrain models to be constructed for each imaging target.

This new camera will part of the payload for the Mars Trace Gas Orbiter, a joint NASA-ESA project that should launch in January 2016. The main goal of the mission to measure the concentration, both in general and locally, of various trace gases in the Martian atmosphere, such as methane. Much as the sub-millimeter observations of Io's atmosphere provided insights into how sulfur dioxide, sulfur monoxide, and sodium chloride got into that moon's atmosphere, this mission is focused on discovering how these trace gases get into Mars' atmosphere, be it through outgassing, photolysis, or most intriguingly, through biology. HiSCI will be used to image possible sources regions for these gases, to better understand their geologic context.

Van Kane has an article up on his blog that describes the Trace Gas Orbiter and the other instruments selected in more detai.

So we (and by we, I mean those at PIRL who do work on HiRISE, unlike myself) are off to Mars again. Congratulations are in order for the HiSCI team for their selection to the payload of NASA's next Mars mission after MAVEN. I hope MRO and the HiRISE camera will still be functioning for you guys to to joint observations. As for myself, I will still be toiling away in the Saturn system, hoping lightning will strike a third time.

Link: UA-Operated Stereo Camera Selected for Mars Mission [uanews.org]

Volcanic Moon of Jupiter Is Not Smelly and is Fun for the Whole Family

2010-08-03T08:43:00.004-07:00

Back in mid-June, Space.com had a news article posted on their website titled, "Volcanic Moon of Jupiter Is Smelly and Bizarre." The article was promoting the results of a paper by Arielle Moullet, Mark A. Gurwell, Emmanuel Lellouch, and Raphaël Moreno that presented results of Sub-Millimeter Array (SMA) surveys of Io's atmosphere. Their observations revealed the presence of the previously observed compounds in the atmosphere like sulfur dioxide (SO₂) and sulfur monoxide (SO) and a compound observed for the first time in gaseous phase, sodium chloride (NaCl), which was predicted to exist based on the presence of sodium and chlorine in the Io Plasma Torus. The SMA data also allowed the authors to map the distribution of these molecules, again including NaCl for the first time. Finally, they analyzed the origin of each species, either sublimation from surface frost (the main source of SO₂), photolysis from larger molecules (the dominant source of SO), or volcanic activity (the primary source for NaCl). We discussed this paper on this blog back in March, and you can read that entry for more information about the results from this paper.

Back to the news article I mentioned at the top. Space.com's reporting generally wasn't too bad. It seemed to focus less on the results of the research than on the importance of it: the need to explain the source of the gases in Io's atmosphere given how small it is. After all, our Moon is about the same size and has a much thinner atmosphere composed mostly of sputtered regolith and outgassed radioactive decay products. Understanding the connection between the atmosphere and Io's volcanic activity seems to be the key, along with how Io's atmosphere interacts with Jupiter's powerful magnetosphere, which acts as a sink for gases in the atmosphere. The article also briefly reminds people about Io's active geology and possible future missions that may further explore this question of the source of Io's atmosphere.

Where the article goes off track is its description of Io's smell and derives mostly from the sentence, "She conceded the moon isn't a very pleasant place, though, because of the rotten-egg smell of the sulfur gases." On the face of it, I could let it pass since it really feels like a throw-away comment. However, many other sites re-reporting what Space.com wrote, particularly this gem from the Mother Nature Network titled, "Jupiter's moon smells like giant rotten eggs" or this one from the Christian Science Monitor called, "Scientists discover that Jupiter moon smells terrible". These articles focused less on the science of the original paper (though both do provide an overview of the moon for readers who were unfamiliar), and more on that comment about how Io smells like rotten eggs, based on this discovery.

To this, I have (and had at the time I saw these stories come out) but one reaction:

Well, let me make it clear lest I am misunderstood. For this purpose, I will employ the caps lock button on my keyboard, so look away if you think you might be offended by its use. IO DOESN'T SMELL BAD. IT DOESN'T SMELL LIKE ROTTEN EGGS. Sorry about that. It had to be done. The known chemical components of Io's atmosphere smell no worse than burnt matches. Sure, the smell of burnt matches may not smell nice say compared to a rose, German Chocolate Cake, or very good barbecue, but I think we can all agree that it is a far cry from rotten eggs or raw sewage. The bad smell that people associate with sulfur actually comes from hydrogen sulfide, not from sulfur, which only has a faint odor. Hydrogen sulfide is produced as a waste product by intestinal and other bacteria and through hydrolysis at volcanic centers on Earth. Its presence in Io's atmosphere has only been hypothesized and it has not been definitively identified. It certainly wasn't observed by Moullet et al. Organic sulfides can also have a pungent odor, but those are much less likely to be present in Io's atmosphere. The other component Moullet et al. observed in the atmosphere of Io was sodium chloride, which is just plain old table salt.

So again, the story here is the all-to-common failure of science reporting on the web, picking up on unimportant trivia (that in this case isn't true) or more often than not, no fact checking and instead just passing along verbatim either what other websites report or copying press releases. In this case, it just passes along misinformation rather than reporting on the original paper that spawned Space.com's story, thus trivializing its results to merely state that "Scientists discover that Jupiter moon smells terrible".

Again, the reports of Io's bad smell are greatly exaggerated. The moon is still a great place to vacation! ;-)

Link: Volcanic Moon of Jupiter Is Smelly and Bizarre [www.space.com]
Link: Jupiter's moon smells like giant rotten eggs [www.mnn.com]
Link: Scientists discover that Jupiter moon smells terrible [www.csmonitor.com]

Io Volcano of the Week: Zal

2010-08-02T18:53:00.000-07:00

This month for my Io Volcano of the Week series, we will be looking at volcanoes that were observed at moderate resolution (160-280 meters or 525-920 feet per pixel) during Galileo's I25 flyby of Io on November 26, 1999. Over the course of the next month, we shall examine such diverse volcanic edifices as Culann, Emakong, Hi'iaka, and Tvashtar. This week, we will be taking a gander at Zal Patera, a large volcanic depression on Io's leading hemisphere.

The difference between Zal Patera and last week's volcano of the week, Pillan Patera, is like night and day. Where Pillan experiences episodic, explosive volcanic eruptions that produce tremendous amounts of lava in a short span of time, eruptions at Zal were more frequent and consisted of smaller bursts of lava. These eruptions also cause smaller changes to the Ionian landscape, but Galileo's observations of the region show that this was not always the case as clear evidence for much larger lava flows are visible within Zal and to its south.

Zal Patera itself is located at 40° North Latitude, 74.5° West Longitude and is 165 kilometers (103 miles) wide. As you can see in the color image above from the C21 mammoth mosaic, this places Zal near the boundary between the red-brown, radiation-deep-fried plains of Io's irradiated North and the slightly-less irradiated, yellow-green plains of Media Regio along Io's equator. Despite the intense radiation this far north on Io, the region around Zal is quite colorful as a result of various allotropes of sulfur, sulfurous compounds such as iron sulfide and sulfur dioxide, silicate materials like lava flows and pyroclastic deposits. Within the patera itself, dark lava flows seem to have flowed out from the western margin of the patera toward the center of the depression. Lava flows of differing ages are visible, slowly brightening as they age due to the persistent influx of falling sulfur and sulfur dioxide. The volcano however, doesn't seem to be strongly segregated from the surrounding plains, with only low scarps marking its edge to the south and east. The western margin is sharp, but it does seem to have much topography related to it, but based on measurements of the shadow of the plateau to its west, we know that the floor of Zal Patera is 0.5 kilometers (1,600 feet) lower than the narrow swath of plains material that separate the plateau from the volcano. Immediately to the south, the terrain is painted with reddish sulfur, a deposit centered around a deep red lineament that runs north from a dark, unnamed volcanic depression (or patera) north to the southwest corner of Zal Patera. In a bit we shall discuss the significance of these colorful patterns. But first, let's look at the structures that surround the volcano.

Zal Patera, shown in the center of the top frame in the mosaic above, is bounded by three mountains, all collectively part of Zal Montes. The northern component is 1.9-2.5 kilometers (6,200-8,200 feet) in height and consists of a large, heavily eroded plateau covering 15,600 square kilometers (6,000 square miles). The erosion is the result of two forces (since of course there is no atmosphere to support physical or chemical weathering): mass wasting and sapping. The first causes portions of the mountain to move down slope, either quickly through landslides or slowly through creep, forming rough landslide lobes at the base of the mountain. This can be most clearly seen on the southeastern edge of the plateau. The latter, sapping, occurs when sulfur dioxide is heated up and escapes from a mountain slope. On North Zal Montes, this creates the sawtooth pattern on its western and northern margins and the pits on top of the mountain, particularly on its southwestern end. South Zal Montes consists of a 225-kilometer (140-mile) long, 7.4-kilometer (24,300-foot) tall ridge that runs roughly north-south. The mountain has an asymmetric profile, like many of Io's uplifted crust blocks, with a steep sloped west margin and a shallower slope on its eastern side. On a whole, this mountain is more rugged than North Zal Montes due its more angled slopes, which encourages mass wasting. A smaller, third mountain is visible to the east of Zal Patera.

So what does these mountains have to do with Zal itself? Proposed by Melissa Bunte and her colleagues in their paper on a geologic map of the region, one theory for how this large volcano formed is that the three structures that make up Zal Montes were part of a single mountain in the past. A right-lateral strike slip fault formed along the eastern margins of what are today east and north Zal Montes and along the western margin of south Zal Montes, displacing these structures due to north-south lateral slip. An extensional fault later formed between the northern plateau and the smaller east Zal Montes and along the northern end of the older strike slip fault. Extensional faulting pushed the smaller mountain to the east, opening up the basin that became Zal Patera. A similar formation mechanism has been used for other volcanoes such as Monan Patera and Hi'iaka Patera.

