Does It Rain Diamonds on Neptune and Uranus?

The methane phase diagram is extremely important to understand the physical properties of the ice giants-Uranus and Neptune. Several experimental studies using diamond anvil cells (DACs) combined with laser heating have reported the formation of diamond from methane at pressures of 10-80 GPa and temperatures above 2000 K, corresponding to the interiors of these planets. These results, however, are probably affected by the presence of metallic heat absorbers, widely used in all previously reported experiments and interacting with hydrogen at high pressure. In the present work, the effect of metallic heat absorbers on the decomposition of methane into diamond was studied at 20-95 GPa and 1300-3700 K using laser-heated DACs with platinum (as hydride-forming) and gold (as non-hydride-forming) metals. In the case of a platinum heat absorber, diamond formation was observed from 50 to 95 GPa near 2000 K simultaneously with platinum hydride formation. In contrast, in the case of a gold heat absorber, diamond formation was not observed below 95 GPa and 3700 K. Thus, the hypothesis of diamond precipitation in the Uranus and Neptune interiors should be reconsidered, taking into account the effect of metallic heater reactivity on the experimentally observed reactions.

Automated summary

The methane phase diagram is crucial for understanding the properties of Uranus and Neptune. Previous studies have suggested diamond formation from methane at high pressure and temperature, but the presence of metallic heat absorbers may have affected the results. This study used laser-heated diamond anvil cells with platinum and gold heat absorbers to investigate the effect on methane decomposition into diamond. Diamond formation was observed with a platinum heat absorber, but not with a gold heat absorber. This suggests a need to reconsider the hypothesis of diamond precipitation in the interiors of Uranus and Neptune, taking into account the reactivity of metallic heat absorbers in experimental observations.