After only three years of close observation from the Cassini-Huygens mission, Titan appears more and more as one of
the key planetary bodies in the solar system for astrobiological studies. Titans does not look any more like a frozen
primitive Earth, but like an evolving planet, geologically active, with cryovolcanism, eolian erosion, clouds and
precipitations, and a methane cycle very similar to the water cycle on Earth. The new data also show a very complex
organic chemistry in the highest atmospheric zones of the satellite, with the formation in the ionosphere of high
molecular weight organics feeding the lower zones, down to the surface. In spite of the low surface temperature, these
organics are probably evolving once in contact with water ice and form organic molecules of biological interest. This
may explain the reflectance spectrum of Titan' surface observed by the DIRS instrument on Huygens. Thus, contrary to
what was expected, the organic chemistry on Titan seems mainly concentrated in the ionosphere, in the aerosols and on
the surface. These astrobiological aspects of Titan are presented and discussed on the basis of the already available
Cassini-Huygens data, as well as the needed post Cassini exploration.
Since the first Voyager data, Titan, the largest satellite of Saturn and only satellite in the solar system having a dense atmosphere, became one of the key planetary bodies for astrobiological studies, due to: i) its many analogies with planet Earth, in spite of much lower temperatures, ii) the already well observed presence of an active organic chemistry, involving several of the key compounds of prebiotic chemistry, in the gas phase but also assumed to occur in the solid phase through the haze particles. And the potential development of a prebiotic chemistry in liquid water, with a possible water ocean in its internal structure, and the possible episodic formation of small liquid water bodies for short but not negligible time duration at the surface (from the melting of surface water ice by impact), iii) the resulting possibility that life may have emerged on or in Titan and may have been able to adapt and to persist. These aspects are examined with some of the associated questions on the basis of the already available Cassini-Huygens data.
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