The high resolution images acquired by Galileo during the November 26, 1999 and February 22, 2000 encounters, the color data taken in July 1999, and near-infrared thermal data from Galileo's Near-infrared Mapping Spectrometer (NIMS) can be combined to create a picture of the style of volcanic activity that occurs at Zal. Zal was seen as an active volcano, based on its emission of near-infrared light, on several, intermittent occasions during the Galileo mission (June 1996, April-June 1997, September 1997, May 1998, and August-October 2001) and during the New Horizons flyby in February 2007. High-resolution thermal data from August and October 2001 provide a clue for the type of activity that occurs at Zal. In these observations, a hotspot was seen lying along the western margin of north Zal Montes (in October 2001, much of this hotspot was blocked from view by the plateau). Combined with the color data which shows a deep red lineament running from a small patera (called "Rustam Patera" in the Bunte et al. paper) along the western margin of south Zal Montes north to southwestern Zal Patera, a lineament which continues north as the western margin, and apparent source of lava flows at the volcano, of Zal Patera, a possible scenario emerges. The original source for lava at Zal Patera is to its south at "Rustam Patera". Lava emerging from this volcano flows northward via lava tubes that run along a fault that may have once broken up the Zal Montes complex. Skylight and the occasional breakout of lava along the 300-kilometer (186-mile) long system allow for the release of gas from the lava, forming deep red deposits along the fault, and fainter red deposits distally, including along the western slope of south Zal Montes. Galileo's SSI camera saw hotspots in eclipse along this fault/lava tube system on several occasions, particularly in May 1998. This system of tubes eventually ends along the western margin of Zal Patera where the lavas finally reach their nadir and flow out on to the surface, likely as part of an inflated flow field, as suggested by the NIMS data which suggest that active lava flows at Zal are less than a meter thick and certainly don't release tremendous amounts of energy like those at Pillan or Surt. Alternatively, the lava flows seen within Zal may form as a result of magma exploiting the fault that split up Zal Montes and formed the depression that became Zal Patera. This exploitation created "Rustam Patera" at the southern end of the fault, allowed sulfurous gases to escape along the southern half of it, and lava to flow out onto Io's surface along the fault's northern half. Future higher resolution observations will be required to distinguish between the two theories, but the predominance of thermal emission at one half of the fault or the other at any given time (rather than both at the same time) may lend support for the latter theory.

Eruptions at Zal Patera are certainly not as explosive as those seen at the last volcano of the week, Pillan, though there was a small plume observed at the volcano during the New Horizons flyby in 2007. However, what it lacks in explosiveness, it makes up for in persistence, providing an outlet for Ionian magma on a regular basis since at least the start of the Galileo mission. Imaging of the region as a whole does show that volcanism along the fault that forms the western margin of Zal Patera can form more voluminous lava flows, as evidenced by the radiating set of bright flows (older silicate lava that have since been coated in sulfur frost) centered around "Rustam Patera". Perhaps one day this volcanic system will reawaken, and fresh lavas will flow along the earlier ones, much like Thor in 2001. The formation mechanism suggested for Zal Patera provides a glimpse at how at least a few of Io's volcanoes may have formed, through the actions of regional plate tectonics.

Next week, we will look at another of the volcanoes Galileo examined during the troubled I25 encounter, Emakong Patera. Until then, I hope you've enjoyed this look at this fascinating volcano.

References:
Schenk, P.; et al. (2001). "The Mountains of Io: Global and Geological Perspectives from Voyager and Galileo". Journal of Geophysical Research 106 (E12): 33201–33222.
Bunte, M. K.; et al. (2008). "Geologic mapping of the Zal region of Io". Icarus 197: 354–367.
Spencer, J. R.; et al. (2007). "Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano". Science 318 (5848): 240–243.
Turtle, E. P.; et al. (2001). "Mountains on Io: High-resolution Galileo observations, initial interpretations, and formation models". Journal of Geophysical Research 106 (E12): 33175–33199.
Davies, A. G.; et al. (2005). "Post-solidification cooling and the ages of Io's lava flows". Icarus 176: 123–137.

Carnival of Space #164 @ Next Big Future

2010-07-29T21:45:00.000-07:00

The latest edition of the Carnival of Space, the 164th, is now online over at Next Big Future. This week, space blogs focused on issues such as an updated Lunar Sample Atlas, terrestrial extremophiles, the Mars rover Curiosity, and the distribution of exoplanets discovered by the Kepler telescope.

Looking elsewhere on the web, The New York Times Art&Design reporter Edward Rothstein reviewed an exhibition at the National Air and Space Museum in Washington titled, "Beyond: Visions of Our Solar System". The art exhibit includes 148 images from various space missions, including an image of Io. New Horizons imaged the Jupiter system from a distance of 2.4 billion kilometers on June 24, 2010. Even from that great distance, Europa and Ganymede were visible and Jupiter was 12 pixels wide in LORRI camera images.

Link: Carnival of Space 164 [nextbigfuture.com/]

Io Volcano of the Week: Pillan - Part Three

2010-07-28T20:13:00.000-07:00

Over the last few days, we have been looking at the geology and volcanic history of Pillan. Pillan experienced a major eruption in 1997 that was seen by the Galileo spacecraft as it was wrapping up its primary mission in Jupiter orbit. In Part one on Monday, we looked at the geology of the region around Pillan Patera and Voyager and Galileo observations of the volcano prior to the 1997 eruption. In Part two yesterday, we focused our attention on Galileo's observations of the eruption itself and how the 10s of cubic kilometers of lava erupted not from the volcanic depression named Pillan Patera, but from a fissure north of the patera. The source fissure was more clearly visible in pre-eruption images, such as those taken in November 1996, as it was largely covered over in images after the eruption. The fissure may be an extension of a fault that bisects Pillan Mons, further indicating that Io's magma often exploit pre-existing faults to reach the surface.

We left off yesterday in March 1998, when Galileo acquired its first detailed view of the eruption site since November 1996. The images showed that lava from the 1997 eruption covered more than 5,600 square kilometers (2,200 square miles) of Io's surface, including the floor of Pillan Patera, where lava poured over the wall of Pillan and down onto the floor of the patera. For a comparison, lava from this eruption covered an area the size of the U.S. state of Connecticut (or the nation of Brunei). Galileo observed the Pillan region using the Near Infrared Spectrometer (NIMS) in May 1998, July 1998, and May 1999. The NIMS observations during this period showed a steady decrease in the power output from the volcano, indicative of cooling lava. By May 1999, the power output seen by NIMS at Pillan was an order of magnitude less than during the peak of the eruption in June 1997 (from Davies et al. 2001). SSI images of Io's trailing hemisphere in July 1999, shown at left, showed slight changes in the Pillan pyroclastic deposit surrounding the eruption site. Most of the western half of the deposit had been subsequently covered over by reddish sulfur from Pele and the northern half was covered up by bright sulfur dioxide frost, likely from sapping from Pillan Mons. Finally, portions of the eastern half of the deposit were covered up by dark pyroclasic and bright plume deposits from the nearby Kami-Nari volcano. These changes further showed that Pillan was much quieter in terms of activity than it was during the eruption in 1997.

13-frame mosaic covering the Pillan flow field. Full-resolution version also available.

On October 11, 1999, Galileo performed its first close flyby of Io since the spacecraft entered orbit around Jupiter in December 1995. The close range of the encounter over Io's trailing hemisphere provided an opportunity for the onboard camera to observe the Pillan lava flow at high resolution (9-18 meters or 30-60 feet per pixel). Unfortunately, due to a camera anomaly, 12 of the 13 images were scrambled, requiring extensive processing to bring out useful information. Despite this processing, most of the images were heavily degraded and included a dark stripe down the center of each frame. The partial full-frame mode image, shown at the lower left of the above mosaic, did not suffer from this anomaly. A rough lava flow emplaced a broad sheet (rather than a channelized flow) was revealed in this mosaic, with pits and lava channels across parts of the flow. Using the length of the shadow along the margin of the flow on the left and right sides of the mosaic, the Pillan lava flow was found to have an average thickness of 8-11 meters (26-36 feet).

The rough texture of the lava surface may have resulted from a number of factors, including the interaction between the hot lava and the cold, volatile-rich surface it flowed over, turbulent flow, and the disconnect between the high effusion rate (the volume of lava flow for each meter of the vent fissure) and the speed of the flow front. In the first case, gas created by the heating of sulfur dioxide frost by encroaching lava flows would burst through the cooled crust of the lava flow. This action would disrupt the cooled lava crust and may form rootless vents that provide gas for the plume seen over Pillan in 1997. One such put can be seen in the left side of the mosaic. Turbulent flow within the lava flow would cause any cooled crust that may have formed to break up into blocks or rafts. These rafts can also disrupt the cooled crust as it is moved downstream by the still molten lava beneath. This can cause gouges to form in the flow field, like the one seen below and to the left of the dark pit on the left side of the mosaic. In the final case, the high lava effusion rate would lead to a crumpling of the lava crust as the flow front advanced at a speed of "only" 0.3 to 1 kilometer per day, the result of the leading edge giving up heat to mobilize surface frost. This would create the rubbly surface texture seen in the October 1999 images.

Galileo observed Pillan from a greater distance on several occasions following the October 1999 flyby of Io. A montage of the SSI observations of Pillan is shown at right. The observations document the gradual fading of Pillan pyroclastic deposits as they are covered by deposits from nearby volcanoes like Pele, Kami-Nari, and Reiden. By December 2000, the deposit was all but invisible except for a faint deposit southeast of Pillan. Interestingly, the multi-spectral color imaging from January 3, 2000 provided the highest resolution color information of Pillan, the source vent of the 1997 eruption, and lava flows. They showed that the flows were greenish in color, perhaps due to the interaction between the still warm lava and sulfur from Pele's plume, producing a layer of impure sulfur mixed with iron (FeS). The observation also showed that the fissure source vent of the 1997 eruption was red in color, similar to other fissures on Io's surface, such as East Girru or the fissure (or lava tube) within the southern end of Lei Kung Fluctus. The Photo-Polarimeter Radiometer (PPR) continued to measure the decreasing power output from Pillan as the Galileo wound down in late 2001 before it plunged into Jupiter in September 2003.

After the end of the Galileo mission, Pillan would be observed from an even greater distance, either from Earth using telescopes such as Keck or the European Southern Observatory or from the New Horizons spacecraft during its encounter in late February 2007. Thermal hotspots were detected at Pillan throughout the 2000s by Keck, suggesting that a low level of volcanic activity continued after the 1997 eruption. Pillan was last detected as a hotspot during a brief observation by Marchis et al. on June 28, 2010. This last observation suggests that Pillan may currently be coming down from a recent eruption.

