Organic Chemistry and Exobiology on Titan

Exobiology is not only the study of the origin, distribution and evolution of life in the universe, but also of structures — including at the molecular level, and processes — including organic chemical transformations — related to life. In that respect, with its dense nitrogen atmosphere, which includes a noticeable fraction of methane, and the many organic compounds which are present in the gas and aerosols phases, Titan appears to be a planetary object of prime interest for exobiology in the Solar system, allowing the study of chemical organic evolution in a planetary environment over a long time scale. We describe here some aspects of this extraterrestrial organic chemistry which involves many physical and chemical couplings in the different parts of what can be called ‘Titan's geofluid’ (gas phase, aerosol phases and surface solid and maybe liquid phases). The three complementary approaches which can be followed to study such chemistry of exobiological interest are considered. Those are experimental simulations in the laboratory, chemical and photochemical modeling and of course observation, using both remote sensing and in situ measurements, which is an essential approach. The Cassini-Huygens mission, that offers a unique opportunity to study in detail the many aspects of Titan's organic chemistry, is discussed and the many expected exobiological returns from the different instruments of the Cassini orbiter and the Huygens probe are considered. This revised version was published online in August 2006 with corrections to the Cover Date.