Liquid Hot Magma Ocean Resolved in Galileo Magnetometer Data

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.]

Paper: Detection of a "Superbolide" on Jupiter

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

Observing Active Volcanic Eruptions Remotely

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]

Life on Io? Not Very Likely.

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]

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

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]

Paper: Mapping Io's Atmosphere with the Submillimeter Array

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]

LPSC 2010: Simulating Io's Auroral Emission in Eclipse

Yesterday, we talked about a model of Io's atmosphere using the Direct Simulation Monte Carlo (DSMC) method.  Today we look at another Monte Carlo (MC) model of Io's atmosphere, this time focusing on simulating the emission of Io's atmosphere during an eclipse.  Like the research discussed yesterday, both the abstract for next week's Lunar and Planetary Science Conference (LPSC) and a new paper in press in Icarus are available.  The LPSC abstract is titled, "Io's UV-V Eclipse Emission: Implications for Pele-type Plumes," by Chris Moore, David Goldstein, Philip Varghese, and Laurence Trafton.  This research will be presented as a talk next Wednesday afternoon, March 3 in the Planetary Atmospheres session.  The Icarus paper in press is titled, "Monte Carlo Modeling of Io’s [OI] 6300 Å and [SII] 6716 Å Auroral Emission in Eclipse," by Chris Moore, K. Miki, David Goldstein, K. Stapelfeldt, Philip Varghese, Laurence Trafton, and R.W. Evans.  Both cover the topic of simulating Io's auroral emission when the satellites goes into eclipse, but under different regimes: the LPSC abstract discusses emissions in the mid-ultraviolet as the result of SO₂ and S₂, while the Icarus paper talks about emissions in the red portion of the visible spectrum from oxygen (from decomposed SO₂).

Last year, this same group published a paper on the dynamics of Io's atmosphere during an eclipse, which occurs each Ionian day when the satellite passes into the shadow of Jupiter.  Each eclipse lasts around 2 hours and 20 minutes.  During this time, no direct sunlight reaches Io surface, though Europa-shine and refracted sunlight from Jupiter's atmosphere can faintly illuminate the surface.  The authors found that Io's atmosphere doesn't completely collapse during an eclipse, as a diffusion layer of non-condensable atmospheric species like oxygen and sulfur monoxide forms near the surface, preventing sulfur dioxide above it from condensing out on to the surface.  With their model from last year's paper in hand, the authors further examined it, seeing how their model results would appear at different emission bands of the species they included in their model atmosphere (SO₂, O, SO, S, and O₂).  They also examined the emission from S₂ gas present in volcanic plumes like Surt and Pele, and the effects of volcanic activity on the other emission bands.

In their Icarus paper, Moore et al. focused on two emission bands, the prominent [OI] oxygen emission line at 630 nm and the much fainter [SII] band at 670 nm (both in the red portion of the visible spectrum).  Thus in the image above, the emission examined would be colored in red, so that covers much of the limb glow.  This limb glow shifts between the north and south polar region of Io, as seen by Galileo and Cassini.  Observations by Trauger et al. 1997 also revealed a high altitude bright spot in the [OI] line over the leading hemisphere, which is actually on the wake side of Io since the magnetosphere of Jupiter spins faster than Io revolves around Jupiter.  Moore's simulation of Io's atmosphere in eclipse was able to match the observed position of the bright wake spot, but not its intensity.  The model suggests that the position of the wake bright spot (which can be seen either above or below the equator) and the polar limb glows are related to the depletion of electrons from the Io-Jupiter flux tube, which effects the 630 nm emission by Io's position above or below the plasma torus, volcanic plumes (particularly large polar plumes), the density of the polar atmosphere, and Io's effect on Jovian magnetic field lines.  The authors also found that the [SII] emission is much weaker than the [OI] oxygen line, in part because of the lower S⁺ (remember, the ratio of oxygen to sulfur in Io's atmosphere is roughly 2 to 1).

