Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..958t&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Saturn's largest moon Titan has long been considered a natural laboratory of prebiotic chemistry given the presence of a dense nitrogen-methane atmosphere and the likelihood of liquid hydrocarbons (e.g. [1]). Several putative liquid hydrocarbon lakes have been recently detected in the polar region of Titan by the Cassini radar [2]. Such lakes may contain organic sediments deposited from the atmosphere and promote further prebiotic chemistry driven by cosmic rays, by which more complex molecules such as nitrogenbearing organic polymer or azides could be produced. Even the possibility of methanogenic life consuming acetylene and hydrogen [3, 4] or silane-based life in hydrocarbon lakes [5] has been speculated. Any consideration of the astrobiological potential of Titan's lakes requires knowledge of the environmental setting of the lakes, as is common in studies of the origin of life on Earth. `Environmental setting' comprises, among others, the temporal variability in composition and temperature or the fate of lakes as such. I investigate the physical properties of the lake and their temporal evolution under present Titan's climatic setting by means of a 1-dimensional lake thermal stratification model [6]. Basic quantities predicted by the model are the lake temperature, density, composition, lake level and thickness of ice, if there is any. The prescribed initial composition of the lake is either pure methane or a methane-ethane-nitrogen mixture and two lake depths have been assumed. Modelling shows that the evolution of the lake primarily depends on the chemical composition of the lake and atmosphere and the balance between inflow and outflow. A pure methane lake rapidly freezes and eventually dries up by sublimation. A mixed lake containing a substantial amount of ethane can evaporate a large amount of methane if the ethane humidity in the atmosphere is not in equilibrium with the ethane concentration in the lake. This will change the lake composition and meanwhile it causes vigorous mixing of the lake down to the bottom. The lake presumably does not freeze at any time. Pure methane ponds that may occasionally form when heavy methane hailstones reach the surface have no chance of surviving since they evaporate, freeze up and eventually dry up. On the other hand, lakes filled with a mixture of methane, ethane and nitrogen are more stable and freezing or drying up can be prevented in most cases. When the ethane humidity in the atmosphere has adjusted to the lake composition, methane evaporation ceases and the lake then undergoes a repeatable seasonal temperature variation and overturning in autumn. Shallow lakes get mixed down to the bottom (holomictic), while deep lakes are merometic, i.e. they have bottom liquid layers which do not intermix. The summer thermal stratification near the lake surface can be destabilized by bottom heating as a result of an enhanced geothermal heat flux, e.g. in the vicinity of cryovolcanoes. Most likely the composition of the lake and atmosphere steadily adjust to each other by a small amount of evaporation, but the lake-atmosphere system can be repeatedly brought out of equilibrium by irregular precipitation. The astrobiological potential appears desolate in a pure methane lake that may temporarily develop as a result of heavy methane hail. There would simply be no time for prebiotic chemistry to proceed in the liquid because of the rapid freezing. Shallow (but not too shallow to allow desiccation) lakes are generally better mixed and a more vigorous exchange of dissolved atmospheric gases and suspension of acetylene sediment on the lake bottom can be expected. Deep lakes may harbour stagnant bottom layers in which neither the temperature and composition changes with time. Also the acetylene sediment on the lake bottom remains undisturbed. References [1] Raulin, F., Dubouloz, N., Frère, C. (1989) Adv. Space Sci., 9 (6), 35-47. [2] Stofan, E. R., et al. (2007) Nature, 445, 61-64. [3] McKay, C. P., and Smith, H. D. (2005) Icarus, 178, 274-276. [4] Schulze-Makuch, D., and Grinspoon, D. H. (2005) Astrobiology, 5, 560-567. [5] Schulze-Makuch, D., and Irwin, L. N. (2004) Life in the Universe: Expectations and Constraints. Springer, Berlin. [6] Tokano, T. (2008) Provisionally accepted for publication in Astrobiology.
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