Clathrate and ammonia hydrates at high pressure - application to the origin of methane on Titan

New high-pressure phase diagrams of ammonia-water compounds and clathrate hydrate are used to model the origin of the methane presently on the surface of Titan, and a new phase of ammonia dihydrate is included in the diagrams to achieve consistency with low-pressure data. Results suggest that an unstable interior configuration results after accretion, with a rock layer positioned above a lower-density rock-ice core, and that the methane migrates upwards when core overturn begins. It is found that insufficient liquid water exists in the upper mantle of Titan to retrap all of the upswelling methane as clathrate, although alternative interpretations of the phase diagram are discussed which permit an ocean thick enough to trap the methane. 43 references