The LPSC abstract examines emission bands by SO₂ and S₂ in the mid-ultraviolet to the visible (250-600 nm) as Io ingresses into an eclipse.  The authors found that the distribution of this emission across Io's sub-jovian hemisphere (the area covered by Trauger et al. 1997 Hubble observations) is strongly effected by volcanic plume activity.  These plumes act as shields for the atmosphere south of them from the electrons from the Jupiter-Io flux tube, reducing the energy of these electrons.  For example, if both the Surt and Acala plumes are active, the northern Surt plume shows up bright due to S₂ emission above 300 nm, while Acala plume south of it appears fainter because molecules in its plume are not as excited by flux tube electrons.  The authors also examined the difference between the western and eastern halves of the sub-Jupiter side of Io at these wavelengths.  With the Surt and Pele plumes active, the emission from Io was much greater than if they were off.  The ratio between the eastern and western halves was greater than one when both are off because of emission from SO2, which as a greater density on the eastern half because before the eclipse, it had been in the afternoon (remember from the article yesterday that Io's sublimation atmosphere peaks in density where the frost temperature is greatest, around 2pm in the afternoon).  When both plumes are one, that ratio becomes less than one as the brightness of the S₂ emission from the Surt plume dominates the brightness on the western side.  With significant differences such as these, the authors suggest that even barely disk-resolved spectra, particularly around 300 nm, can be useful for determining the activity of volcanic plumes on Io from Earth-based data.

These two papers explore Io's auroral emission at various wavelengths from the mid-ultraviolet to the visible using a simulation to explain the observations we have on hand.  They show that the auroral glow of Io's atmosphere is affected by volcanic plume activity, such that observations from Earth can be used to determine the presence or absence of different plumes, Io's position in the magnetosphere, and the density of Io's atmosphere.  These simulations also explore the various chemical species in Io's atmosphere and how even minor constituents like oxygen, formed from the disassociation of sulfur dioxide, can have a strong effect on its auroral, so vividly seen when Io is in eclipse.

Link: Io's UV-V Eclipse Emission: Implications for Pele-type Plumes [www.lpi.usra.edu]
Link: Monte Carlo Modeling of Io’s [OI] 6300 Å and [SII] 6716 Å Auroral Emission in Eclipse [dx.doi.org]

LPSC 2010: Modeling Io's Atmosphere in Three Dimensions

Okay, it is about time I finished up my coverage of the abstracts covering Io science for next week's Lunar and Planetary Science Conference.  Today I am going to talk about "Modeling the Sublimation-Driven Atmosphere of Io with DSMC" by Andrew Walker, Sergey Gratiy, Deborah Levin, David Goldstein, Philip Varghese, Laurence Trafton, Chris Moore, and Benedicte Stewart.  A paper in press in Icarus was published last month by the group covering this topic, "A comprehensive numerical simulation of Io’s sublimation-driven atmosphere".  This post will act as a summary of both the LPSC abstract and the paper.  The LPSC paper will be presented as a talk next Wednesday afternoon, March 3 in the Planetary Atmospheres session.

In their model, Walker et al. used the Direct Simulation Monte Carlo (DSMC) method for simulated Io's rarefied atmosphere in three dimensions.  Previous modelers explored Io's atmosphere as a single dimension, looking at how column density and temperature changes over the course of a day in response to changes in surface temperature, or as a two dimensional model that looked at how these parameters changed across a single latitude, axi-symmetric with the sub-solar point.  With a three dimensional model, the authors were able to explore the effects on Io's atmosphere from volcanic plume activity at known volcanoes like Pele and Prometheus, plasma bombardment heating from above, planetary rotation, sub-solar temperature (115-120 K), the residence time of fine-grained sulfur dioxide frost on bare rock, and variations in frost temperature and areal coverage.  The DSMC method models individual sulfur dioxide molecules (usually representative of the total number of molecules), which is useful when the atmosphere has such low density that the mean free path of sulfur dioxide molecules exceed that the length over which many gas properties propagate.  Similar modeling was performed by Austin and Goldstein 2000, though this new model includes the inhomogeneous frost coverage mapped by Galileo NIMS. This also allows the authors to graph variations in the translational, vibrational, and rotational temperatures (related to the different emission bands of sulfur dioxide based on motions of the S-O bonds), density and column density (number of sulfur dioxide molecules per cubic centimeter or over a square centimeter of Io's surface, respectively), and flow rate (expressed in the article as mach number).

Since the authors primarily modeled the sublimation component of Io's atmosphere, the column density and many of the other properties of the lower atmosphere were related to the temperature and areal coverage of sulfur dioxide frost on the surface as this part of the atmosphere would be in vapor-pressure equilibrium with that frost.  Because of the difference between the position of the peak frost temperature and the sub-solar point, ~30° to the east or 2pm local time, the column density near the surface peaks to the east of the sub-solar point.  This lag in peak frost temperatures results from the thermal inertia of SO₂ frost.  Changing the sub-solar peak temperature from 115 K to 120 K causes a five-fold increase in the peak atmospheric column density from 4.7×10¹⁶ cm^–2 to 2.7×10¹⁷ cm^–2.  This brackets the lower and upper bounds for the atmospheric column density measured by earlier observers of Io's atmosphere.  Compare this to the column density to the Earth's, which is ~3×10²⁵ cm^–2.

