Phase Behavior of Clathrate Hydrates in the Ternary H₂O–NH₃–Cyclopentane System

Abstract

Titan, Saturn's largest satellite, is the only icy moon with a dense atmosphere that is composed mainly of N 2 . 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 N 2 . 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.

Reply to the ‘Comment on Cage occupancy of methane clathrate hydrates in the ternary H2O–NH3–CH4 system’ by S. Alavi and J. Ripmeester, Chem. Commun., 2022, 58, DOI: 10.1039/D1CC06526B

“…As delineated in the Communication, 1 the preferential destabilization of large cages in sI methane clathrate hydrates formed from freezing of aqueous solutions would be consistent with most observations on the H 2 O–NH 3 –CH 4 , 1 H 2 O–NH 3 –THF, 3,4 H 2 O–NH 3 –CP, 5 and H 2 O–NH 3 –C 2 H 6 6 systems. This mechanism would only be applicable to the temperature-concentration domain where incongruent behaviour of hydrates occurs in presence of an NH 3 aqueous solution.…”

“…3–5 (1) Incongruent melting/dissociation of the hydrates has been observed in the temperature range ∼200–270 K from optical images, 3 Raman spectroscopy, 3–5 calorimetry, 4,5 and XRD. 5 This implies a persistent equilibrium between fractions of liquid (of varying NH 3 concentration) and crystals that change with temperature as hydrate crystals progressively grow and melt. No changes to the Raman signature or XRD patterns of CP hydrate were observed throughout this process while the ammonia concentration in the remaining liquid changes, which seems irreconcilable with a substantial incorporation of NH 3 inside the structure.…”

“…A recent experimental study 1 of methane clathrate hydrates in the ternary H 2 O-NH 3 -CH 4 system reported a variation in cage occupancy as function of ammonia concentration. It proposed a mechanism, based on its results as well as other works, [3][4][5][6] in which the large cages of structure I (sI) methane hydrates are preferentially destabilized in the presence of aqueous ammonia as a more robust explanation than the previously proposed (e.g. ref.…”

A simple gas introduction system for cryogenic powder X-ray diffraction

“…Jensen et al (2010) and Scarlett et al (2017)]. We study XRD of volatile organics, clathrates and hydrated salt minerals at low temperatures, as a means of exploring the chemistry of planetary materials Cable et al, 2020;Vu et al, 2019;Petuya et al, 2020). The primary instrument we use is a Bruker D8 DISCOVER DaVinci, optimized for powder samples held in capillaries cooled with an Oxford Cryosystems 800 cryostream.…”

Vertical compositional variations of liquid hydrocarbons in Titan’s alkanofers

“…The fast destruction rate of atmospheric methane predicted by pre-Cassini works (Flasar et al 1981;Yung et al 1984) lead scientists to hypothesize the existence of a subsurface source of methane. One potential endogenic source of methane could be the complete or partial dissociation of a clathrate hydrate reservoir (e.g., Lunine & Stevenson 1987;Tobie et al 2006;Choukroun et al 2010;Muñoz-Iglesias et al 2018;Petuya et al 2020) In addition, the possible presence of liquid reservoirs, more or less deeply buried under Titan's surface, has been proposed. For instance, Griffith et al (2012) interpreted dark patches observed in equatorial regions as an emergence of liquid.…”