Methane thermodynamics in nanoporous ice: A new methane reservior on Titan

Abstract

A porous, icy regolith has been proposed to house a methane/ethane/ nitrogen liquid reservoir on Titan. The thermodynamics of such a mesoporous reservoir with a pore diameter fixed at 2 mm are equivalent to the previously proposed surface ocean. This work shows that if such a porous hydrocarbon/nitrogen reservoir contains a significant volume within pores of nanometer dimension, then the modification of methane thermodynamics through the Kelvin effect significantly changes the reservoir-atmosphere interaction on Titan. We measure the capillary uptake of pure methane liquid into well-characterized ice grown at temperatures and partial pressures simulating Titan at present. From these results, a model of a nanoporous reservoir housing the methane/ ethane/nitrogen solution is constructed. Modifying a previously published analytical climate model for Titan to incorporate such a reservoir, we report calculations of the coupled reservoir-atmosphere system. This modification leads to a very sensitive dependence of the state of the atmosphere on the total reservoir volume. In particular, assuming a nanoporous volume distribution reported here, calculations show that such a reservoir must comprise at least 1.5% of a 10 km deep ice regolith in order to yield a climate consistent with that currently observed on Titan. In contrast to the mesoporous model, it reduces the sensitivity of the system to temperature, mitigating the runaway greenhouse effect observed in the previous model. Furthermore, the model indicates that in a past epoch a surface ocean subsided into the reservoir as the total surface liquid volume reduced with time. Copyright 2007 by the American Geophysical Union.