I should point out at this point that this group published a companion paper (Gratiy et al.) that actually showed up in the Icarus in press page first, and was discussed here last month.  This paper compared their model of Io's atmosphere to actual observations taken a ultraviolet, infrared, and millimeter wavelengths.  One note that Walker et al. does make is that the variation in column density with latitude doesn't seem to match the Hubble Lyman-α observations, which showed a sharp drop-off in atmospheric density poleward of ±45°.  They suggest that this could be because of differences in frost temperature from the assumed cos^1/4(ψ) latitudinal variation.

In other results, the authors found that heating from the Io plasma torus inflates the upper atmosphere of Io and keeps the nightside atmosphere from completely freezing out.  Plasma from Jupiter's magnetosphere only penetrates down to an altitude of 1 km at the point of peak frost temperature, and the altitude decreases the further you get from that point, reach the surface at the poles and on the nightside.  This actually means that the low altitude translational temperature of the SO₂ in the atmosphere is higher on the nightside (where plasma reaches all the way to the surface due the lower atmospheric density) and particularly along the terminator.  At the terminator, the higher density dayside atmosphere interacts with the low density nightside atmosphere, leading to supersonic gas flow just past the dusk terminator and near the poles.

Tomorrow we will take a look at another LPSC abstract and Icarus paper by this group on modeling Io's auroral emission.

Link: Modeling the Sublimation-Driven Atmosphere of Io with DSMC [www.lpi.usra.edu]
Link: A comprehensive numerical simulation of Io’s sublimation-driven atmosphere [dx.doi.org]

Carnival of Space #138 and other news

The 138th edition of the Carnival of Space, a weekly series highlighting the best in the astronomy and space blogosphere, is now online at Nancy Atkinson's new blog.  You know the drill.  Some great posts the Mars rover Opportunity's latest travels, Martian dunes as imaged by HiRISE, and the Southern Cross from various exoplanets.

I also wanted to take this quick post opportunity to point a few other news items.  There was a paper in Nature Geosciences titled, "Origin of the Ganymede–Callisto dichotomy by impacts during the late heavy bombardment," by Amy Barr and Robin Canup.  I haven't downloaded this paper, but a great summary can be found at ScienceBlog.  Essentially, they found that Ganymede's closer distance to Jupiter and thus being deeper in Jupiter's gravity well, led to more and larger cometary impacts than on Callisto early on in the history of the Solar System.  These impacts helped to cause more complete differentiation on Ganymede by bring rock closer to the core and water closer to the surface.  Fewer large impacts on Callisto meant that it was less differentiated.  Today, Callisto has a more ancient surface with few signs of internal activity, while Ganymede has a convecting iron core producing an internal magnetic field and a surface covered in lanes of grooved terrain separating more ancient terrain.

Finally, there are a couple of new Io-related papers in press in the journal Icarus.  The first is "A Comprehensive Numerical Simulation of Io’s Sublimation-Driven Atmosphere" by Andrew Walker et al.  This paper details a new Monte Carlo simulation of the atmosphere of Io.  The other is "Monte Carlo Modeling of Io’s [OI] 6300 Å and [SII] 6716 Å Auroral Emission in Eclipse" by Chris Moore et al.  This paper will also look at modeling of Io's atmosphere.  In this case, the authors are attempting to recreate observations of Io's aurorae, seen in eclipse in May 1997.  Both these papers have associated LPSC abstracts so I will combine the discussion of them into one post in the next week.

Link: Carnival of Space #137 [noisyastronomer.com]
Link: SwRI researchers offer explanation for the differences between Ganymede and Callisto [www.scienceblog.com]

Paper: Verifying a new model of Io's atmosphere by simulating multi-spectral observations

In November, the paper "Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model" was published in press in the journal Icarus.  The authors for this paper are Sergey Gratiy, Andrew Walker, Deborah Levin, David Goldstein, Philip Varghese, Laurence Trafton, and Chris Moore.  This paper takes a look at a computer model of Io's atmosphere, consisting of a composite of sublimation and volcanic sources and published in Walker et al. (submitted to Icarus but not yet available online), and attempts to validate the model by comparing simulated observation derived from the model to real observations published over the last decade.  I have to admit that this paper covers a topic that is definitely out of my wheelhouse, so I may not cover the findings of this paper with as much depth as I have given to other papers recently, but I will do the best I can to provide a summary.

