Galileo's I24 Flyby of Io - A Look Back: Results

Today, we finish up our look back at Galileo's I24 flyby of Io that occurred 10 years ago last Sunday. I had hoped to get this done a few days ago but present day encounters of Titan and Tethys had taken priority.  While last week's Cassini encounters felt almost routine, though you can never presume that just because you've seen an area many times before that you won't learn something new, the flyby we've been looking back at here in this blog was definitely not so.  In our last installment, we saw how Galileo's engineers and scientists had to overcome the spacecraft going into safing hours before the flyby was to occur, scrambled images, and a stuck spectrometer grating to pull off a fairly successful flyby.

In this final installment, we take a look at some of the data that was returned by Galileo and what it taught us about Io.  To put this data in a kind of video timeline, I created a Youtube video using Celestia and put together in Adobe Premiere Pro that I posted last week.  Check it out if you haven't done so already.

SSI


Despite the degradation of the majority of images acquired by Galileo during orbit I24, many of these images were still usable following a reconstruction effort at JPL using an alorithm in National Instrument's LabVIEW software.  For example, cooled lava flows, pits, and channels are visible in this 13-frame mosaic covering portions of the Pillan lava flow.  The eruption that formed much of the terrain visible here occurred two years earlier in a massive event that produced not only a 3100 sq. km lava flow, but also a huge pyroclastic eruption.  Lava from the eruption would then cascade over the edge of Pillan Patera (to the southwest of the area seen in the mosaic) to cover the floor of that depression with dark basaltic lava.  The rough texture, pits, and channels seen across portions of the flow field suggest potentially turbulent flow of Pillan's lavas, as well as a strong interaction been the emplaced lavas and the SO₂ frost that coated the ground prior to the lava flowing over the surface.

Observations at other flow fields, such as the mosaic covering Prometheus shown above, revealed a different story.  Rather than a rough textured surface, SSI images revealed a patchwork of bright, sulfur dioxide frost, fresh dark flows, and older flows.  The irregular margins of this flow field and the similar Zamama and Amirani fields suggested much thinner flow lobes (~1 meter versus 8-10 meters) than what was seen at the more rapidly emplaced Pillan flows.  These flow fields, Amirani, Prometheus, and Zamama, are interpreted to compound flood basalt flow fields.  Rather than being formed in one giant eruption, lava at these volcanoes is emplaced in the form of "small" breakouts on top of previously erupted lava.  When lava from a breakout interacts with sulfur dioxide frost on top of the older lava, the sulfur is heated up and blasts its way through the meter-thick flow lobe, helping to form part of the plumes at these volcanoes.


Several of SSI's observations were dedicated to the other major landform type on Io, its many tall mountains.  This includes the high resolution mosaic of Ot Mons, an older mountain in the middle of Colchis Regio.  This mosaic and other lower resolution observations of Io's mountains closer to the terminator to the east revealed that nearly all of these structures are at some stage of degradation.  This is despite the fact that these mountains are all less than one million years old due to Io's high resurfacing rate.  The mosaic of Ot Mons revealed a structure covered in 100-meter tall, rounded hills with a mix of bright and dark material, with the dark material located mainly in topographic lows.  The lower resolution mosaics of mountains such as Skythia Mons, Gish Bar Mons, and Monan Mons, revealed evidence for large landslides and significant faulting across the top of these mountains.  In addition to evidence for various stages of degradation, evidence for layering was observed at several mountains, including one west of Donar Fluctus (seen near the center of the ZAMAMA02 mosaic).

These SSI observations provided further evidence that Io's mountains generally consisted of tilted crustal blocks.  The observed layering would then be old lava flows, pryoclastic material, and sulfur and sulfur dioxide ices that form layers in Io's upper crust.  The observed degradation shows that these mountains begin to fall apart soon after they form as a result of sulfur dioxide sapping from those ice layers within the mountains and Io-quakes that must be fairly common.

