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

Abstract

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 c...