Effect of ammonia on the stability of clathrate hydrates: experimental study and implications for Titan

The likely presence of clathrate hydrates on Titan has long been inferred from cosmochemical, thermal, and thermodynamic models. As gas-laden icy structures (up to 15 mol%), they may be internal reservoirs of methane and other atmospheric gasses, and their dissociation during cryovolcanic activity could be a main contributor to the replenishment of Titan's atmospheric methane. However, the controversial nature of the few cryovolcanic features tentatively detected on Titan by the Cassini spacecraft questions our current understanding of the modalities of outgassing processes. The paucity of data on the stability of clathrate hydrates in presence of ammonia (likely source of Titan's atmospheric nitrogen) hinders our capability to address outgassing processes. We are conducting an experimental study in the ternary systems H₂O-CH₄-NH₃ and H₂O-THF-NH₃. Phase diagrams of the former are constructed under pressures up to 100 bars using a high-pressure cryogenic calorimeter, while the latter is investigated at atmospheric pressure using a liquid nitrogen cooled cryostage coupled to a microscope and a Raman spectrometer. The main preliminary results are: 1/ although ammonia does affect the stability of clathrate hydrates, its influence appears lower than on the melting of water ice; 2/ the dissociation proceeds incongruently, similarly to the incongruent melting of water ice in the H₂O-NH₃ system. These results and their implications will be presented at the meeting. This work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Copyright 2013, Caltech. Support by the NASA Outer Planets Research Program and government sponsorship acknowledged.