Coupled Atmosphere-Ocean Models of Titan's Past

Abstract

We have developed a coupled atmosphere and ocean model of Titan's surface. The atmospheric model is a 1-D spectrally-resolved radiative-convective model. The ocean thermodynamics are based upon solution theory. The ocean, initially composed of CH 4, becomes progressively enriched in ethane over time. The partial pressures of N 2 and CH 4 in the atmosphere are dependent on the ocean temperature and composition. We find that the resulting system is stable against a runaway greenhouse. Accounting for the decreased solar luminosity, we find that Titan's surface temperature was about 20 K colder 4 Gyr ago. Without an ocean, but only small CH 4 lakes, the temperature change is 12 K. In both cases we find that the surface of Titan may have been ice covered about 3 Gyr ago. In the lakes case condensation of N 2 provides the ice, whereas in the ocean case the ocean freezes. The dominant factor influencing the evolution of Titan's surface temperature is the change in the solar constant—amplified, if an ocean is present, by the temperature dependence of the solubility of N 2. Accretional heating can dramatically alter the surface temperature; a surface thermal flux of 500 erg cm -2 see -1, representative of small levels of accretional heating, results in a ∼20 K change in surface temperatures.