Ocean-Atmosphere Stability on Titan

R. D. Lorenz, J. I. Lunine (LPL, U. Arizona), C. P. McKay (NASA Ames)

We explore the sensitivity of atmospheric pressure and composition on Titan to temperature and the surface inventory of methane, nitrogen and ethane, using the thermodynamic equilibrium methods in McKay et al (1993). The results are compared with the surface temperature to be expected from the radiative effects (greenhouse) of the resultant gas mixture using a radiative-convective model and empirical fits thereto. Where the pressure/temperature curve for the radiative model intersects that of the ocean, Titan has an equilbrium state. McKay et al. (1993) found a stable equilibrium for present conditions: exploring a wider range of volatile inventory, and insolation (both of which have varied over Titan's history), we find instances where the curves cross twice, indicating stable and unstable equilibria, and cases where equilibrium is attained at end points - where the atmosphere has either rained or frozen out onto the surface giving pressures much lower than present, or where the ocean has boiled dry.