All was quiet at Pillan during the New Horizons encounter in 2007. The fading of the pyroclastic deposit from 1997 was complete by that point, and the floor of Pillan was once again covered in a thin coat of sulfur frost from the Pele plume, making it show up bright in the LORRI camera images. Only the still dark lava flow north of Pillan Patera suggested that anything had happened at Pillan between 1979 and 2007. No thermal hotspot was observed at Pillan by either LORRI during two eclipses of Io by Jupiter or by the LEISA near-infrared spectrometer.

In 1979, Pillan was a quiet volcano that was not even worthy of a name. But in 1997, a major eruption there allowed scientists to track the changes it created on Io's surface at a close range. The eruption became the archetype for fissure-fed eruptions on Io, producing large-scale surface changes, areas of incandescent lava, and lava fountains at the source vent. Similar, even more powerful eruptions would later be seen at Surt and Tvashtar. Potentially, the East Girru eruption seen by New Horizons may be most similar to what happened at Pillan in 1997, though the short duration of the encounter prevented follow-up observations of East Girru.

Thanks for reading this week's premiere of my "Io Volcano of the Week" series! If you haven't already, I encourage you to read Parts One and Two of my profile of Pillan. Next week I will be profiling a volcano that should only require one post to discuss: Zal Patera.

Io Volcano of the Week: Pillan - Part Two

2010-07-28T00:12:00.001-07:00

The Pillan eruption must be shown montage style

Yesterday, we took a look at the volcano Pillan Patera and its surroundings prior to its eruption in 1997. In our look back, I took you all the way to April 1997, on the eve of the eruption. To that point, Pillan had been a fairly nondescript, 75-kilometer (47-mile) wide patera that only been seen as a weak thermal hotspot on a pair of occasions in November 1996 and February 1997. This would change beginning in May 1997, when the Near-Infrared Spectrometer (NIMS) onboard the Galileo spacecraft detected a much brighter hotspot at Pillan, suggesting that a major eruption had begun. Later modeling by Davies et al. suggested the presence of lava with a temperature of at least 1500 K covering an area of 0.2 square kilometers (50 acres). Such high temperatures are consistent with large lava fountains or turbulent lava flows, where massive amounts of extremely fresh lava could be detected. Despite the detection of a bright eruption at Pillan by NIMS during Galileo's G8 orbit, Pillan looked no different in distant images taken by the SSI camera than it had in earlier orbits. This suggests that the Galileo observations on May 7, 1997 captured the eruption at a very early stage, before any large lava flows had formed. This is validated by the low-temperature component in the Davies et al. paper on the eruption styles of Pele and Pillan, which found the area of fresh lava flows 5-10 minutes old covered an area of 31 square kilometers (12 square miles). This provides another link to the East Girru eruption seen by New Horizons 10 years later. New Horizons also appeared to have caught that eruption in its early stages, before it had a chance to produce any major surface changes. Perhaps if we were to revisit Io today, a fading pyroclastic deposit and lava flows field would be seen around the East Girru fissure.

Galileo returned to the inner Jupiter system in late June 1997 during orbit C9, allowing Io to be observed in daylight and in eclipse on June 28, 1997. In the case of the daylight image, shown above, a new plume ~110 kilometers tall was observed over Pillan on the limb of Io. Before that observation, no plume had ever been observed at Pillan. In fact, when Hubble imaged the Pillan plume on July 22, 1997, it was initially thought that the plume came from the nearby Reiden Patera. Unlike the plumes from other outburst eruptions such as at Grian Patera in 1999 or Tvashtar Patera in 2007, rather than being a large, gas-rich plume, Pillan's plume was much more dust-rich, approaching Prometheus's prominent plume in brightness and mass (see Geissler and McMillan 2008). By analogy with similar plumes on Io, this suggests that the Pillan plume resulted from the interaction between hot silicate lava and cold sulfur dioxide frost on the surface of the plains the lava flowed over, rather than direct outgassing from a volcanic vent. Pillan was also seen in eclipse images taken 16 hours later. Again, evidence that an intense eruption was taken place at Pillan was observed. An intense hotspot was seen over the volcano in both SSI and NIMS data (though the low resolution of the later meant that the NIMS pixel covering Pillan also covered the active Pele volcano).

Initial modeling from the SSI and NIMS C9 observations provided tight constraints on the eruption temperature of Pillan's lavas, suggesting temperatures that were above the range of terrestrial basalts, but were instead in the temperature range of ultramafic lava, last seen on Earth two billion years ago. These temperatures, 1800-1900 K (lower limits actually), also caused problems for modelers when it came to the amount of melt required in the Ionian asthenosphere. Tidal heating models required a mantle with a much smaller melt fraction than would be required by the super high eruption temperatures estimated for Pillan. However, more recent modeling by Keszthelyi et al. 2007, that better accounts for lava fountaining, now place a lower limit on the eruption temperature at Pillan closer to 1610 K, within the range of terrestrial basalts.

The first visible images of the effects of the eruption came on September 19, 1997, when Galileo returned to the inner Jupiter system for the 10th time. This multi-spectral set (10ISIOGLOC03) revealed a dark spot more than 400 kilometers (250 miles) across and a fresh lava flow with an area of 3,100 square kilometers (1,200 square miles). This data allowed researchers to better understand the geologic context for the eruption. Rather than being located with Pillan Patera, the eruption started from a fissure located to the north of the volcano. Fresh lava flows extended south and east from the southern end of this fissure. The dark spot that surrounded the Pillan lava flows were likely the result of pyroclasic flows that hugged the ground and deposited a thin layer of dark basaltic tephra. This hypothesis is supported by the visible and near-infrared spectra of this deposited, as derived from this image set. Geissler et al. 1999 found evidence for iron-rich orthopyroxene (enstatite end-member), a mafic mineral, within the dark deposit. NIMS and SSI observations of Ionian hotspots in September 1997 also revealed that the eruption was still on-going, with significant areas of incandescent lava still visible.

Galileo took a few more peaks of Pillan in late 1997 and early 1998, as the spacecraft began its first extended mission. In November 1997, SSI observed Pillan's plume, fainter and slightly taller than it was June of that year, as well as the thermal emission from the volcano during an eclipse observation. This time, two hotspots were seen: one corresponding to the source fissure and a brighter one to the south corresponding with the southern portion of the flow field and Pillan Patera. This suggested that lava from the Pillan eruption was flowing over the edge of the patera and down onto the floor of Pillan Patera, like a two-kilometer tall lava fall. Sure enough, this was confirmed four months later, when Galileo SSI observed Pillan at only 2.6 kilometers (1.6 miles) per pixel. The floor of Pillan Patera had darkened, likely from lava from the eruption flowing down from the north and covering the 2,500-square-kilometer (950-square-miles) patera floor. Thus, by March 1998, the Pillan eruption of 1997-1998 had resulted in 5,600 square kilometers (2,200 square miles) of new Ionian terrain. Assuming an average thickness of the lava flow was 10 meters (a reasonable number given later observations), then more than 56 cubic kilometers (13 cubic miles) flowing from the source vent during the eruption. For comparison, the Laki eruption in Iceland in 1783, one of the largest eruptions ever observed on Earth, produced more than 14 cubic kilometers (3.4 cubic miles) of lava.

Believe it or not, there is just too much to talk about with respect to Pillan to cover in just TWO blog posts. More specifically, I am tired and hungry, so I am going to cut this installment short. But have no fear, I will be back tomorrow with a discussion of the high resolution observations of the Pillan flow field from Galileo and more recent observations of the volcano.

Until tomorrow...

Io Volcano of the Week: Pillan - Part One

2010-07-26T23:06:00.001-07:00

Io's Black Eye - Thanks to Pillan

Beginning this week, we will take a look at one of Io's hundreds of volcanoes each week. For this premiere post, we will take a look at Pillan, a volcano that is notable for its large eruption during the Galileo Nominal Mission in 1997. The eruption resulted in a dark, pyroclastic more than 400 kilometers (250 miles) across, a fresh lava flow with an area of 3100 square kilometers (1,200 square miles), and a deposit created by a dusty plume 100-120 kilometers (60-75 miles) in height that partially covered the iconic red ring plume deposit of Pele. The massive eruption, one of the most significant observed by Galileo in terms of energy output and the areal surface coverage of the fresh lava, began in May 1997 and was in full swing by June 28 when the SSI camera onboard Galileo detected a bright hotspot at the volcano when the instrument observed Io while the satellite was in the shadow of Jupiter. And to think, nothing strange had ever been noted at this volcano.


Voyager and Galileo images of Pillan before its eruption

Pillan Patera was first seen during the Voyager 1 encounter in March 1979, the first time Io's surface had been seen in any kind of detail. Voyager scientists were astonished by the sheer number of volcanic features on Io's surface. However, Pillan was not of much interest. The volcanic depression was not even named until shortly before the eruption in 1997. It was seen at a resolution of 500 meters (1,640 feet) per pixel in the large south polar mosaic, so the lack of interest wasn't for lack of good images of the region. Image number 1 at right shows the region around Pillan. You can see Pele, a persistently active lava lake near lower left. Pillan wasn't really distinguishable from the surrounding terrain, if it weren't for the two kilometer high margin that separates the floor of the patera from the surrounding plains. The image does provide some details about the nearby terrain that was not seen at this kind of detail by Galileo. To the north of the speech-bubble shaped patera is a mountain now named Pillan Mons. This mountain is bisected by a pair of fractures and appears to be in a state of collapse with a landslide deposit on the northeast side of the mountain. This deposit is frequently coated in bright sulfur dioxide frost, perhaps from sapping from the mountain. Between Pillan Patera and Pillan Mons is the V-shaped source fissure of the 1997 eruption. During the Voyager mission, this fissure was surrounded by bright material, likely older lava flows that have been coated in sulfur and sulfur dioxide after an earlier eruption. Interestingly, some of these flows have the same shaped as the 1997 flows. Once again on Io, eruptions have happened the exact same way before, and they will do so again.