NIMS

While NIMS, Galileo's Near-Infrared Spectrometer, was in ride-along mode for most of its observations and it lost much of its spectral resolution due to the stuck grating, it was able to make a number of exciting observations.  One of its first observations of the flyby covered portions of Loki Patera (shown at right) while the volcano was on Io's nightside.  This observation revealed warm material within fissures in the bright island on the patera floor as well as within the darker, lava lake surface to the east.  This data revealed dark material with color temperatures ranging from 305 K on the patera floor to 350 K in the island cracks.  The NIMS team determined that assuming a starting temperature of 1475 K, the temperatures correspond to surface ages of about 127 days and 39 days, respectively, suggesting that the patera floor dates back to a prior Loki eruption, while the lava in the island cracks may date from the early stages of a Loki eruption at that started in September 1999.  The uniformity of the surface temperature of the caldera floor along with PPR data taken a few minutes earlier has been used as evidence that the dark patera floor of Loki consists of one large lava lake that overturns episodically.

As Galileo passed Io, it was able to turn its cameras toward Io's dayside.  This allowed NIMS to examine the distribution of sulfur dioxide frost across Io's anti-Jupiter hemisphere as well as search for hotspots and examine the fine scale thermal features at several Ionian volcanoes, including Prometheus and Amirani.  For example, the image at right shows the region surrounding the volcano Prometheus.  In this data from the REGION01 observation, two hotspots are seen at Prometheus, one associated with the vent in the east and the current breakout region to the west.  Based on this and higher resolution observation, as well as co-analysis with SSI data, it has been suggested that lava at Prometheus erupts along a fissure that bounds the eastern end of the flow field.  This lava then travels west via covered lava tubes until they reach a local topographic low where the lavas then erupt again onto the surface in the form of dark breakouts.  Higher resolution data also revealed a third hotspot between the two, from an area of activity in the center of the lava flow.  The dark ring around Prometheus in the 4.2 micron data, in the middle of an SO₂ absorption band, results from plume fallout, not only of the present day plume, but also from the previous eastern location of the plume, as seen by Voyager in 1979.  NIMS also used its first opportunity to image Io at high resolution to search for small and faint hotspots and observations like the one above and below revealed several of these thermal features, including the first hotspots observed at volcanoes like Tien Mu Patera and Steropes Patera.

I know I haven't covered results from Photopolarimeter-Radiometer (PPR), Galileo's mid-infrared mapper, but it is getting rather late and I want to get this post out the door.  PPR further provided coorelations between thermal emission and dark materials.  During I24, PPR observed Loki near the start of a new overturning event at the lava lake there, as evidenced by increased thermal emission near the southwestern end of the patera, where these events typically start before moving around the patera in a counter-clockwise motion.  PPR also observed older flows at Zamama and Pillan, providing estimates for the age of these flows.

I hope you have all enjoyed this look back at Galileo's I24 flyby, which occurred 10 years ago on October 11, 1999.  Galileo would go on to encounter Io five more time as it wound down its mission at Jupiter, helping to revolutionize our knowledge of this exciting satellite.

Water on Dry Worlds

Updated 09/24/2009 12:03 PM MST based on info from this morning's press conference:

The internet is abuzz this evening regarding the possible discovery of wide-spread "water" or hydroxyl molecules of the surface of Earth's moon, a discovery made by spectrometers on three different spacecraft: M³ on Chandrayaan 1, VIMS on Cassini, and Deep Impact.  The papers, if I understand correctly, will be published later today in this week's issue of the journal Science and I have not had a chance to look at them.  There will also be a press conference later today at 2pm EDT (11am MST) discussing these results.

Why do I bring these results up here on this blog?  Well, according the few reports I have been able to find online, like this one here from Universe Today, from Bad Astronomy, and from NASA Watch, this discovery was made by finding a weak absorption band near 3 microns, associated with water and the hydroxyl ion (OH^-), concentrated mostly near the moon's high latitude.  The absorption band found on the Moon is very weak, suggesting a very low concentration of water or OH^- in the moon's soil.  The M³ instrument team suggests a concentration of as much as 770 ppm has been observed on the sunlit side of the Moon, according to the NASA Watch posting.  While the discovery isn't quite Moon-shattering, previously water ice (or hydrogen anyway) had only been observed within cold-traps in permanently-shadowed craters near the poles.

