sp9b00309_si_001.pdf (970.48 kB)
Phase Behavior of Clathrate Hydrates in the Ternary H2O–NH3–Cyclopentane System
journal contribution
posted on 2020-04-03, 21:29 authored by Claire Petuya, Mathieu Choukroun, Tuan H. Vu, Christophe Sotin, Ashley G. DaviesTitan,
Saturn’s largest satellite, is the only icy moon
with a dense atmosphere that is composed mainly of N2.
Methane, the second most abundant constituent, would be depleted in
only 30–100 million years 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 analogue 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 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 the 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 replenishment
of atmospheric methane.