Galileo entered orbit around Jupiter in 1995 and imaged Io on a semi-monthly basis starting in June 1996. Many of these early images, intended to monitor changes in the plume deposit around Pele, also revealed apparently changes at Pillan Patera. Were these real changes? During Galileo's first orbit in June 1996, Pillan Patera appeared brighter than its surroundings (see image 2 above). However, it had darkened considerably by September 1996 during the next orbit. Was this due to an eruption? Galileo would observed Pillan at even higher resolution during C3, the next orbit in November. This time, Pillan had the same albedo as its surroundings. Was this due to rapid surface changes? Turns out, it was due to an odd phase function of the surface materials on the floor of the patera. When Pillan was viewed at low phase angles, like G2 and later in E6 in February 1997, the surface appears dark. When viewed at higher phase angles, the surface appears brighter. This is the result of a thin layer of bright sulfur dioxide frost coating dark silicate materials on the patera floor. This makes the floor of Pillan quite forward scattering. You can see a similar effect taken to extremes near Loki in New Horizons images. In terms of volcanic activity, some slight activity was seen by the Near-Infrared Mapping Spectrometer (NIMS) in November 1996 and February 1997, but neither detections approached the level of activity seen later in the nominal mission.

Two Fissures: Pillan from Nov 1997; East Girru from Oct 1999

Special attention should be paid to the clear filter images from November 1996, the highest resolution data set of the Pillan region from Galileo prior to the eruption. The image clearly shows the carrot-shaped ^ of the 1997 eruption source. It appears as a thin, dark fissure in this image, which would be around six kilometers wide based on this data. Surrounding this thin fissure is a diffuse region of darkish material. A similar albedo pattern is seen at other Ionian fissures, such as within Lei Kung Fluctus, and most intriguingly at East Girru, shown paired with the November 1996 image of Pillan. East Girru was the source of a bright eruption that was just getting going during the New Horizons flyby in late February 2007.

Unfortunately, I am going to have to cut this installment short (I want to get to Best Buy for the midnight sale of Starcraft II). Have no fear, tomorrow, when I am not working (or playing Starcraft II), I will continue this tale of fire fountains and black eyes tomorrow evening. We will explore the significance of the shape of the Pillan eruption source vent. We will also look at Pillan's impact on our knowledge of Io and its interior. Finally, we will look at how the model it inspired was undone.

JPL webcast and chat on Outer Planet Satellites Tonight

2010-07-22T17:17:00.001-07:00

JPL will be host a webcast and chat on their Ustream channel at http://www.ustream.tv/nasajpl covering outer planet satellites like Io. The topic is called "Moons: The Weirdest Planets in Our Solar System" and has the following description:

Moons are fantastic worlds - with features unlike anything seen on Earth: giant sulfur-spewing volcanoes, globally cracked ice-covered surfaces, liquid lakes of hydrocarbons, and colossal watery plumes. Many of these worlds also happen to be the most likely places for life to evolve outside the Earth. How cool is that?

The webchat takes place at 7pm PDT (10pm EDT) tonight.

Link: Moons: The Weirdest Planets in Our Solar System [www.ustream.tv]

"Wonders of the Solar System" Airing in the US

2010-07-21T12:46:00.004-07:00

The popular BBC Two series, "Wonders of the Solar System", is finally making its way to this side of the Atlantic. The first episode, "Empire of the Sun" premieres on the Science Channel on Wednesday, August 4 at 9pm EDT/6pm PDT. The show, hosted by Brian Cox, was well received when it aired in the UK earlier this year. It covers various aspects of planetary science, from the Sun in the first episode, the formation of the Solar System and Saturn's rings in the second, planetary atmospheres in the third, geology in the fourth, and the role of water for life on Earth and elsewhere in the fifth. A significant section on Io, its volcanic activity, and the terrestrial lava lake analog at Erta'Ale will air in the fourth episode, "Dead or Alive."

Thankfully, while it looks like they have changed the music, the world-wide version will have the same host as the BBC one, though given that he is an on air host, I don't really see how else you can do it. But at least they are not replacing him with Oprah.

In addition to airing next month, the series will also be released on Blu-ray and DVD on September 7. Strangely enough, the Blu-ray version is cheaper on Amazon.com than the DVD. Good news for me ;-)

Tip o' the hat to Ashley Davies for the tip!

Here is the trailer for the new series:

Observing Active Volcanic Eruptions Remotely

2010-07-21T00:59:00.002-07:00

Well, I should finally get to writing up an article on a new paper out this month in the Journal of Volcanology and Geothermal Research on Io titled, "The thermal signature of volcanic eruptions on Io and Earth." The authors for this paper are Ashley Davies (pictured at right with the nice manly man-beard), Laszlo Kestay (formerly Keszthelyi), and Andrew Harris. Unfortunately, my delay in writing something up about this paper for the blog has meant that other bloggers have had time to scoop me. Seriously, how was I to know that someone would write up a post about an Io paper before me? This hasn't really happened before. You can read Emily Lakdawalla's excellent discussion of Ionian volcanism and this paper's treatment of it on her Planetary Society Blog. Davies emailed out copies of the paper to various Ionians and Emily, which as Emily states, is something other scientists should take note of. However, I have to one up Emily just a little bit. He told me about the paper personally last week :-p So there! Though maybe my parody ads for Io have had some effect after all?

Anyways, enough with my professional jealousies [humor using hyperbole alert!], let's get to the paper, shall we. In the article, the authors investigate methods for identifying volcanic eruption styles using low spatial resolution, near-infrared data. This is particularly useful for Io, as the only thermal observations available of Io have a resolution of at best a few kilometers per pixel but more typically have pixel sizes of tens to a couple hundred kilometers across. The problem of spatial resolution is compounded in more recent data from the Keck telescope, where observations are typically acquired at only a few wavelengths between one and five microns. Just last week, Franck Marchis showed off data on his blog showing Io at three such bandpasses from the Keck telescope at 2.1, 3.8, and 4.7 μm. Davies and his two co-authors also examined satellite data of terrestrial volcanoes, which could then be compared with observational ground-truth. This provided a way to test their method.

Davies and his co-authors determined that by examining the ratio between thermal output at two and five microns of a volcano in Galileo NIMS or ground-based data and tracking how that ratio changes with time, they could characterize the style of volcanic activity. These different styles include open-channel or insulated lava flows (like those seen at Kilauea), lava fountains, lava lakes, lava domes, silicic lava flows, though the latter two, while important volcanic features on Earth, have not been identified on Io. This works because the peak wavelength for the thermal emission of a lava flow or lava lake shifts to longer and longer wavelengths as it cools. Basaltic lava that has only been cooling for one second has a peak thermal emission wavelength of two microns, while the thermal emission of lava that has been cooling for more than seven hours (or two hours on Earth) peaks around five microns.

So more vigorously active eruptions will have more fresh lava exposed than older, more quiescent eruptions, and thus will have greater 2 μm : 5 μm ratio. More active volcanic eruptions include Tvashtar's back in 1999 and 2007, when lava curtains were observed at one of its constituent paterae. The eruptions of Pillan in 1997 and Surt in 2001 also fit this model, with Surt having a 2 μm : 5 μm ratio of 2. Also typical of these outburst eruptions is their short duration. Over time, the 2 μm : 5 μm ratio at these eruptions decreases as the fire fountaining ceases and the thermal emission becomes dominated by large areas of cooling lava. The volcano I profiled on Sunday maybe in this stage. High 2 μm : 5 μm ratios can also be found at vigorous lava lakes such as Pele, where smaller lava fountains balance out the emission from the cooled lava crust that covers most of the lake. Similar activity such as this can be seen at a much smaller scale at the Erta'Ale lava lake in Ethiopia, shown at night in the image at the top of this post and at above right.

Quiescent eruptions, such as those with insulated lava flows (where lava flows from the source to the flow front via lava tubes) or episodically overturning lava lakes, have much lower 2 μm : 5 μm ratios as their thermal emission is dominated by cooling lava with only small areas of recently emplaced lava. Such activity can be seen at Io's large, persistent flow fields like Amirani, Zamama, and Prometheus or the multitude of volcano depressions like Altjirra Patera.

Combined with analysis of terrestrial data, the authors found that low 2 μm : 5 μm ratios typified volcanic eruptions with "older surfaces, increasing insulation [more lava flowing through lava tubes to breakouts], and quiescent emplacement". Eruptions with a high 2 μm : 5 μm ratio (> 0.5) suggest the presence of "younger [flow] surfaces, decreasing insulation, and more violent emplacement. Eruptions with greater overall radiant fluxes have increasing effusion rates and lava with lower viscosity and less silicon dioxide (less silicic). This ratio must also be combined with repeat observations for temporal coverage. This allows for the disambiguation between vigorously active lava lakes, open-channel lava flows, and lava fountains, for example, which are active for different timescales.

Finally, the authors provide suggests for applying their method to data from future spacecraft to the Jupiter system and Io. For example, they suggest that temporal resolution trumps spatial and spectral resolution for monitoring the progress of a volcanic eruption, particularly for understanding processes at different temperature regimes (though high spatial resolution observations are great for spotting small scale features like skylights over active lava tubes). For example, observations with a temporal resolution on the order of 1-10 seconds, or less, are useful for obtaining temperatures from vigorously active lava bodies such as lava fountains. Observations with scales on the order of a few minutes to hours are useful for monitoring changes in the flow rate at open-channel lava flows, while lava lakes and insulated lava flows can be observed on a daily to weekly basis. They also suggest that thermal imagers on future spacecraft use a few, select wavelength windows such as 2 and 5 microns, and others in the thermal infrared between 8 and 12 microns for monitoring different volcanic eruption styles.

Link: The thermal signature of volcanic eruptions on Io and Earth [dx.doi.org]

The Moon Your Moon Could Smell Like

2010-07-19T18:13:00.001-07:00

Like many of my fellow netizens, I was amused last week by a guy in a towel answering random requests from Twitter and Facebook fans via a series of Youtube videos. These clips were part of a viral marketing campaign for a body wash by Old Spice, that have included more traditional commercials that have played online and on TV. Like last week's viral video series, these videos have been widely parodied, including my favorite for a library at BYU.

Anyways, like many of my fellow netizens, I have created some parody videos, this time advertising the exploration of Io, based on the Old Spice ads. Enjoy!

