Latitudinal Distribution of Ethane Precipitation on Titan Modulated by Topography and Orbital Forcing and Its Implication for Titan’s Surface Evolution

Abstract

Abstract A general circulation model with geography constrained by Cassini is used to predict how ethane precipitation in Titan’s lower stratosphere varies with latitude, season, and orbital forcing over the past 100 kyr. Ethane precipitation is generally more prevalent near the winter pole, where stratospheric ethane is transported downward toward the cold trap, and this general pattern is relatively insensitive to orbital parameter variations and geography. However, eccentricity-driven seasonal temperature variations modulate the seasonal asymmetry of ethane precipitation to some extent. The annual ethane precipitation does not monotonically increase from equator to pole but maximizes at selected sites, preferentially over empty deep basins such as Hagal Planitia. Local enhancement of ethane precipitation is caused by katabatic winds from plateau to basin and an associated regional-scale thermally direct circulation over the slope, which induces strong adiabatic cooling near the tropopause. The observed putative ethane clouds off the poles are evidence that ethane condensation is affected by topography. Preferential ethane precipitation over basins may increase the irregularity of Titan’s shape by isostatic crustal subsidence after substitution of enclathrated methane by percolated ethane.