A similar absorption band was found on Io using ground-based spectroscopy (Salama et al. 1990) and Galileo NIMS (Carlson et al. 1997 and Cataldo 1999) observations.  In these measurements, a weak absorption band in Io's near-infrared spectrum at 3.15 microns was observed to be ubiquitous across its surface with a concentration of 4 ppm according to Carlson et al. 1997 and 1000 ppm according to Salama et al. 1990, on the order with what has been observed on the Moon.  This absorption band is associated with the O-H stretch transition.  Such an atomic bond between an oxygen and hydrogen atom would be found in water, hydrated minerals, or the hydroxyl ion (OH^-).  Small concentrations of this band have also been observed.  An absorption band near 3 microns, attributed to water ice crystals or hydrates mixed with sulfur dioxide frosts, was seen to the north and west of Gish Bar Patera by the Galileo NIMS instrument during the October 2001 I32 encounter (Douté et al. 2004).  The low spectral resolution of NIMS at the time (12 spectral measurements spread out between 1 and 5 microns) makes this result a bit tenuous, but if true would indicate that concentrations of possible water ice on top of the low background levels exist on the surface of Io.

So where does the "water" come from on these two, supposedly dry worlds?  For the Moon, two possible mechanisms are likely.  The first would be recent cometary impacts, which would bring their water to the Moon's surface near the site of these impacts.  Concentrations within the ejecta blankets of several small craters on the moon provide further evidence for this hypothesis, but the pattern of the hydroxyl absorption within the ejecta seems to be more consistent with material from the target body rather than material from the impactor.  The widespread distribution of water or hydroxyl ions across the moon's sunlit surface suggests another explanation.  In this scenario, charged particles, in the form of hydrogen ions and transported from the Sun by the solar wind, impact the Moon's unprotected surface (remember that the Moon is outside Earth's magnetic field most of the time).  These hydrogen ions split oxygen atoms from silicate molecules in the Moon's soil, and combine with newly freed oxygen ions to form hydroxyl ions or water.  As the day progresses and the Moon's surface heats up, these new molecules themselves split up, freeing the hydrogen to space and returning the oxygen to the soil.  The process of water formation from the combination of hydrogen from the solar wind and oxygen in the lunar soil kicks back up the surface starts to cool down in the late afternoon and evening.  Alternatively, the water molecules may become excited and be transported to Moon's polar regions, where they are deposited within those aforementioned cold-traps.

For Io, the solar wind can't reach its surface due to Jupiter's strong magnetic field.  So where does its water come from?  Again, oblique cometary impacts could be a source of water for Io.  The two recent cometary impacts on Jupiter in 1994 and again this year would suggest that Io could be hit by water-rich cometary bodies on a regular basis.  This could certainly be the source for the concentration found near Gish Bar Patera.  For the global ubiquitous concentrations of water or hydroxyl ions, another mechanism maybe necessary.  For example, low concentrations of water might be present in Io's magma, like here on Earth.  Water vapor would then be released during volcanic eruptions and water ice would be deposited on the surface, however, no water vapor has ever been observed within Io's plumes.  Another possibility could be that hydrogen ions from Jupiter's magnetic field break off oxygen from sulfur dioxide and silicate compounds on the surface then combine with them to form OH^- or water, akin to the preferred scenario for the Moon.

This discovery of OH molecules on the Moon is certainly interesting, and just goes to show everyone that water is quite common place in the solar system, even in the driest of places.

Graphic above by University of Maryland/F. Merlin/McREL.

Link: Water on the Moon...?  Yep. It's Real. [blogs.discovermagazine.com]

Paper: Spectroscopy of Io Eclipse Reappearances

Last week, a new paper was published in press (the paper has been approved for publication, but hasn't found a slot in the dead-tree version of the journal yet) in the journal Icarus discussing spectroscopic observation of Io as it emerged from the shadow of Jupiter. The paper is titled "Eclipse reappearances of Io: Time-resolved spectroscopy" and was written by Dale Cruikshank, Josh Emery, Katherine Kornei, Giancarlo Bellucci, and Emiliano d'Aversa.

In this new paper, the authors discuss spectroscopic observation of Io acquired using NASA's Infrared Telescope Facility (IRTF) in Hawaii during five eclipse reappearances in April, May, and June 2004. These observations were intended as a follow-up to results from Cassini VIMS observations in Bellucci et al. 2004 taken during that spacecraft's Jupiter flyby during New Year's 2001 that showed a brightening of Io's surface in the near-infrared and a deepening of several strong sulfur dioxide absorption bands following Io's emergence from Jupiter's shadow. This result continues a 40-year-long mystery concerning the interaction between Io's atmosphere and its surface during and after an eclipse by Jupiter.

