Polymerization and diamond formation from melting methane and their implications in ice layer of giant planets

High-pressure and high-temperature experiments of solid methane were performed using a laser-heated diamond anvil cell. X-ray diffractometry and Raman spectroscopy revealed the melting conditions to be above approximately 1100 K in a wide pressure range of 10–80 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.