Io Volcano of the Week: Tvashtar - Part Two

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

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

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

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

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

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

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

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

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!

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]