Unlike lunar eclipses, when the Earth passes between the Sun and our Moon and which happen about once a year, or every 13 orbits of the Moon around the Earth, eclipses of Io by Jupiter occur about once each Ionian day. This is due the large size of Jupiter compared to Earth and the much lower axial tilt of Jupiter and its main satellite system. Each Ionian lunar eclipse lasts about 2 hours and 22 minutes. During this time, the temperature of Io's surface cools due to the sudden lack of sunlight. As Io cools down as the eclipse progresses, atmospheric Sulfur dioxide (SO₂) condenses onto the surface. Check out a post I wrote earlier this year on another paper for more details on this process.

Depending on the amount of SO₂ that condenses onto the surface, the fresh frost should be visible shortly after Io emerges from behind Jupiter's shadow as a brightening of Io's surface compared to its appearance prior to being eclipsed, and it should quickly dim as the frost sublimates from the surface now that the Sun is able to heat it up. In addition, the strong SO₂ absorption bands at 3.56 μm, 3.78 μm, 4.07 μm, and 4.37 μm would be deeper than they were prior to the eclipse and should become shallower during the first 60-90 minutes after each eclipse and particularly in the first 15 minutes as the fresh, fine-grained SO₂ frost sublimates back into the atmosphere. Results from multiple studies using ground-based and spacecraft observations over the last 40 years, since Binder and Cruikshank 1964 revealed a brightening of Io of 10 percent following an eclipse by Jupiter, have been inconsistent with some showing such a brightening, and others showing none. As explained in this new paper, Nelson et al. 1993 found that post-eclipse brightenings are likely to be rare as a fresh SO₂ frost layer several millimeters thick would be required to explaining the magnitude of the brightenings that were seen, and it would take longer than 15 minutes to sublimate that layer away. In addition, modeling of Io's atmosphere during an Io eclipse by Moore et al. 2009 suggests that SO₂ condensation onto the surface would be curtailed to some degree by atmospheric heating by the Io plasma torus and by non-condensable species like Sulfur monoxide preventing SO₂ in Io's upper atmosphere from condensing.

Cruikshank et al. examined their observations taken at IRTF and found no evidence of changes in Io's albedo or the area of three SO₂ absorption bands at 3.56 μm, 3.78 μm, and 4.07 μm. What changes were observed were either the result of the rotation of Io during the 60-90 minutes of each observation run, were found in one absorption band but not in the other three, or were the result of observation noise or the thick airmass of Earth's atmosphere. Therefore, the authors were not able to confirm the VIMS results published by Bellucci et al. 2004. The authors suggested that the two conflicting results could be due to the background frost coverage in the area observed by the two groups of researchers. VIMS observed Io's trailing hemisphere which is thought to have the least abundant SO₂ frost coverage while the Cruikshank et al. group observed the sub-Jupiter hemisphere, SO₂ abundance is higher. The lower SO2 abundance would have made condensed SO₂, even if in a very thin layer, more noticeable compared to the sub-Jupiter hemisphere.

In other results, Cruikshank et al. observed additional SO₂ absorption bands between 2.11 and 2.24 μm, including a faint one at 2.198 μm that the authors thought they were first to see in Io's near-infrared spectrum. Another weak absorption band at 2.1255 μm was mapped by Laver and de Pater and the results of that study were published earlier this year and discussed on this blog. Cruikshank et al. also observed Io's emission spectrum while the satellite was still in the shadow of Jupiter during the observation run on June 22, 2004. They did not find convincing evidence for condensed SO₂ in Io's atmosphere, which would be expected in Io's volcanic plumes. This negative result could be the result of the temporal variability of Io's plumes.

Finally, the table of contents for the October 2009 issue of Icarus has been published online. No Io-related papers in this issue, but there are a series of papers covering Jupiter's Oval BA, also known as Red Spot Jr.

Link: Eclipse reappearances of Io: Time-resolved spectroscopy (1.9-4.2 μm) [dx.doi.org]