Phase Diagram of the Ternary Water–Tetrahydrofuran–Ammonia System at Low Temperatures. Implications for Clathrate Hydrates and Outgassing on Titan

Titan’s icy shell is expected to contain predominantly methane clathrate hydrates, water ice Ih, and possibly ammonia hydrates, beneath a cover of diverse organics formed via atmospheric photochemistry. The dissociation of clathrate hydrates has long been inferred as a potential replenishment mechanism for atmospheric methane; however, pure methane clathrates would be stable all the way to the surface. The melting of ammonia hydrates and subsequent interaction with methane clathrates could favor the dissociation of clathrates at much lower temperatures, due to the strong antifreeze effect of ammonia. To better understand the phase behavior of clathrate hydrates in the presence of ammonia, we have developed phase diagrams for the ternary system water–ammonia–tetrahydrofuran at 1 bar and in the temperature range 77–280 K via differential scanning calorimetry and Raman spectroscopy. We have been able to determine how ammonia promotes the start of a partial dissociation of THF–clathrates at temperatures far colder than the liquidus. We have also established that this ternary system exhibits a complex chemistry, with multiple phases forming in thermodynamic equilibrium because of a phase separation between a THF-dominated liquid and a H2O–NH3 dominated liquid. In addition to the expected THF–clathrates, we report the formation of other mineral phases such as ammonia hydrates, a new THF–NH3-rich phase, and potentially mixed THF–NH3 clathrates. Partial dissociation of ∼10% of the clathrate reservoir would release to Titan’s atmosphere methane amounts sufficient to sustain the hydrocarbon cycle for 650 My, which is commensurate with the age of the present atmosphere.