Experimental investigations of gas hydrates of CH4 and CO2 at high-pressure conditions in the interior of Titan

Gas hydrates plausibly occur in large icy satellites of Jupiter and Saturn, and their presence has important implications for the chemical evolution of these planetary bodies [1]. Despite the importance of gas hydrates, especially inside Titan, their behavior under conditions relevant to large icy moons, where pressure exceeds a few hundred MPa, remains poorly understood and is still the focus of scientific debates [2, 3, 4, 5, 6, 7]. Previous high-pressure (HP) studies have applied mainly X-ray and neutron powder diffraction and Raman spectroscopy [1] and have focused solely on pure end-member compositions, while mixed clathrates – incorporating multiple types of guest molecules – are envisaged to occur in Nature. In this work, we have aimed to explore the behavior of pure CH₄ and mixed CH₄:CO₂ gas hydrates under HP using single-crystal X-ray diffraction (SC-XRD) in diamond anvil cells. The samples were synthesized in a pressurized vessel from controlled gas mixtures and their chemical compositions subsequently analyzed by gas chromatography and Raman spectroscopy at the Laboratory of Planetology and Geosciences in Nantes, France. Diffraction experiments were conducted at the ID27 beamline of the European Synchrotron Radiation Facility (Grenoble, France). The evolution of pure CH₄ hydrate was investigated at ambient temperature up to 2.47 GPa. In agreement with previous observations, above 0.88 GPa, CH₄ hydrate transformed from a cubic sI structure to a sH hexagonal structure (P6/mmm; a = 11.9546(7) Å; c = 10.023(6) Å; V = 1240.5(7) ų). However, upon further compression, a previously unreported hexagonal structure was found above 1.28 GPa.
The new structure, so-called sH-II, crystallizes in the P-62m space group with unit cell parameters, a = 11.6203(1) Å; c = 9.836(8) Å; V = 1150.2(9) ų. The change in symmetry arises from the ordering of guest molecules within the large cage of the clathrate structure. Interestingly, considerable softening of the sH-II crystal structure was observed above 2 GPa. The sample with mixed CH₄:CO₂ (~69:31) composition was compressed at room temperature up to 2.12 GPa. Below 1.75 GPa, sI and sH clathrates hosting only CH₄ wereobserved, while CO₂ presumably remained dissolved in liquid water. Above this pressure, the mixed clathrate containing CO₂ and CH₄ was formed. The mixed CH₄:CO₂ clathrate adopts a new sH-II structure, found in the pure CH₄-water system (sp. gr. P-62m), with the following unit cell parameters: a = 11.7652(5) Å; c = 9.828(4) Å; V = 1178.2(5) ų. In the case of the mixed clathrate, the large cage hosts three sites of CO₂ and two sites of CH₄ (Fig. 1).

Figure 1:  5¹²6⁸ cage of mixed sH-II clathrate containing two CH₄ molecules, in the upper and bottom parts, and three CO₂ molecules around the waist. The red spheres represent O atoms; red lines indicate hydrogen bonds; the brown spheres represent carbon atoms.

In this work, the usage of synchrotron-based SC-XRD allowed us for the first time to provide unambiguous evidence of the presence of two different guest molecules in the structure of clathrate hydrate under conditions relevant for the interior of Titan, and other large icy moons (Ganymede and Callisto), as well as to resolve the long-lasting controversy of the HP structure of CH₄ clathrate above 0.8 GPa. We demonstrate that SC-XRD is a powerful tool that enables tracking of subtle pressure-induced changes in the clathrate hydrates, in particular changes in occupancy and ordering of guest molecules. These new results have implications for the evolution of the CH₄/CO₂ reservoir in the thick hydrosphere of Titan, with potential impact on the thermal structure of the hydrosphere and the replenishment of atmospheric methane.

Acknowledgments: We acknowledge the financial support provided by the Agence Nationale de la Recherche (ANR) through the project CAGES (“High pressure clathrate hydrates in large ocean worlds”, ANR-23-CE49-0002, PI A. Pakhomova).

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