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The Planetary Society Blog

By Emily Lakdawalla

LPSC, Thursday: Icy satellites

Mar. 15, 2007 | 17:33 PDT | Mar. 16 00:33 UTC

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There was a short session at the Lunar and Planetary Science Conference this afternoon devoted to the icy worlds of the solar system not presently considered cool enough to be seen with Titan or Enceladus. That's only half a joke -- there were three previous complete sessions during the week with talks on those two moons of Saturn, whereas this six-talk half-session contained all but one of the oral presentations there were on any other icy body, including Europa and Pluto! (The talk wasn't even about Pluto -- it was about Charon.) There were, of course, posters to be seen on these other bodies, but I found it really striking that there were only two talks this year on Europa. Anyway, Anne Verbiscer has come through for me with notes on the talks from this session. Thanks, Anne! --ESL

by Anne Verbiscer

Brad Dalton spoke about near-infrared spectral modeling of Saturn's icy satellites from both ground-based and Cassini VIMS spectra. He first presented his results on Tethys, where he modeled the surface entirely with large grains (on the order of a few tenths of a millimeter in diameter) of water ice. The Cassini VIMS spectrum of Phoebe shows evidence of iron, cyanide, carbon dioxide, and water ice, and there are cyanide features in the less icy regions of Dione. He showed a beautiful map of Dione with a color-coded overlay showing the regions where the spectral signatures of water ice, carbon dioxide, and cyanide each dominate the spectrum. Water ice is more prevalent along the fractures (what was formerly known as "wispy terrain" from Voyager days), with small amounts of carbon dioxide in "patches". Cyanide is more prevalent in the areas that are not fractured. In map of Hyperion color-coded the same way, he interprets the dark areas as lag deposits, not impact craters. Carbon dioxide is distributed across the surface in the "country" rock, whereas cyanide is more localized.

Amanda Hendrix presented Cassini UVIS observations of Iapetus and Phoebe in the far ultraviolet (FUV), from 110 to 190 nanometers. In the FUV as in visible light, the polar cap of Iapetus is bright and the leading hemisphere is dark. In the FUV, water ice has a characteristic absorption at 165 nanometers; she examined the strength of this feature as a function of latitude. From 0-30 degrees north, the feature is the weakest; it's a little stronger between 30 and 40 degrees; and is strongest between 40 and 50 degrees. Interestingly, when she compares the strength of this feature on Iapetus with that on Phoebe, the bright terrain on Iapetus is a closer match to Phoebe than the dark terrain, leading her to surmise that it is probably not "pure Phoebe" material that contaminates the dark side of Iapetus. In fact, Iapetus' light terrain matches Hyperion better than it does Phoebe. On the other hand, Iapetus' dark terrain doesn't match Hyperion or Phoebe; both Phoebe and Hyperion are too water-rich to match the dark terrain on Iapetus. She presented spectral models in which Hyperion is composed of 55% water ice and 45% Triton "tholin", and Iapetus' dark terrain as 5% water ice and 95% Triton "tholin". Their detection of water ice at the lowest, warmest latitudes of the dark apex region on Iapetus suggests an ongoing or recent implantation/coating process, which is consistent with the lack of fresh craters in the thin layer of dark material. Hyperion may be a better candidate for the source of Iapetus' dark material, primarily because of the color (red) similarities. They cannot rule out an endogenic source of dark material or a water-rich layer under the dark material as a source of the water. There will be a good Iapetus flyby in September which will provide coverage of the trailing (bright) hemisphere.

Roland Wagner presented a talk on the global geology of Rhea (from Cassini ISS) which agrees with much of the results already presented at this meeting by Moore and Schenk. Rhea shows no clear evidence for cryovolcanic resurfacing as seen on Dione (see abstracts by Moore, etc.). His digital elevation maps (DEMs) reveal degraded impact structures as well as extensional, compressional, and shear tectonic features. The observed tectonic structures may have been produced by phase changes, from Ice I to Ice II. Rhea has, of course, one ray crater on its leading hemisphere, which is difficult to date because of the order of magnitude difference between crater frequencies in areas surrounding the crater. (Ray craters are not common on Saturnian satellites.) Rhea shows similar tectonic features as those seen on Dione, but its endogenic evolution ceased earlier in its history.

Wes Patterson discussed "non-transform structural discontinuities (NSDs)" on Europa, primarily Belus Linea. The terrestrial analogs to NSDs are overlapping spreading centers which are almost exclusively observed in areas of thin oceanic lithosphere. On Europa, we see obvious segmentation at Belus Linea which has a bright interior flanked by dark material on either side. (These are the old Voyager "triple bands" which Galileo showed were not really bands, but instead complex ridges.) The dark material flanking these complex ridges may represent erupted volatiles brought to the surface via water-filled cracks initiated at the base of the ice shell. (This mechanism has previously been proposed (and published) by Crawford and Stevenson.)

Kevin Zahnle presented "Io Attacks!" (which I'm really interested in the analogies to the same idea in the Saturnian system that I just published in Science last month with Enceladus' "cosmic graffiti art"... but that's another story)... Basically the idea here is that ejecta from Io are launched into orbit around Jupiter and about 8% of this material actually hits Europa, just about everywhere except the polar regions and on the trailing hemisphere. Impact velocities are 3.5 kilometers per second. His "off the shelf" theory provides a good match to the observed size-number distribution of small craters on Europa. He refers to the resulting craters as "sesquenaries"... neither primary nor secondary... So, Ionian basalts are an important source of vitamins and minerals on Europa!

Finally, Steve Desch presented a talk on cryovolcanism on Charon and other KBOs (a subject relevant to my own poster on observations and models of Charon near-infrared spectra incorporating ammonia hydrate). Steve interprets the 1.65 micron band of crystalline water ice as the hallmark of cryovolcanic activity. [I'll note as an aside here that this spectral feature is prominently seen in just about all bright, icy Saturnian satellite spectra, such as those of Rhea, and I just wrote above about how there is no evidence for cryovolcanism on Rhea, so there's a problem here...] Crystalline water ice should amorphize on relatively small timescales because of exposure to cosmic rays, etc. The spectral feature of crystalline water ice is seen on KBOs larger than 500 kilometers in diameter, and not in the spectra of smaller KBOs. He presented thermal evolution models of a differentiated Charon.