Overcoming hydrophobicity: A maximally filled methane clathrate hydrate under conditions of early Titan and ocean-exoplanets

The hydrophobic effect, the primary driver of methane confinement within crystalline water cages, stabilizes vast reservoirs of methane clathrate hydrates across Earth and other planetary bodies. While high pressure is known to drastically alter both the stoichiometry and intermolecular dynamics of gas hydrates, understanding these transformations has been hindered by the limited resolution of previous studies. Here, using in situ high-pressure single-crystal X-ray diffraction, we demonstrate a pressure-driven increase in methane content and the emergence of a previously unknown clathrate hydrate stable at pressures of 1.3-2.1 GPa. The novel phase, MH-IIb, is a maximally filled clathrate phase with an H ₂ O:CH ₄ ratio up to 3.40. We show that the transition to MH-IIb is driven by methane–methane steric repulsion, which, with pressure, overcomes classical methane–water hydrophobic interactions and forces formation of unconventional C–H···O hydrogen bonds. Our results indicate that the high-pressure, maximally filled clathrate MH-IIb may serve as a methane reservoir in the outer part of the differentiating core of early Titan, thereby impeding degassing and accounting for the delay in the formation of Titan’s methane-rich atmosphere. Beyond the solar system, this pressure-induced methane-trapping mechanism extends to water-rich exoplanets, allowing their deep interiors to retain vast carbon reservoirs, suggesting that atmospheric surveys may fundamentally underestimate these planets' total carbon inventory.