Paleoclimate Evolution on Titan After Episodic Massive Methane Outgassing Simulated by a Global Climate Model

Abstract

Titan's paleoclimate after the onset of the putative last major methane outgassing event 700 Myr ago is simulated by a global climate model. If the atmosphere was methane-depleted prior to outgassing, outgassed methane initially causes warming due to increased greenhouse effect. Further outgassing leads to methane snowfall, which in turn cools the troposphere and surface by an ice-albedo feedback and thereby initiates a lengthy ice age. Formation of ice sheets begins in the polar region, but with increasing methane inventory the entire globe is eventually covered by surface methane frost as thick as 100 m, with local accumulation on elevated terrains. Among various time-dependent input parameters the methane inventory by far exerts the greatest control over the climate evolution. As Titan's climate transitions from a dry state via a partially ice-covered state to a globally ice-covered state, the circulation and precipitation pattern change profoundly and the tropospheric temperature further decreases. Globally ice-covered snowball Titan is characterized by weak meridional circulation, weak seasonality and widespread snowfall. Frost ablation begins after the end of outgassing due to photochemical destruction of atmospheric methane. It is conceivable that Titan's polar seas resulted from melting of the polar caps within the past 10 Myr and subsequent drainage to the polar basins. Surface methane frost could only melt when the frost retreated to the polar region, which led to global warming by lowering of the surface albedo at low latitudes and increased greenhouse effect.