On the global distribution of neutral gases in Titan's upper atmosphere and its effect on the thermal structure

Abstract

Using a time‐dependent general circulation model of Titan's thermosphere, we calculate the global distribution of neutral gases by winds and diffusion. Our calculations suggest that solar driven dynamics effectively redistribute constituents, causing considerable diurnal and seasonal changes in gas abundances. Subsidence causes an accumulation of lighter gases on the nightside, with nighttime CH4 mole fractions at equinox near 1400 km reaching up to 50%. The reverse happens on the dayside, where lighter gases are depleted, giving minimum CH4 mole fractions near 1400 km of around 12%. The vertical transport time scales are around 5–10% of a Titan day, so these extrema in gas abundances are shifted with respect to local noon and midnight by up to 4 hours Local Solar Time (LST). The strong horizontal variations in gas abundances, combined with the local time shifts of their extrema, have an important impact on the thermal structure and lead to a shift of the nighttime minimum from local midnight towards early morning hours (0330 LST). This coupling between gas distribution and thermal structure on the nightside occurs via dynamical processes, primarily through changes in adiabatic heating. The redistribution of gases effectively controls, through changes in mean molecular weight, the pressure gradients, which in turn control the horizontal and vertical winds, and thereby adiabatic heating and cooling. On the dayside, changes in solar EUV absorption due to the redistributed gases occur but are comparatively small. Although it is possible with our calculations to identify important processes, Voyager and ground based observations of Titan are currently not sufficient to constrain the dynamics of Titan's upper atmosphere, but comparisons with forthcoming Cassini observations are highly anticipated.