More on the recent Keck Observations of Io

2010-07-18T13:22:00.001-07:00

On Friday, I posted a note about Franck Marchis' observations of Io using the Keck Telescope's adaptive optics system on June 28, 2010. These observations allow us to sneak a peek at the ongoing volcanic activity on Io's anti-Jovian hemisphere. Their images revealed hot lava at several volcanoes like Pillan Patera (the most intense hotspot seen on that date), Isum Patera, Marduk, Prometheus, and Volund (not Zamama as previously suspected). These hotspots were seen in both 3.8 μm and 4.7 μm wavelength images, in the near infrared. Hotspots were seen in only the 4.7 μm wavelength image, indicative of more cooled lava, at Rata Patera, Culann Patera, near Kurdalagon Patera, Tupan Patera, near a patera at 42.5 South Latitude, 172.5 West Longitude, and at a patera located at 6 degrees South, 190 degrees West. All but the last of these hotspots was seen before by either Galileo or New Horizons as active volcanoes. None of these hotspots were seen in the 2.1 μm image, suggesting that none of these volcanoes were vigorously active on June 28. The 2.1 μm instead showed Io's surface in reflected sunlight. Silicate materials show up as dark in the 2.1 μm image, while sulfurous materials show up as bright.

Now as Franck pointed out, this isn't the most exciting set of results. Save one new hotspot, all the others were seen as active before and none are currently in an outburst phase. But regardless, it is important to observe Io more often to understand what is typical in terms of volcanic activity on the satellite. How often do outbursts occur? You are not going to know that if some observing days reveal none. Exciting, okay, maybe not, but VERY useful.

Anyways, I was a bit curious about the one exception to this "boring" fest, the new hotspot at 6 degrees South, 190 degrees West. The patera this hotspot seems to be associated with is 60 kilometers by 45 kilometers in size. The image at left shows various views taken by Galileo during its mission between November 1996 and October 1999. As you can see, no changes were observed at this volcano during the Galileo mission, nor did the volcano look any different in New Horizons images taken in February 2007. While the 2.1 μm image from Keck has a very low resolution (150 km or so), there does seem to be dark spot associated with this volcano that would indicate the emplacement of dark lava, whereas before, the floor of the patera was covered with sulfurous materials.

Very intriguing stuff that at least one volcano on Io seems to have reawakened.

Link: Keck AO Observations: Io Volcanism - “Mornes plaines” [www.cosmicdiary.org]

New Io Data! Hallelujah, praise Jebus!

2010-07-16T14:29:00.000-07:00

Believe it or not, but public information on recent volcanic activity on Io has been pretty limited. In fact the last report on Io's volcanic activity that is available to discuss here came during the New Horizons encounter back in late February 2007, more than three years ago. Thankfully, that information drought has finally, mercifully come to an end! Franck Marchis posted on his blog at Cosmic Diary results and images he and one of his undergraduate assistants, Keaton Burns, of Io on June 28.

Using the adaptive optics system on the W.M. Keck II telescope on the Big Island of Hawai'i, Marchis was able to image Io at several near-infrared wavelengths during a break between searching various asteroids for satellites (a project Marchis has previously been quite successful with). On his blog, Marchis posted a set of images taken at 2.1 μm (Kp), 3.8 μm (Lp) and 4.7 μm (Ms) of Io's anti-Jovian hemisphere. The 2.1 μm image shows mostly reflected sunlight, but the other two reveal thermal emission from Io's volcanoes. More volcanoes are visible in the 4.7 μm image because cooler (read older) lava can be detected at longer wavelengths. Both the 3.8 μm and the 4.7 μm images revealed emission at the usual suspects like Marduk, Prometheus, and Zamama, with the most intense hotspot located at Pillan. Looking at the image myself, you can also see additional, fainter hotspots can be seen at Rata, Culann, and Isum, as well as a new hotspot near a volcano at 6 degrees South, 190 degrees West.

I need to head out the door right now, so I will definitely have more when I get back.

Link: Keck AO Observations: Io Volcanism - “Mornes plaines” [www.cosmicdiary.org]

No Debris Field from the June 3 Impact

2010-06-16T10:15:00.001-07:00

A new set of images of Jupiter, taken a few days after the June 3 impact of a small asteroid or comet, was released on the website for the Hubble Space Telescope today. The research group, which includes Mike Wong, Heidi Hammel, and Amy Simon-Miller, focused on finding a small debris field that might have resulted from the impact, similar to the ones seen from last year's asteroid impact and the Shoemaker-Levy 9 impacts in July 1994.

To date, no such debris field has been spotted by amateur, ground-based telescopes, but it had been hoped that with Hubble's superior resolution, it might be able to spot a small, dark spot from the impact. Instead, as you can see above, even with Hubble's larger eyes, no impact scar is visible. This suggests that the impactor was too small to penetrate very deep into Jupiter's atmosphere, but instead burned up in the upper atmosphere, akin to meteor fireballs on Earth. Since these events are so brief, it is possible that similar Jovian meteors maybe fairly common, but because of their brief duration (< 3 seconds), they just hadn't been noticed before. Leigh Fletcher on his Twitter feed has noted that the Keck, Gemini, and IRTF observatories at Hawaii's Mauna Kea and VLT in Chile have also obtained views of the impact site, so maybe they will be able to spot a residual thermal hotspot from the site. He seems to suggest that no such scar has been seen though.

While it failed to detect an impact scar from the Jovian meteor, the Hubble data was able to provide images of the changes that have occurred in Jupiter's atmosphere over the last six months. The South Equatorial Belt (SEB), normally the reddish-brown southern complement to the similar North Equatorial Belt (NEB) which bracket the bright Equatorial Zone (EZ), brightened during the end of 2009. From ground-based scopes, this makes Jupiter appear as if it only has one dark belt, as opposed to the normal two. This also makes the Great Red Spot more visible since it is surrounded by mostly bright clouds. Hubble detected a high-altitude layer of ammonia clouds over the SEB, obscuring the darker clouds below. The Hubble image also points to the beginning of the end for these high clouds, as a series of dark spots along the southern margin of the SEB were also seen. Similar spots were seen near the end of earlier SEB brightenings.

Link: Hubble - Mysterious Flash on Jupiter Left No Debris Cloud [www.hubblesite.org]

World Cup

2010-06-12T08:20:00.000-07:00

Well, it is that time again, when I pay attention to soccer (or football where the NFL isn't the dominant league), and I wonder why the vast majority of my countrymen couldn't careless. But no matter, I will be watching in glorious high-definition anyway. And I will be watching today's game between Good and Evil, I mean the US and England.

Here's wishing the US National Team, godspeed and beat those limey !@$!s.

While this post is off-topic, please note that I did pose in front of a map of Io (you can see it just to the right of me in the background...)

Life on Io? Not Very Likely.

2010-06-11T14:56:00.002-07:00

It is starting to seem like most places in the Solar System might be host to their own native form of life, using whatever liquids might be available.

And maybe pigs can really fly. Earlier this week, it was the possibility of life on Titan that was making news, with Cassini scientists trying to bring people's expectations back down to Earth, so to speak. Space.com ran a news story yesterday titled, "Jupiter's Volcanic Moon Io Could be Target for Life", suggesting the possibility that life in some form may exist on Io. This article is based on a paper published in the Journal of Cosmology by Dirk Schulze-Makuch of the University of Washington State titled, "Io: Is Life Possible Between Fire and Ice?". The Journal of Cosmology is a new publication started up last year, focusing on general physics and space science topics. They also provide open access to all their published papers, so unlike other papers I talk about here, this one is free for all of you to read by following the link.

Well, let me briefly discuss what is presented in the paper before I go into any editorializing or critiquing. The paper examines Io as a potential abode for life, both currently and in the past, despite the obvious environmental roadblocks to the development of life. A primary focus is whether one of the known chemical components of Io's surface and shallow sub-surface, such as hydrogen sulfide, sulfur dioxide, or sulfuric acid, could be used as a solvent for an alternative form of life native to Io. Schulze-Makuch also suggests that water may have been used as a solvent for microbial life early in Io's evolution when it may have been more Europa-like (active silicate core covered with a thin layer of water and water ice). Finally, he explores possible energy sources for life, such as geothermal heat or from Jupiter's magnetic field. The authors states that, "One possible microbial survival strategy in this type of environment would be that microorganisms remain in a dormant-type of state most of the time and are reverting back to a vegetative state only when heated by nutrient rich lava flows." The authors notes that a potential habitat for life could be lava tubes in Io's sub-surface, similar to those suggested for Mars.

Well, before I rip this article a new one, let me make it clear that the author does state in his conclusion that the likelihood of life on Io "has to be considered low". This sort of article I think was intended to explore whether the assumption that there is absolutely no chance for life on Io is valid. Schulze-Makuch describes some scenarios for how life on Io might have gotten started and what kind of form it might take. He even points out a number of critical issues, such as the lack of carbon on Io, with the exception of possibly some carbon dioxide in Io's volcanic plumes. However, the low residence time for carbon-based molecules in Io's atmosphere and surface thanks to the radiation environment may have something to do with it, but still, Io is not know for its carbon, unlike Callisto.

Now is it time for me to rip into it? Oh please, can I? I ever so want to...

Now it is no secret that I am not a big fan of astrobiology. In fact, I think that it is at best a mislabeled field of science and at worst it is pseudo-science, based more on speculation and grant-hunting than on reality. Even though it would be nice to add Io to the list of places to the possible abodes for life in the Solar System, the only reason at this point would simply so that we can shoehorn Io exploration into NASA's goal of studying Solar System habitability than actually advancing Io science. I just don't see where you can go to follow-up on it without looking blatantly self-serving. Besides, do advocates for Ionian exploration such as myself want to bother with the planetary protection policies that other targets have problems with.

So go ahead, read the Schulze-Makuch article. Just take it with a grain of salt ;-)

Link: Jupiter's Volcanic Moon Io Could be Target for Life [www.space.com]
Link: Io: Is Life Possible Between Fire and Ice? [journalofcosmology.com]

Latest on Yesterday's Jupiter Impact

2010-06-04T10:28:00.000-07:00

Yesterday at 20:31:29 UTC, astronomers Anthony Wesley and Christopher Go observed a bright flash within Jupiter's faded south equatorial belt. This bright flash is thought to be the fireball of an impacting meteor in Jupiter's upper atmosphere. Since the impact, both astronomers have posted videos and processed images online showing this flash. Wesley's images and video have been posted to his website. The link to his video is at the bottom of the page. Note that his video is an AVI container and you may have issues viewing it in Windows Media Player. It played just fine in VLC. Additional news from Wesley going forward maybe posted on the Ice in Space forum first. Christopher Go has posted processed images and video on his website.

