Workshop on the Habitability of Icy Worlds (2014) 4065.pdf

THE OF TITAN. J.I. Lunine, Center for Radiophysics and Space Research, Cornell Universi- ty, 402 Space Sciences Bldg., Ithaca NY 14853, [email protected].

Introduction: Titan is the only solar system world and energy. Once refrozen, these sites may preserve outside of the which has an active volatile cycle the results of such prebiotic experiments for study. involving stable surface liquids. The primary compo- Most interesting is the third environment, the polar nent existing in gaseous, liquid and possibly solid form lakes and seas, which are almost certainly mostly is methane; secondarily ethane (a product of methane ethane and methane. The extent to which nonaqueous photolysis high in the ) participates as well. liquids may be the solvent for an exotic biochemistry The background atmospheric gas is molecular nitro- has been extensively treated elsewhere [4], but Titan gen, with a surface temperature of 94 K(equatorial) provides a test of such ideas. Is life, of whatever basic and pressure of nearly 1.5 bars. biochemical variety, a natural outcome of the existence The equivalent of the terrestrial hydrologic cycle of organic molecules, abundant free energy, and sur- on Earth is the methane cycle on Titan [1]. The drop- faces with catalysts? Or is it a phenomenon peculiar to ping temperature with allows methane to con- water? The detection of complex polymers associated dense out as at of 8 km (equatorial) with Titan’s surface lakes and seas, as evidence of and above; precipitation of methane rain, with an ad- chemical self-organization on the way to (or across the mixture of nitrogen and possibly ethane, occurs in par- threshold of) a nonaqueous form of life, would answer ticular latitudes according to season, with some evi- this question. dently intense storms scouring the surface [2]. Unlike Analysis of multiple Cassini data sets [5] suggests Earth’s hydrologic cycle, in which very little water an unexpected and puzzling sink of hydrogen at the escapes to the photolytically active and surface of Titan. While a biological explanation must mesosphere, the temperature of Titan’s is not be seriously contemplated until abiotic physical- such that large amounts of methane end up in the strat- chemical hypotheses are evaluated and (possibly) falsi- osphere and above, where conversion (with nitrogen fied, this problem illustrates the seductively complex and a little oxygen) to hydrocarbons, nitriles, and car- nature of Titan’s surface-atmosphere system. bon (mon+di)oxide occurs. Hydrogen is lost to space Future exploration: Operation of the Cassini Sat- and the products rain out on the surface as solids and urn Orbiter until solstice in 2017 is essential to under- (for ethane, propane and butene) liquids. Ample evi- standing how the northern seas interact with the cli- dence for both exists in the equatorial dunes made of mate during the time of maximum insolation, as well organic particles, and the polar lakes and seas. as to provide further opportunities to map the surface The density of Titan, along with the near-identity and probe the composition of seas, sands and skies. of bulk properties with Jupiter’s Ganymede and Callis- Beyond Cassini, the next mission arguably ought to be to, strongly imply an interior made of nearly equal a lander to explore in situ the surface organic chemis- parts water and silicate (presumably with metals). try to understand the extent to which it has evolved Some of the silicate may be hydrated, while strong beyond the products of atmospheric charged particle evidence for a liquid water layer exists primarily in and ultraviolet chemistry. Alternatively, a balloon or gravity data of Titan [3]. Given this, the extent of the specially-equipped orbiter would be capable of map- ice layers in Titan may be more limited than previously ping terrains whose geological provenance is poorly thought from Voyager data. Indeed, it is possible for understood thanks to the limited spatial and spectral the water ocean to be in contact with underlying rock. resolution of the Cassini remote sensing investigations. Environments of Astrobiological Interest: Three References: environments on Titan are likely to be of interesting in [1] Lunine J. I. and Atreya S.K. (2008) Nature Ge- either hosting life or preserving evidence of chemical oscience, 1, 159-164. [2] Turtle E. P. et al. (2011) Sci- processes leading to life. Deepest is the liquid water ence, 331, 1414-1417. [3] Iess L. et al. (2012) Science, ocean, which likely has large amounts of organics and 457-459. [4] Benner S.A. et al. (2004) Curr. Opinion possibly dissolved salts from leaching associated with Chem. Biol., 8, 672-689. [5] Strobel D.F. (2010) Ica- early hydration. In the crust itself, sites of impacts or rus, 208, 878-886. cryovolcanism may remain melted for centuries of millennia, protected by a cap of insulating ice. Within these sites, organic molecules reacting with water may undergo self-organization to an extent limited by time