While I have not read the Walker et al. paper covering the details of this new model since it hasn't been accepted by Icarus and posted online, according to this paper, it is a 3-D global rarefied gas dynamics model that uses both volcanic plumes and sublimation of surface sulfur dioxide (SO₂) frost as sources for the gas in Io's atmosphere.  The model takes into account changes to the column density of the atmosphere as a result to time-of-day changes to the surface temperature, distribution of SO₂ surface frost, distribution of volcanic plumes (though they use persistent volcanic thermal hotspots as plume sites rather than confirmed plume locations or the locations of large surface changes), plasma heating from Jupiter's magnetosphere, and heat loss to space.  For this paper, the authors used a backward monte carlo method to simulate how their model atmosphere would appear in different types of observations.

In Gratiy et al. 2009, the authors compared their model results to three observations: disk-integrated, high-spectral resolution measurements in the mid-infrared near 19-µm published by Spencer et al. 2005; disk-resolved, far-ultraviolet observations in the hydrogen Lyman-α band published in Feldman et al. 2000; and disk-integrated, millimeter wavelength observations published in Io After Galileo in the Io's Atmosphere chapter by Lellouch et al.

First, the authors compared their model output to observations published in Spencer et al. 2005.  For that paper, Spencer observed 16 SO₂ ν₂ vibrational band absorption lines using the TEXES mid-infrared spectrometer at the NASA Infrared Telescope Facility.  This was disk-integrated spectra centered around 19 microns.  From this data, the authors found a significant longitudinal asymmetry in absorption band depth, a measure of the average column density over the hemisphere sampled, with a low band depth (1%) compared to the continuum level at 315°W (the hemisphere centered near Ra and Loki) and a much stronger bands (7%) at 180°W (the hemisphere centered near Colchis Regio).  This indicates that Io's atmosphere is denser over the anti-Jupiter hemisphere than over the hemisphere centered on Ra and Loki.  This distribution correlates to the distribution of surface frost, but it also correlates with the distribution of volcanic plumes, as Gratiy et al. notes.  The authors of this new paper found that a combined model with sublimation and volcanic sources with long residence times (5000 seconds) for condensed SO₂ on bare rock.  They also note that their model and the data from Spencer et al. seem to support a thermal lag where the surface temperature and thus frost sublimation peaks 3.5 hours, or 30° of rotation, after local noon.  This is akin to the terrestrial experience of the air temperature being warmer in the mid- to late-afternoon, as opposed to right at noon when the sun is at its highest point in the sky.  However, if you look at the PPR day-side data from I31, shown at right from Rathbun et al. 2004, there maybe some question of whether that thermal lag is real.

The next dataset the authors compared their model to was the Lyman-α data disk-resolved observations published in Feldman et al. 2000 and Feaga et al. 2009 (the latter paper was discussed here last year).  This data was acquired using the Space Telescope Imaging Spectrograph (STIS) on Hubble between 1997 and 2001.At these far ultraviolet wavelengths, areas where Io's atmosphere are denser absorb sunlight, appear dark in Lyman-α images.  Sunlight is better able to reach the surface and reflect back into space in order to be seen by Hubble.  Thus, areas where Io's atmosphere is thinner appear brighter in Lyman-α images.  An example of one of these images is shown at top.  Gratiy et al. could not reproduce this data with their model, suggesting that they do not properly simulate the latitudinal variation in the column density of Io's atmosphere (thicker at the equator, thinner at the mid-latitudes and poles).  In particular, they had difficulty reconciling the observed sharp increase in Lyman-α brightness, and therefore the sharp decrease in atmospheric column density, 45° North or South of the equator, and with the utter lack of atmosphere beyond 60° North or South latitude.  This cutoff, the authors suggest, is more consistent with the distribution of surface changes and volcanic hotspots, as opposed to surface frost.  However, there don't seem to be variations in the Lyman-α images resulting from known volcanic plumes.  The lack of an east-west asymmetry on either side of the central meridian in Io's equatorial brightness in the far-ultraviolet data that would be expected from the Walker et al. model suggests that the surface thermal inertia is much lower than they used for that model.