The question now is whether this impact left a dark scar like the Shoemaker-Levy 9 impacts in 1994 and the July 2009 asteroid impact. Gary Spiers posted times when the impact site cross the central meridian as viewed from Earth on his blog. The first opportunity at 5:04 UTC was imaged in western and southern Europe by several observers. While the seeing wasn't as good as what those further south like Wesley and Go obtained the previous Jovian day, they don't seem to show any sign of an impact scar. A summary by John Rogers of the British Astronomical Association has been posted on ALPO-Japan with several "after" images from the UK, France, and Italy. The next opportunity at 15:00 UTC was observed by several observatories on Mauna Kea in Hawaii (Gemini, Keck, and IRTF), according to Leigh Fletcher (who was involved in the followup observations at Mauna Kea after last year's impact) on Twitter. No results have been posted yet, so hopefully their keener eyes will be able to spot a small impact scar.

Link: Anthony Wesley's 2010 Jupiter Impact Page [jupiter.samba.org]
Link: Christopher Go's Jupiter Imaging site [jupiter.cstoneind.com]
Link: New impact on Jupiter before & after by John H.Rogers [alpo-j.asahikawa-med.ac.jp]

Paper: Ground-based observations of the variability of Io's volcanoes

2010-06-04T00:06:00.000-07:00

Today, a new paper was published "in press" (accepted and revised, but not yet in a paper issue) in the journal Icarus titled, "Ground-based observations of time-variability in multiple active volcanoes on Io" by Julie Rathbun and John Spencer. In this paper, the two authors summarize the results they obtained by observing Io using NASA's Infrared Telescope Facility on more than 100 occasions between June 1997 and the end of 2005. They focus on variations in the thermal output of three volcanoes: Loki, Kanehekili, and Janus, as well as output from smaller volcanic centers like Grian Patera.

For their analysis, Rathbun and Spencer observed Io in the near-infrared at 2.26, 3.5, and 4.68 microns both in disk-resolved images while Io was in Jupiter's shadow and in sunlight. An example of an image taken while Io was in sunlight is shown at left. It was taken in November 1999 when the volcano Tvashtar Paterae erupted (seen much closer up by Galileo). In both cases (in eclipse and in sunlight), the spatial resolution of the observation is generally too low to pick up any but the brightest hotspots. The authors also measured the brightness at 3.5 microns of an eclipsed Io as it passed behind the dark limb of Jupiter. By noting the times when dips in the occultation light curve occurred, caused by Jupiter occulting a volcanic hotspot, the authors were able to constrain the location and intensity of an erupting volcano. Unfortunately, these would be one-dimensional fits of Jupiter's limb projected on the surface of Io. This method is also limited to finding hotspots on Io's Jupiter-facing hemisphere.

Three of the most persistent hotspots on the sub-Jupiter hemisphere are Loki, Kanehekili, and Janus. Rathbun and Spencer used their eight-year span of ground-based observations to chart variations in the amount of energy (in terms of Gigawatts) output by these volcanoes. Loki, Io's most powerful volcano, experienced periodic increases in power output between 1990 and 2001. In 2002, Rathbun and her colleagues suggested that this periodicity was due to a crust over a large lava lake foundering after becoming too thick, starting a wave of overturning crust that spreads counter-clockwise around the patera starting from the southwest corner of the volcano. However, the authors note in this paper that this pattern ended after 2001 (around the time Rathbun published her paper describing the periodicity) as Loki's power output leveled out in 2001-2002 a bit below the average between the earlier active and inactive episodes, before weakening between 2005 and 2007. Their extended history of Loki observations suggests that there have been no brightening events since 2001. The authors concluded that the measured brightness of Loki at 3.5 microns, and the derived brightness at 2.26 and 4.68 microns (taken by subtracting the total power output of Io in eclipse when Loki is shown by the occultation data to be inactive from the power output of Io when Loki is active) is consistent with the author's thermal model of Loki.

Kanehekili and Janus are two volcanoes on Io's leading hemisphere located within Media Regio. Ground-based observations by Rathbun and Spencer were unable to distinguish activity between these volcanoes are their proximity and Galileo observations of both of them as persistently-active volcanoes. The authors found that the 3.5 micron brightness of the region containing Janus and Kanehekili remained fairly consistent between 1996 and 1998 at a level similar to that of Loki in 2003 and 2004, before trending downward. A significant increase was observed early in 2002, though the authors couldn't distinguish between an increase in activity at either volcano, or another volcano at that longitude. I will point out that Marchis et al. 2005 observed a fairly bright hotspot at Janus in December 2001 using the Keck telescope, a few months prior to the Rathtbun and Spencer observations, and a very powerful eruption at Janus in January 2003. Combined with the observations of variations in the brightness of Janus and Kanehekili at shorter wavelengths by Galileo SSI and NIMS noted by Rathbun and Spencer, this indicates that the high-temperature component of the eruptions at these two volcanoes can vary greatly, even if the lower-temperature one stays comparatively consistent.

Finally, the authors examined shorter-lived volcanic eruptions from other sources they found in their data. These sources show significant variations in 3.5 micron brightness from near the background brightness to some of the brightest events seen in their decade of observing, such as an eruption of Grian Patera in June 1999. The observed variations are consistent with non-persistent volcanic activity creating fresh, cooling lava that emits light in the near-infrared. The authors noted weaker variations were observed in the mid-infrared by the PPR instrument on Galileo, which was sensitive to cooler, older lava flows.

Link: Ground-based observations of time-variability in multiple active volcanoes on Io [dx.doi.org]

Meteor fireball spotted in Jupiter's Atmosphere

2010-06-03T16:53:00.001-07:00

Less than a year after he first spotted an impact scar on Jupiter's south polar region, astronomer Anthony Wesley spotted a bright flash at 20:30 UTC today (about three hours ago from the time of this post) that lasted about 2 seconds in Jupiter's faded south equatorial belt. The brightness of the flash indicates that it was mostly likely caused by a small asteroid or comet striking Jupiter's upper atmosphere and burning up. Wesley indicates that don't seem to be any markings left by the impact, like those seen after the SL9 impacts in 1994 and last July's impact, though it occurred late in the day at the impact site, and we may not see anything until the area rotates into view three hours from now.

The original thread where Wesley posted this image is located at the Ice in Space web forum. He plans to post a video of the impact in the next few hours as well. I want to give a tip o' the hat to Emily Lakdawalla for alerting us to this observation on her blog. Coincidentally, she points out that the Hubble telescope team has released several images of the site of last year's impact, as the scar became sheared by the planets's winds.

Another place to check for up-to-date news on this is at the Unmannedspaceflight.com forum, where several regulars are already discussing the impact.

UPDATE 5:18 PM MST: A video by Christopher Go in the Philippines is now online. Didn't seem to last very long.

Link: Jupiter impact, June 3 2010 [www.iceinspace.com.au]

Last Month's OPAG Meeting

2010-03-20T21:46:00.000-07:00

The presentations for last month's meeting of NASA's Outer Planets Assessment Group (OPAG) were posted online last week. The focus of the meeting, which took place on February 8 and 9, was the president's NASA budget for FY2011 and the projected budgets over the next few years, and the Europa/Jupiter System Mission (EJSM).

In the first day of the meeting, the presenters focused on the new NASA budget and the ongoing, planetary science decadal survey. James Green and Curt Niebur, both from NASA Headquarters, presented on the new budget and how it impacts ongoing and upcoming outer planet missions. Green made it clear that NASA's Planetary Science Division was committed to funding EJSM through at least pre-phase A, and the budget reflects that. NASA will then look to the new decadal survey for how to proceed with flagship mission program. Personally, I don't see why they need to wait, the flagship mission was selected competitively. I don't see why we need to go through this non-sense again. These issues won't effect budgets until the out-years in the current budget projections... Anyways, Green also reported that funding for the restart of Pu-238 production is back on track now that NASA and the Department of Energy have developed a plan outline the "role and contribution of major users of Pu-238", as requested by Congress last year after nulling out such restart funding in last year's budget. Niebur's presentation provides a breakdown of planned funding for EJSM in the president's budget: "FY11: $20M, FY12: $72M, FY13: $64M, FY$14: $53M, FY15: $63M." This is fine for phase pre-A and A, but as we get passed instrument selection from the AO, those budgets should be going up not down, as seen now in FY13, FY14, and FY15, if a 2020 launch is still planned for. John Spencer's presentation on the satellites part of the decadal survey showed a series of mission concepts that are being evaluated inclusion in the survey, including a New Frontiers-class Io Observer.

The Europa/Jupiter System Mission presentations from the meeting's second day provide additional details on cooperative observations by the two spacecraft of the mission. For example, they plan to use a radio link between the Jupiter Europa Orbiter and the Jupiter Ganymede Orbiter to probe the upper atmosphere of Jupiter.

I will post more on the EJSM presentations tomorrow.

Carnival of Space #145 @ Crowlspace

2010-03-15T22:55:00.000-07:00

The Crowlspace blog is hosting this week's Carnival of Space, the 145th edition. The Carnival of Space presents the best posts across the span of space-related blogs. So don't forget to check it out and get caught up on the latest news! This week, read up on handling lunar samples, Opportunity leaving Concepcion crater, Herschel topography, and inappropriate nebulae.

The Tale of Gish Bar Patera

2010-03-13T04:27:00.006-07:00

I am rather surprised to find that in the two years I have had this little slice of the internet, not once have I written a post discussion the name sake for this blog, Gish Bar Patera, except for the post earlier this week presenting a version of the above image. So, tonight, I thought I would provide a discussion of the geology of this volcano and the volcanic activity observed there. Much of this description is based on an abstract for a poster I presented at LPSC in 2003. However, this post does include some new science as I included NIMS data from August 2001 (I31) in this analysis, whose data at 1.1 microns allows for reasonable comparisons with the Galileo's cameras visible data (providing further evidence that the flow seen in October 2001 formed after the August 2001 flyby, which was only inferred in my abstract). I've also provide a better description of the Galileo nominal mission data, which provides additional weight to my arguments about changes at Gish Bar between July and October 2001 (there was some debate at the time about these changes being due to phase angle effects).

Geology

Gish Bar Patera is a volcanic depression located on Io's leading hemisphere at 16.2° North, 90.3° West. The depression has an irregular shaped, as wide as 146 kilometers (91 miles) in the north-south direction and 131 kilometers (81 miles) in the east-west direction. The margins of the patera present signs of terracing along the patera walls as well as multiple floor levels, suggesting either multiple collapse events (akin to the volcanic crater atop Olympus Mons) or post-formation mass wasting of the patera wall and several volcanic sills, respectively. Compared to Chaac Patera, a volcano discussed here in January, Gish Bar is a shallower depression, with a floor only 500 meters below the surrounding plains.

