Composition of Titan's Atmosphere
Résumé
As a result of the 13-year exploration of the Saturn system by the Cassini-Huygens mission (2004–17), there has been a revolution in our understanding of Titan, Saturn's largest satellite. Titan's atmosphere shows the most complex chemistry in the solar system. A chain of chemical reactions is being initiated in the upper atmosphere by ionization and dissociation of the major species, nitrogen and methane, primarily by solar UV photons. The produced photochemical molecules and aerosols have a strong impact on the radiative budget of Titan's atmosphere, and hence on its temperature profile. Furthermore, transport by global dynamics, which reverses each half Titan year, greatly affects the distribution of these compounds. These complex couplings between chemistry, radiation, and dynamics make Titan's atmosphere an ideal laboratory to understand physical and chemical processes at play in atmospheres in general, particularly those showing the presence of photochemical haze such as Pluto or the increasing number of detected exoplanets showing hazy atmospheres. Finally, the study of Titan's chemistry has astrobiological implications as it naturally produces complex organic nitrogen species that settle to the surface to produce an organic soil. This material is postulated to be exposed to aqueous melt pools produced through meteoritic impact or cryovolcanism, or even to be transported to an underground ocean of liquid water. Therefore, we may entertain the idea that, in certain regions at the (sub-)surface of Titan, the results of chemistry leading to life as we know it may be present.
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