In the final comparison, Gratiy et al. compared their model to disk-integrated millimeter-wave SO2 emission line profiles obtained at IRAM 30-meter telescope in Spain, published in Io After Galileo in the Io's Atmosphere chapter by Lellouch et al. and disk-resolved data in Moullet et al. 2008.  The authors determined that strong atmospheric winds explain the wider SO₂ emission lines in the IRAM data compared to what would be expected from thermal Doppler effects alone.

To be honest, this was a difficult paper for me to get through, hence why it took a month and a half for me to get this summary up.  So, I apologize for not explaining the paper's conclusions as well as I could have.  Basically, the authors hope that by comparing their rarefied gas dynamics model of Io's atmosphere with real observations they can make some improvements to that model that also provide new information about Io, such as the presence and strength of atmospheric winds, the surface thermal inertia, and the relative contribution of frost sublimation and volcanic plumes to Io's atmosphere.

Link: Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model [dx.doi.org]

Paper: Geomorphologic Mapping of Hi'iaka and Shamshu Regions

On Friday, a new paper was published in press in the journal Icarus titled, "Geologic mapping of the Hi’iaka and Shamshu regions of Io".  In press articles are those that have been reviewed and approved for publication, but have not yet been published in the print journal.  This paper, by Melissa Bunte, David Williams, Ronald Greeley, and Windy Jaeger.  This paper discusses the results of a geologic mapping project based on the 25ISTERM__01 mosaic of the Hi'iaka region and the 27ISSHMSHU01 mosaic of Shamshu region [I've also uploaded labeled versions of these two mosaics, Hi'iaka and Shamshu, to help people with this discussion].  These two regions are dominated by a patera with a floor partially covered in dark lava flows surrounded by several large mountains.  This paper is part of a series of geomorphologic mapping projects using medium-resolution Galileo mosaics of Io for their basis.  We previously covered papers discussing maps of Prometheus and Zal.  Other regions mapped by the ASU group include: Chaac-Camaxtli, Culann-Tohil, Zamama-Thor, and Amirani-Skythia-Gish Bar.  In March, I covered this group's LPSC abstract covering the mapping of Hi'iaka and Shamshu, so bare with me if I repeat some things from that post..

For this paper, the authors created two geomorphologic maps of the regions surrounding Hi'iaka and Shamshu Paterae, two volcanoes near Io's equator on the leading hemisphere.  These maps were based primarily on two SSI mosaics from Galileo's I25 and I27 encounters with Io in November 1999 and February 2000, respectively.  Color information and low phase brightness information was taken from the global color mosaic produced by the USGS.  Geomorphologic maps are a type of geologic maps where different surface units are identified and mapped so that relationships between these different units can be identified.

In the case of the Hi'iaka and Shamshu regions, four basic units types were identified: plains material, mountain/plateau materials, patera floor materials, and lava flow materials.  These units were also identified in the mapping of other regions on Io, though diffuse material, recent surface coatings that are derived from volcanic or sapping processing and identified in other mapped regions, was not identified in the Hi'iaka and Shamshu regions.  It should be noted though that diffuse materials are often transient; red diffuse material was observed in a faint partial ring of material surrounding Hi'iaka during Galileo's first orbit of Jupiter, suggesting the presence of a plume at Hi'iaka shortly before June 1996.

These four basic units were further sub-divided into different sub-units based on their albedo, color, surface texture, or structural contacts.  For example, mountain units are divided into lineated (ridged/grooved material, generally found at higher altitudes, bounded with plains by scarps), mottled (hummocky, often with lobed margins), undivided (intermediate in texture and altitude between the mottled and lineated types), and plateau (flat-lying terrain with smooth or hummocky textures) types.  Based on the spatial relationships between these units, the authors theorize that these types represent different stages of degradation on Io's mountains.  Lineated mountain units tend to be higher standing and have sharper unit contacts with the surrounding plains, suggesting that terrains of this unit have undergone the least amount of degradation.  The ridges and grooves, such as those seen above at north Hi'iaka Montes, are the result of down slope slumping of a mobile surface layer atop the mountain (see Jeff Moore's 2001 paper on slope degradation on Io).  Remember that mountains are thought to be tilted crustal blocks, the ridged and grooved top surface of the mountain is the result of the upper few kilometers of the mountains, which is a mix of basalt and sulfurous materials that was once part of the upper few kilometers of Io's flat plains.  Once that upper layer sloughs off, the unit transitions to the undivided or mottled types (though the mottled types are likely a mix of old mountain and the mass wasted materials surrounding the mountain).

