Clathrate and Ammonia Hydrates at High Pressure: Application to the Origin of Methane on Titan

Abstract

The origin of methane at the present surface of Titan is modeled in light of new high-pressure phase diagrams of ammonia-water compounds and clathrate hydrate. Using recently published experimental data on the ammonia-water system at kilobar pressures, temperature-composition slices of the phase diagram are constructed at a series of pressures up to 12 kbar. A new phase of ammonia dihydrate is proposed and incorporated in the diagrams, to allow consistency with low-pressure data. These results, along with the high-pressure phase diagram of methane clathrate hydrate recently caculated by J. I. Lunine and D. J. Stevenson (1985a, Astrophys. J. Suppl.58, 493–531) are applied to a model for the origin of the methane presently on the surface of Titan. Using simple bounds on the accretional temperatures and postaccretional state of an ammonia-rich Titan, we show that an unstable interior configuration is likely immediately after accretion, in which a rock layer is positioned above a lower-density rock-ice core. When core overturns begins the methane in the core, which is released from the clathrate structure by virtue of the high pressures, migrates upward. A model for the cooling and freezing of an ammonia-water ocean in the upper mantle of Titan, based on the phase diagram, is applied and it is concluded that insufficient liquid water exists to retrap all of the upwelling methane as clathrate. However, alternative interpretations of the phase diagram permit an ocean thick enough to entrap the methane. For the bulk of the range of plausible accretion models, enough methane is available from the interior to account for the present-day surface hydrocarbon abundance; however, the amount of nitrogen extruded in this model may be much smaller.

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