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

• Methanol enters a clathrate hydrate structure in presence of a promoter. • Methanol concentration increases upon compression and reaches a unity. • Incorporation of methanol strongly affects pressure-induced crystallization pathway. • Methanol enclathration explains its absence on the surfaces of the 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. Methanol is a volatile of a particular interest due to its expected presence in various icy bodies of the outer solar system and its disputable role in clathrate hydrates formation. However, the clathration process of methanol in complex compositions and under high pressures representative of these ocean worlds still remain unexplored. Herein, using high-pressure – low-temperature in situ single crystal X-ray diffraction, we demonstrate that high pressure facilitates incorporation of methanol in clathrate structure in presence of a promoter, with formation of mixed methanol-bearing clathrate hydrates. 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.