Three-Dimensional Modeling of the Tropospheric Methane Cycle on Titan

Abstract

The tropospheric methane cycle on Titan including advection, diffusion, condensation, precipitation, evaporation, and surface source is simulated by a three-dimensional general circulation model. The model takes into account the different nucleation of methane in Titan's troposphere, and assumes condensation whenever the saturation ratio (relative humidity) exceeds a critical value of either 100 or 150%. The simulation with the latter critical saturation is shown to be more compatible with the Voyager data. The meridional circulation accounts for a seasonal methane transport from the winter to summer hemisphere in the mid-troposphere, causing a higher supersaturation in late summer and early autumn. Condensation preferentially occurs in this season at 15 km altitude at low latitudes. The occasional character may be explained by the high critical saturation ratio, which cannot be often exceeded. Falling rain evaporates during the descent, so no rain arrives at the surface. The hydrological cycle of methane on Titan takes place mostly within the atmosphere. The latitudinal methane distribution near the surface cannot be reproduced by transport, evaporation, condensation, and combinations of them, indicating some undetermined surface processes affecting the methane distribution. The zonal and meridional circulation is sensitive to the methane distribution.