Tuesday, August 31, 2010

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 13001350 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.

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