Addressing Open Issues about the Structural Evolution of Methane Clathrate Hydrate

Methane is widespread in the Universe, and its occurrence is intimately connected with that of water, often as clathrate hydrate, likely the priority form in which methane is stored in icy moons, water-rich exoplanets but also in the depths of Earth's oceans. Arrangement and stability range of the crystalline structures, decomposition conditions, and miscibility of the resulting dense fluid mixtures are crucial for modeling the static and dynamic properties of these complex extraterrestrial environments and for identifying possible prebiotic reactive events under transient favorable conditions of pressure, temperature, and irradiation. Here, we report a high-pressure study of methane hydrate up to 4 GPa and 550 K. Ex situ synthesis of crystalline methane hydrate allowed the analysis of homogeneous samples by state of the art Raman and FTIR spectroscopy, accessing information which considerably expands and modifies our knowledge of the crystalline structures, of the decomposition conditions, and of the molten fluid's characteristics in a wide pressure and temperature range.