PHOTOCHEMICALLY DRIVEN COLLAPSE OF TITANS ATMOSPHERE

Saturn's giant moon Titan has a thick (1.5 bar) nitrogen atmosphere, w hich has a temperature structure that is controlled by the absorption of solar and thermal radiation by methane, hydrogen, and organic aeros ols into which methane is irreversibly converted by photolysis. Previo us studies of Titan's climate evolution have been done with the assump tion that the methane abundance was maintained against photolytic depl etion throughout Titan's history, either by continuous supply from the interior or by buffering by a surface or near surface reservoir. Radi ative-convective and radiative-saturated equilibrium models of Titan's atmosphere show that methane depletion may have allowed Titan's atmos phere to cool so that nitrogen, its main constituent, condenses onto t he surface, collapsing Titan into a Triton-like frozen state with a th in atmosphere.