The composition of Titans atmosphere : a meteorological perspectivefn1

One of the most important and intriguing issues concerning Titans atmosphere is the methane cycle. Although the broad outline of the breakup of methane by photolytic and catalytic dissociation to produce a suite of more complex hydrocarbons and nitriles (additionally requiring the dissociation of N2) is understood, several fundamental questions exist, and many details remain obscure. The spatial distribution of stratospheric hydrocarbons and nitriles has been inferred from Voyager thermal infrared spectra, but only at northern spring equinox. The photochemical, radiative, and dynamical processes underlying this distribution are not well understood. Additionally, methane itself is important both radiatively and (possibly) dynamically in the troposphere. Voyager infrared and radio-occultation measurements place some constraints on the vertical and horizontal distribution of methane, and suggest that it may be supersaturated in the upper troposphere. The presence of convective activity associated with methane condensation is unknown, but recent work indicates that a meager amount of convection (compared to Earth) could be sufficient to transport heat and methane vertically and account in part for the vertical thermal structure of Titans troposphere and provide a major source of methane for the stratosphere. The additional measurements provided by Cassini over a four-year period will greatly improve our understanding of these issues. Emphasis in this paper is on the combination of radio occultation measurements and thermal infrared spectra, with their different sensitivities to atmospheric constituents.