Volcanism on the floor of the patera is focused in two areas: a large flow field in the north, west, and central portions of the volcano to the northwest and a possible lava lake within the southeast portion of the patera. The flow field consists of multiple flow lobes, suggesting that this portion of the patera consists of a compound flow field similar to Amirani. These flow lobes have different albedos and colors, suggesting a spectrum of surface ages or possibly different compositions. A bright orange flow in the northern potion of the patera is a possible sulfur lava flow. The other flows are consistent with the deposition of sulfur and sulfur dioxide on a cooling lava crusts of different ages. As we shall see a little later, dark, fresh flows have been observed in this flow field from orbit to orbit, while older flows fade. The southeastern end of the patera consists of a possible lava lake, 62 by 25 kilometers (38 by 16 miles) in size, with evidence for overflow events from three lava flows exterior to it. The sharp boundaries of this area suggests topographic control for the material in it, consistent with the lava lake interpretation.

To the north of Gish Bar Patera is an 11-km (36,000-foot) tall mountain named Gish Bar Mons. Gish Bar Patera and the volcano north of the mountain, Estan Patera, both appear to have led to increased mass wasting on the north and south sides of the structure. Despite this, very little debris from the mountain is apparent in the northern end of Gish Bar Patera. However, large landslides are visible to the west of the mountain, suggesting at least some degradation as the top layers of the shallow-sloped side of it sloughed off.

Volcanic History

Volcanic activity was first observed at Gish Bar Patera in the form of a thermal hotspot by the NIMS instrument during Galileo's first orbit on June 26, 1996 (orbit G1). It was also observed as a low-intensity, near-infrared hotspot in November 1996 (C3) and September 1997 (C10). Perhaps the most intense eruption of Gish Bar Patera during Galileo's nominal mission occurred in December 1996 (E4), when the SSI camera observed an intense hotspot at the volcano in an observation of Io while the moon was in the shadow of Jupiter. Jani Radebaugh, for her dissertation, correlated Galileo's eclipse observations with Io's surface features. Assuming the twist angle of the camera is correct, her E4 results suggest that the southwestern and central portions of the patera were the source of the eruption. E4 and later C10 sunlit images revealed small, dark flows in these areas, though it is possible they predate the December 1996 eruption as no images with sufficient resolution to resolve them exist before it.

The next Gish Bar imaging opportunity for Galileo was during the C21 non-targeted encounter on July 2, 1999. Gish Bar was imaged in three-colors at a resolution of 2.6 km/pixel in the violet and IR-756 filters and 1.3 km/pixel for the green filter (used in the graphic above). No surface changes are clear in the C21 data (compared to the clear-filter frame taken almost two years earlier), except for a new dark spot on the eastern wall of the southeastern "sub-patera". This is consistent with the lack of observed thermal activity at Gish Bar between September 1997 and July 1999.

In late 1999, Galileo flew by Io twice, on October 11 then on November 25. During the October flyby (I24), Galileo observed Gish Bar at 500 meters per pixel, shown above at full-resolution in the Geology section above. Two notable changes, shown in the ratio image at right, had occurred at Gish Bar Patera between the July and October 1999. Much of the southeastern "sub-patera" had darkened except for a small "island" area. Within the flow field in the center of the patera, a fresh, 22-kilometer long, 3-kilometer wide, lava flow was also visible. A possible origin for this fresh lava was an outburst eruption that occurred on August 2, 1999. However, the low spatial resolution of the available ground-based data makes it difficult to assign a specific volcano to the eruption. Within the Galileo AMSKGI01 mosaic, no other major changes are visible that would be consistent with such a major eruption, but the terrain to the east of Gish Bar was not covered in October 1999 to provide additional comparisons with the pre-eruption images from C21. However, the amount of fresh lava covering the floor of the lava lake (or "sub-patera") and the new flow lobe would be consistent with such a major outburst.

Gish Bar was observed again by Galileo during a pair of flybys in 2001, on August 6 (I31) and October 16 (I32). While high-resolution camera images were not obtained in the August flyby due to a camera malfunction, the near-infrared spectrometer acquired an observation of Gish Bar with a resolution of approximately 7.3 kilometers per pixel. This data revealed thermal emission within the lava lake and across parts of the flow field. However, most of the emission was at longer wavelengths, consistent with cooled lava flows. The shorter wavelength data provides the highest resolution data of the volcano between October 1999 and October 2001. No surface changes are apparent, though it would appear that many of the flows on the floor of Gish Bar had faded since October 1999.

Two months later, Galileo's camera observed Gish Bar once again, this time at 250 meters per pixel. A fresh lava flow, 1430 square kilometers in size, was visible across much of the western half of the patera. This covered much of the green colored terrain seen in the C21 color data. The flows seen in the center of the patera in the C21 and I24 data had faded due to inactivity and later sulfur deposition as the flows cooled. The lava lake was still covered mostly with dark material, suggesting continued activity after October 1999. The fresh lava flow observed at Gish Bar is thought to have formed sometime after the August 2001 flyby, as a significant increase (two orders of magnitude in emitted power) in Gish Bar's thermal emission was observed by NIMS during the I32 flyby, and the flow was not seen in the NIMS short-wavelength data in August. Assuming the eruption started shortly after the I31 flyby, the average coverage rate for the eruption was 230 square meters per second, similar to the coverage rate inferred for the Pillan eruption in 1997 or twice Kilauea's coverage rate at its peak. This was a major eruption indeed! Marchis et al. detected a hotspot using the Keck telescope in December 2001, showing that the eruption continued at least another couple of months.

Conclusion

Gish Bar Patera displays some fairly unique behavior for a volcanic depression. The patera itself is one of the largest on Io, large enough to have different portions of the volcano display different eruption styles. The southeastern potion of Gish Bar appears to consist of a large lava lake. Much of the rest of the patera floor is covered by a lava flow field, not unlike Amirani or Prometheus, that is topographically confined by the walls of the patera. The flow field is built up by multiple flow lobes, that form during major eruptions that occur every few years. Thus, unlike Amirani and Prometheus, Gish Bar is not persistently active. Its multi-colored surface and the adjacent mountain, Gish Bar Mons, make this region a compact version of many of the geologic themes that are common on Io. It certainly makes it one of the most fascinating features on Io, and that's why I presented a poster about it seven years ago, and that's why I named my Io blog after it.

Link: Gish Bar Patera, Io: Geology and Volcanic Activity, 1996-2001 [www.lpi.usra.edu]

Carnival of Space #144 @ Discovery News

2010-03-11T00:38:00.000-07:00

The Discovery Channel's Space News blog is hosting this week's Carnival of Space, the 144th edition. Ian O'Neill went with an Oscar theme this time around (can't imaging why...). The Carnival of Space presents the best posts across the span of space-related blogs. So don't forget to check it out and get caught up on the latest news!

Two more Europa mosaics

2010-03-10T01:48:00.000-07:00

Okay, two more Europa mosaics then I'm done. Honestly, I just made these because I hadn't seen them online and I have been been reading from the recently published Europa book and these help me follow along. But I promise, I won't desecrate this blog with any more of this vile Europan filth anymore ;-)

15ESREGMAP01 - This mosaic covers the terrain north from the 17ESREGMAP01 mosaic presented yesterday. This mosaic runs from Mehen Linea in the top frame, across the intersection of Minos and Udaeus Lineae, and finally to the lenticulated terrain northwest of Dyfed Regio. A portion of the middle of this mosaic was covered by the higher resolution mosaic, 19ESRHADAM01. This mosaic has a resolution of 228 meters per pixel.
15ESREGMAP02 - This mosaic covers the terrain north from the 17ESREGMAP02 mosaic presented yesterday. This mosaic covers the famous, mitten-shaped chaos region, Murias Chaos as well as two good sized impact craters, Brigid (three quarters of the way down) and Maeve (in the top frame).

That's it. No more. I am done with Europa. I need a shower. I feel unclean now.

19ESRHADAM01 - Galileo Mosaic of Europa

2010-03-09T17:14:00.000-07:00

Just because Europa (or her publicist) on Facebook asked nicely, I present the following mosaic, 19ESRHADAM01. This four-frame mosaic was taken on February 1, 1999 and was intended to cover a portion of Rhadamanthys Linea, a double-ridge on Europa's northern anti-Jupiter hemisphere. The view at right is just southwest of the prominent intersections of Udeaeus, Minos, and Cadmus Lineae. Clicking on the image at right will take you to a smaller version of the image, or you can download the full-resolution version by clicking here [3.57 MB PNG image]. The full-resolution version has a pixel scale of 62 meters per pixel and is centered near 35° N, 225° W. The two background frames come from the 15ESREGMAP01 mosaic.

The target of the mosaic was Rhadamanthys Linea, which runs left-to-right in the bottom half of the image, was only covered by the bottom corners of the four frames. This mosaic instead, highlights a region of lenticulated terrain. Lenticulae are region of small chaos or depressions on Europa surface, often appearing darker than the surrounding terrain. Many of these dark chaos regions disrupt or destroy the pre-existing ridged terrain, like the two-lobed dark area in the second frame from the left. Another disruptive feature is a fracture that runs through all four frames from left to right, that cuts into pre-existing ridges.

And now for something completely different...

2010-03-09T03:05:00.003-07:00

Here are some Europa mosaics from September 1998 for all of you. Don't ask me why I made these mosaics of Europa this evening...

17ESREGMAP01 - 233 meter per pixel mosaic of a north-south strip centered around 210° West, running south from around 15° N to 65° S Latitude. The mosaic covers portions of Belus Linea near the top of the mosaic, runs south covering Castalia Macula and portions of Argadnel Regio, then finishes up over portions of Onga, Katreus, Agenor, and Astypalaea Lineae.
17ESAGENOR03 - 46 meter per pixel mosaic across portions of Agenor Linea. This includes the western seven frames out of ten due to a gap in the mosaic.
17ESSOUTHP01 - 43 meter per pixel mosaic across portions of the south polar region of Europa near 79° S, 128° W.
17ESREGMAP02 - 211 meter per pixel mosaic of a north-south strip centered around 77° West, running sourth from around the equator to the limb near the south pole. The mosaic covers portions of Euphemus Linear near the top of the mosaic, across an area of chaotic terrain on Europa's leading hemisphere, south to a series of ridges (such as Sarpedon Linea). Several impact craters are visible, including: Cliodhna (middle top), Uaithne (fresh crater on top of a dark ridge about three quarters of the way down), and Grainne (larger crater in bottom left frame).

