Phase behavior of clathrate hydrates in the ternary H2O-NH3-Cyclopentane system

Titan, Saturn’s largest satellite, is the only icy moon with a dense atmosphere which is composed mainly of N2. Methane, the second most abundant constituent, would be depleted in only 30-100 Myr by active photochemistry, suggesting replenishment from Titan’s interior. Under Titan’s near-surface conditions, clathrate hydrates are the stable form of methane and ice together, making them a likely methane reservoir. Cassini-Huygens observations suggest that ammonia is the main source of Titan’s atmospheric N2. Ammonia is known to decrease the melting point of water ice and some clathrate hydrates such as those of tetrahydrofuran. The present study investigates the interaction of ammonia with cyclopentane clathrate hydrates (atmospheric analog for methane clathrates) via a detailed examination of phase behavior using micro-Raman spectroscopy, differential scanning calorimetry, and X-ray diffraction. The results show that ammonia has the same effect on the stability of the cyclopentane clathrate as on tetrahydrofuran clathrate and on ice by lowering the dissociation temperature by several tens of degrees and inducing incongruent melting. Ammonia does not interact directly with cyclopentane, and does not appear to be incorporated into cyclopentane clathrate structure, whether in the lattice or within the cages. A similar effect could be expected for methane clathrates. The presence of ammonia in Titan’s crust would thus destabilize methane clathrates, resulting in outgassing and the replenishment of atmospheric methane.