Quantum molecular dynamics simulations of methane-water mixture at extreme conditions in Uranus and Neptune

Based on first-principles molecular dynamics simulations, the equation of state and Hugoniot curve of a methane-water mixture are investigated at relevant conditions in Uranus and Neptune. Under a strong shock wave, the compression ratio of the methane-water mixture is significantly higher than that of pure methane. Calculations of the pair-correlation function show that methane molecules are dissociated by ionized water above 4000 K. Above 6000 K, the methane and water molecules are completely dissociated, and many short-lived chemical species such as ${\mathrm{H}}_{3}\mathrm{O}$ and $\mathrm{OH}$ ions, C--O bonds, and free H atoms are formed. These results may provide a reasonable material basis for the unusual magnetic fields and the thin-shell dynamo models in Uranus and Neptune. The temperature and density ranges of our simulations are also relevant to dynamic shock experiments, and the results may provide reasonable models for future laboratory studies.