Color Gish Bar Mosaic from I32

2010-03-08T03:03:00.000-07:00

Last week, I posted a new version of the global color mosaic the Galileo spacecraft acquired in July 1999. This week we take a look at a mosaic from a Galileo flyby in October 2001 of the blog namesake, the volcano Gish Bar Patera (right frame), and the terrain to the west of it. The mosaic has a resolution of 267 meters per pixel and is centered near 17° N, 95° W. In this version, I have combined the brightness information from the three frame mosaic (32ISGSHBAR01) with the color data from the C21 mosaic from July 1999.

Be sure to DOWNLOAD [3.56 MB PNG image] the full-resolution version (Blogger automatically shrinks the version linked above).

A few things of note in this color mosaic. First, in the two years between the time the color data was taken and the higher resolution clear filter mosaic, several volcanic eruptions and other surface changes occurred at Gish Bar. Between July and October 1999, the southeastern portion of the patera darkened, possibly due to a major eruption in August that year. During the October 2001 flyby, Gish Bar was undergoing another major eruption, which likely generated the dark flow on the western side of the patera. Some of the dark flows seen in the July 1999 data have since brightened, which appear purple-ish in this mosaic. As a result of the surface changes, some of the colors seen in the patera should probably taken with a grain of salt, which is why I preferred not to do this before.

Outside of Gish Bar Patera, the terrain is mostly yellowish, with a few key variations seen. For example, the mountain Monan Mons, the southern portion of which is seen in the left frame, has bright material across much of the debris deposits to the east of it, suggesting that sapping of sulfur dioxide within the mountain may have played a role in the degradation of that structures. There also appear to be color differences between the bright yellow plateau to the west of Gish Bar (seen in the middle frame with the Y-shaped dark fracture running down the middle of it) and the surrounding, greenish plains.

Interesting stuff.

Paper: Mapping Io's Atmosphere with the Submillimeter Array

2010-03-06T19:36:00.002-07:00

Another day, another Io atmosphere paper. Today, we take a look at a paper by Arielle Moullet, Mark A. Gurwell, Emmanuel Lellouch, and Raphaël Moreno titled, "Simultaneous mapping of SO2, SO, NaCl in Io’s atmosphere with the Submillimeter Array." This paper was posted online Thursday in the Papers in Press section of the journal Icarus. In press papers are articles which have gone through peer review, were revised, and have been approved for publication, but have not been published in the print journal yet. This paper discusses results from sub-millimeter wavelength observations of Io's leading and trailing hemisphere. Using these observations, the authors were able to map the distribution of three chemical species in Io's atmosphere, sulfur dioxide (SO₂), sulfur monoxide (SO), and sodium chloride (NaCl). The authors then used these maps and the intensity of the observed emission lines to model the sources for these atmospheric components. For additional information beyond this summary, check out Moullet's presentation from the Harvard-Smithsonian Center for Astrophysics SMA Science Symposium 2009.

Io has a very thin atmosphere (1-10 nanobars at the surface) composed mostly of sulfur dioxide, along with its disassociation products, sulfur monoxide and atomic sulfur and oxygen. Other gases observed in Io's atmosphere or thought to exist in Io's atmosphere include sodium chloride, potassium chloride (KCl), diatomic sulfur (S₂), and sulfuryl chloride (Cl₂SO₂). Io's atmosphere has significant density variations with time of day, being densest near the sub-solar point and thinnest on its night side, and with position on the surface. The distribution of SO₂ gas was mapped across Io's surface using Hubble ultraviolet images of Io at the hydrogen Lyman-α line by Feaga et al. 2009, finding Io's sulfur dioxide gas in the atmosphere to be densest within 40 degrees of latitude from the equator and on Io's anti-Jupiter hemisphere. Adaptive optics observations at Keck in 2002 showed SO gas at several volcanic centers, which de Pater et al. 2007 suggested was related to volcanic activity. One of major questions these observations are trying to answer include identifying the dominant source of Io's atmosphere, whether it is sublimation of surface frosts, as atmospheric sulfur dioxide is thought to be in vapor-pressure equilibrium with the surface, or direct volcanic outgassing.

In this new paper, Moullet et al. 2010, the authors used observations taken at the Submillimeter Array (SMA) atop Mauna Kea in Hawaii, a set of eight, 6-meter-wide radio antennas acting as a radio interferometer. The system observes at millimeter and sub-millimeter wavelengths between 0.3 and 1.7 millimeters (or frequencies between 180 and 700 GHz), at between the infrared and microwave portions of the electromagnetic spectrum. For these observations, Moullet et al. observed Io in June 2006 and July 2008 at 338 and 346 GHz, at emission bands for sulfur dioxide, sulfur monoxide, and sodium chloride. These observations were disk-resolved, allowing the authors to examine the hemispherical distribution of each gas species, and in the case of SO₂ and SO, the high spectral resolution and signal-to-noise ratio allowed for more detailed analysis of their total emission.

For SO₂, Moullet et al.'s SMA data at 346.523 and 346.652 GHz is consistent with the results from Fegea et al.'s Lyman-α data and Spencer et al. 2005's infrared data, further indicating that the sulfur dioxide in Io's atmosphere is predominantly located at equatorial latitudes and Io's anti-Jovian hemisphere. Both constraints on the distribution of SO₂ are similar to the distribution of sulfur dioxide frost on Io's surface and volcanic plumes. To see which source of SO₂ dominates, volcanic outgassing or sublimation of surface frosts, the authors modeled the appearance of Io's 346.652 GHz SO₂ line emission using a hydrostatic model of Io's sublimation atmosphere and another modeling emission purely from the volcanic plumes noted by Geissler et al. 2004. Using an appropriate number of active plumes, the authors found that volcanism does not match the distribution and shape of SO₂ gas seen in the SMA data (except for the slight northward offset in emission seen over the anti-Jovian hemisphere in 2006). In addition, to match the amount of emission observed, an unrealistic number of active volcanic plumes (40-230 Prometheus-type, or 5-20 Pele-type plumes, on each hemisphere) would be required. Even if all 16 plumes noted by Geissler et al. were active, they would still only account for 5-11% of the emission on the leading side, and 13-18% on the trailing side. The hydrostatic models of the frost sublimation component of Io's SO₂ atmosphere better match the distribution and emission flux seen in the SMA data. The derived SO₂ column densities (an average of 2.3-4.6×10¹⁶ cm⁻² for the leading hemisphere, and 0.7-1.1×10¹⁶ cm⁻² for the trailing side), gas temperatures, and variations with Io's distance from the Sun provide further evidence that the main source for sulfur dioxide, the dominant component in the satellite's atmosphere, is vapor-pressure equilibrium sublimation of SO₂ surface frosts. A minor fraction of the SO2 also comes from volcanic gas plumes (though not all gas emissions for volcanoes form plumes).

Moullet et al. also mapped the distribution of sulfur monoxide and sodium chloride in Io's atmosphere for the first time. Both gases were predominately found on Io's anti-Jupiter hemisphere, as SO₂ was. The signal-to-noise ratio for the SO data was sufficient for the authors to perform an analysis on the source of that gas, similar to the one they performed on sulfur dioxide. Two sources were considered: photolysis of SO₂, when sulfur dioxide is broken up by solar ultraviolet photons into SO and O, and direct volcanic emission. In the case of volcanic activity, they found that, using a reasonable level of plume activity, volcanism is the source of 2.5% of the SO observed in the SMA data, using a model that included four volcanic plumes. Using more plumes, or Pele plumes, would result in models that was more extended spatially than the emission maps from the data acquired. Even using a favorable, 16 plume model, volcanic plumes account for 13-40% of the total SO emission, assuming a 10% mixing fraction in the plumes. This at least suggests though, that volcanism is a more significant source of sulfur monoxide than sulfur dioxide. The dominant source, however, is photolysis from SO₂, as the modeling performed by Moullet et al. suggests. The authors note that the "relative contribution is not easy to assess in the absence of precise estimates on SO lifetime in the atmosphere."

The signal-to-noise ratios of the NaCl maps were much lower than those for the SO2 and SO data, but radiative transfer modeling of NaCl in Io's atmosphere shows that the data is consistent with a volcanic origin. This is particularly the case since NaCl has a short lifetime in Io's atmosphere due loss from condensation on dust particles and photolysis into atomic sodium and chloride. The amount of NaCl emission observed is consistent with a lower-bound, mixing ratio in volcanic plumes of 0.6% on the trailing hemisphere, and 2.5% on the leading side, similar to thermochemical modeling done by Fegley and Zolotov 2000. Moullet et al. could not rule out other sources for the NaCl emission, such as sputtering.

This paper seems to provide further evidence that most of the gases in Io's atmosphere result from the sublimation of sulfur dioxide surface frosts, with direct volcanic emission playing a minor role. Other major atmospheric components such as sulfur, oxygen, and sulfur monoxide result from the photo-disassocation of sulfur dioxide into its elemental components by solar ultraviolet photons, though a larger percentage of the sulfur monoxide in the atmosphere may come from direct volcanic outgassing. Finally, some atmospheric components not related to sulfur chemistry, such as sodium chloride, could be entirely explained by volcanic outgassing, though other sources such as sputtering are also possible, but weren't modeled by the authors as the distribution of surface deposits of minerals other than sulfur and sulfur dioxide are very poorly constrained. According to their presentation from April 2009, the authors stated they planned to map the distribution of other species such as potassium chloride, silicon oxide, and disulfur monoxide in the summer of 2009 using the APEX antenna in Chile, and will submit a proposal to observe Io's atmosphere at much higher resolution using the ALMA telescopes when they are finished in 2012.

Link: Simultaneous mapping of SO2, SO, NaCl in Io’s atmosphere with the Submillimeter Array [dx.doi.org]
Link: Presentation - Mapping of SO2, SO and NaCl emission in Io's atmosphere [www.cfa.harvard.edu]