A similar age progression in sub-units was also noted in patera floor and lava flow materials based on color and albedo (for dark flow/patera floor materials, darker equals younger).  The paper only mentions briefly the debate whether some bright or yellowish lava flows are primary or secondary sulfur flows, or silicate flows with chemical altered surfaces.  The authors (and I agree) that these flows are likely old silicate flows.  This supported less in this region but at Thor, where an outburst eruption generated lava flows that overlapped those of an older, yellowish flow.  Exceptions to this could include a bright white flow to the south of Shamshu Patera and the orange floors of "Mekala Patera" and western Hi'iaka Patera, which could be due to mobilized sulfur dioxide and sulfur, respectively.

As I mentioned in my post from March on the researchers' LPSC abstract on this subject, the authors examined the hypothesis that the three mountains that surround Hi'iaka Patera had at one point been part of a single structure that had split apart as a result of strike-slip and extensional faulting.  This fault would have also resulted in the creation of pull-apart basins that would become Hi'iaka Patera and orange-color patera to the lower left of north Hi'iaka Montes (named Mekala Patera in the paper).  The authors note that the freshest lava flows on the floor of Hi'iaka appear to emanate from the eastern and southern margins of the patera, a pattern consistent with this break-up scenario.  This model of regional-scale plate tectonics was also applied to the mountains in the Shamshu region (with Shamshu Patera being presented as a pull-apart basin) with less convincing results, but it does suggest a new line of investigation to see just how prevalent this style of plate tectonics is on Io with mountains forming, breaking apart, and shifting around across Io's surface.  How many other close groupings of mountains were the result of a single mountain being broken apart by strike-slip and extensional faulting?  I should note that several Ionain mountains have been observed with canyons running down the centers as a result of extensional faulting, like Ionian Mons (upper left), Mongibello Mons, and Danube Planum.

This paper summarizes the results of geomorphologic mapping of the Hi'iaka and Shamshu regions on Io based on regional-scale imaging by Galileo, finding units consistent with those found in other regional mosaics of the satellite.  The geologic history of these regions based on their mapping supports earlier research into this area by authors like Turtle et al. 2001 and Jaeger et al. 2001 that Hi'iaka Patera formed as the result of Hi'iaka Montes breaking apart by strike-slip and extensional faulting.  Magma then ascending to the surface using these same faults that bounded the pull-apart basin.  At this point, the volcanic activity at both Hi'iaka and Shamshu appear to be dominated by compound lava flows, unlike the lava lakes seen at Pele and Loki.  The mountains were then further degraded by gravitational mass wasting, SO₂ sapping from ice layers within the mountain, and thermal erosion from nearby volcanoes.  Thermal erosion by flowing lava is thought to be responsible for Tawhaki Vallis, a channel observed to the right side of the image at top, as proposed by Schenk and Williams 2004.  Each of mountains in the region exhibit signs of degradation, from the ridges and grooves atop north Hi'iaka Montes and north Shamshu Montes, indicative of a slumping mobile surface layer, to hummocky-textured and lobe-edged landslide deposits, indicative of gravitationally- or sapping-induced mass wasting.

I think that leaves one more region to map, Tvashtar.

Link: Geologic mapping of the Hi’iaka and Shamshu regions of Io [dx.doi.org]

A Few New Io/Jupiter System Papers

Sorry for the hiatus there is posts over the last couple of months.  I've just been a bit busy with Cassini plus I wanted to take a bit of a break after posting so much in September and October.  I wanted to post a quick note letting you all know about three new Io/Jupiter System related papers currently in press in the journal Icarus.

The first is titled, "Geologic mapping of the Hi'iaka and Shamshu regions of Io" by Melissa Bunte, David Williams, Ronald Greeley, and Windy Jaeger.  This paper is the latest in a series that covers the geologic histories of some of the regions observed at high- and medium-resolution by Galileo during its flybys in the late 1990s and early 2000s.  Back in March, I covered the authors' LPSC abstract on this research so you can read up on that while I slowly get through this paper and post a summary over the weekend (hopefully, there is a major Titan flyby today whose data comes back Sunday morning).

The other Io paper is titled, "Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model" by Sergey Gratiy et al.  Yeah, that's going to take me a bit longer to get through.  Hopefully I can post something next week.

The final paper that caught my eye in Icarus is titled, "Global geological mapping of Ganymede" by G. Wesley Patterson et al.  This paper discusses the completed Ganymede global geologic map and presents research on the observed geologic units on that satellite.  Again, I've only flipped through the article, and maybe later this month I can write up something about it.