Biology
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p22a..04l&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P22A-04
Biology
[0406] Biogeosciences / Astrobiology And Extraterrestrial Materials, [5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6281] Planetary Sciences: Solar System Objects / Titan
Scientific paper
The photochemical conversion of methane into heavier organics which would cover Titan’s surface has been a principal motif of Titan science for the last 4 decades. Broadly, this picture has held up against Cassini observations, but organics on Titan turn out to have some surprising characteristics. First, the surface deposits of organics are segregated into at least two distinct major reservoirs - equatorial dune sands and polar seas. Second, the rich array of compounds detected as ions and molecules even 1000km above Titan’s surface has proven much more complex than expected, including two-ring anthracene and compounds with m/z>1000. Radar and near-IR mapping shows that Titan’s vast dunefields, covering >10% of Titan’s surface, contain ~0.3 million km^3 of material. This material is optically dark and has a low dielectric constant, consistent with organic particulates. Furthermore, the dunes are associated with a near-IR spectral signature attributed to aromatic compounds such as benzene, which has been sampled in surprising abundance in Titan’s upper atmosphere. The polar seas and lakes of ethane (and presumably at least some methane) may have a rather lower total volume than the dune sands, and indeed may contain little more, if any, methane than the atmosphere itself. The striking preponderance of liquid deposits in the north, notably the 500- and 1000-km Ligeia and Kraken, contrasts with the apparently shallow and shrinking Ontario Lacus in the south, and perhaps attests to volatile migration on astronomical (Croll-Milankovich) timescales as well as seasonal methane transport. Against this appealing picture, many questions remain. What is the detailed composition of the seas, and can chemistry in a nonpolar solvent yield compounds of astrobiological interest ? Are there ‘groundwater’ reservoirs of methane seething beneath the surface, perhaps venting to form otherwise improbable equatorial clouds? And what role, if any, do clathrates play today? What are the inscrutably amorphous midlatitude terrains made of ? Is Titan’s bulk crust made of water ice? Only a few places show evidence of water ice bedrock (notably fluvial debris, and the fresh-looking Sinlap ejecta blanket). This may relate to another puzzle - the apparently karstic depressions, many of which are filled with hydrocarbon lakes, found in the polar regions. Low solubility makes etching of hundred-meter deep depressions in ice rather improbable. Perhaps the crust comprises shock-processed organic material from early in Titan’s history, as well as photolysis/radiolysis products forming today. Are the flow-like features detected in a number of places such as Hotei, ‘classical’ ammonia-water cryolavas as claimed, merely geomorphological delusions, or are they some reprocessed organic material heated or squeezed in the interior ? A terrestrial analog might be the salt glaciers of Iran, or in sci-fi the ‘waxworms’ of Titan in Arthur C. Clarke’s ‘Imperial Earth’. Classic cryovolcanism (and impact melt) is of course of interest for the known prebiotic synthesis pathways via hydrolysis of tholins, but some recent work suggests that at least trace prebiotic synthesis may occur in the gas phase via oxygen-bearing molecules sprinkled into the upper atmosphere by ablation of icy meteoroids and O+ ions from Enceladus. In any case, Titan presents us with an abundant and rich inventory of organics.
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