Methanol storage in high-pressure clathrate hydrates as a prolonged source of methane in large ocean worlds

Large ocean worlds in the solar system and in other planetary systems likely contain significant fraction of volatile and organic compounds, which condition their habitability potential. Some of these compounds are known to combine with water molecules to form clathrate hydrates under pressure. However, the clathration process in complex compositions and under high pressures representative of these ocean worlds still remain unexplored. Herein we report on the first experimental results on the characterization of the pressure-induced transitional pathway of a mixed clathrate hydrate. We studied crystalline compounds forming in two systems: binary tetrahydrofuran (THF)-H2O that served as a reference system, and ternary THF-methanol (MeOH)-H2O in order to address the role of methanol at high-pressure. We show that high pressure facilitates the incorporation of methanol in clathrate structure, with formation of two mixed clathrate hydrates, THF•xMeOH•17H2O (sII) and THF•0.75MeOH•10H2O. The preferential trapping of methanol at high-pressure offers an explanation for the non-detection of methanol on surfaces of large ocean worlds, while more common on small icy worlds. Moreover, the hydrogenolysis of enclathrated primordial methanol into methane presents a novel hypothesis for the deep-seated endogenic source of methane in Titan and potentially for other ocean worlds.