Phase changes of methane hydrate under high-temperature and pressure and inference of interiors of giant icy planets

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This study intended firstly to clarify phase changes and properties of methane hydrate and solid methane under high-pressures and high-temperatures comparable to those of interiors of icy planets and satellites, and then to infer the states and internal structures of the icy bodies based on the experimental results. High-pressures and high-temperatures were generated by using laser heated diamond anvil cells (LHDAC), and characterization of the samples were made by X-ray diffractometry and Raman spectroscopy. For methane hydrate, the high-pressure phase of filled ice Ih structure survived at least up to 90 GPa. The reasons for the remarkable high-pressure stability were examined. The orentaional ordering of guest methane molecules and symmetrization of hydrogen bond of host molecules might substantiate the outstanding stability of the filled ice Ih structure under extremely high pressure. For solid methane, X-ray diffractometry and Raman spectroscopy revealed the melting conditions to be above approximately 1100 K in a wide pressure range of 10 to 80 GPa. Above 1100 K, polymerizations occurred to produce ethane molecules and further polymerized hydrocarbons. Above 3000 K, diamond was produced. These changes proceeded depending on temperature rather than on pressure. The present study provides some experimental supports to theoretical predictions that the middle ice layer of giant planets such as Neptune and Uranus melts into a hot methane-rich ocean, and will help to improve our understanding of the chemical process of the giant planets' interiors.