Abstract: Saturn's largest moon Titan is a complex world presenting unique characteristics such as a substantial atmosphere, ongoing organic chemistry and a frigid surface carved by hydrocarbon flows. Clathrate hydrates have long been inferred to constitute a major crustal component of its icy shell and may contribute significantly to the atmospheric methane’s replenishment by releasing their entrapped gas via their dissociation or substitution by ethane.
The dissociation of clathrate hydrates and subsequent CH4 outgassing would require interaction with inhibitors and/or destabilizing events (e.g., thermal plume, or impact). Clathrates of methane and tetrahydrofuran (THF), its ambient pressure analog, undergo partial dissociation at temperatures as low as ~200 K in the presence of ammonia, a strong antifreeze agent likely present in Titan’s outer ice shell. Ammonia is known to generate partial melting of water ice between the eutectic point at 176 K (at 1 bar) and the liquidus temperature. In Titan’s subsurface, NH3-bearing liquids may interact with methane clathrate hydrates and could potentially trigger clathrate dissociation. In addition, the ternary H2O-THF-NH3 system has been shown to exhibit a great chemical complexity with the reported formation of several crystallization stages upon cooling, including a binary THF-NH3 clathrate and an unknown THF-NH3-rich phase.
In order to establish the effects of NH3 on clathrates and better assess their contribution to methane outgassing and exchange processes on Titan, we use laboratory and modeling studies. In particular, we investigate the ternary H2O-THF-NH3 system and the CH4-C2H6 replacement kinetics in clathrate hydrates under Titan-like conditions. Our X-ray diffraction data are in agreement with previous Raman spectroscopy and calorimetry studies and confirm the partial dissociation of THF clathrates in aqueous solutions of ammonia between 200 and 270 K. Transposing this effect to methane clathrates implies that the presence of small amounts of ammonia in Titan’s icy shell could trigger partial melting of these subsurface methane reservoirs. Furthermore, these new data enabled the characterization of a previously unknown THF-NH3-rich phase observed below 220 K. We also report the successful formation of ethane clathrate hydrates from the reaction of liquid ethane with methane clathrates and ice. This supports the hypotheses that liquid ethane can be trapped in Titan’s subsurface, and that methane could be released as ethane replaces it in clathrate hydrates.
About the speaker: Elodie holds a PhD in planetary sciences from UCLouvain (Belgium) where she studied methane trapping and global clathrate stability regions in the Martian crust. After a first postdoc in Belgium, during which she studied the surface and subsurface water environment on Mars, Elodie joined JPL in June 2021 to study the stability of clathrate hydrates in the presence of ammonia under conditions relevant to the outer Solar System. Her research involves experimental investigation and thermodynamic modeling to better constrain methane outgassing and subsurface – atmosphere exchanges on Titan.
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