Hydrogen-Water Mixtures in Giant Planet Interiors Studied with Ab Initio Simulations

Abstract:

With the extraordinary discovery of more than 1700 confirmed exoplanets and even more candidates from the Kepler survey, the need for more accurate structure and evolution models becomes very relevant. One important issue for gas and ice giant planets is to determine the behavior of hydrogen-water mixtures in their envelope. It is currently unknown whether a planet contains a dense, hot steam atmosphere or a water ocean that is well separated from a hydrogen atmosphere. Laboratory measurements demonstrated the molecular hydrogen and water phase separate at low pressure and temperature. Recently, computer simulations predicted that water and metallic hydrogen form a homogeneous mixture at pressure of 10 megabars and 3000 K [Wilson, Militzer, ApJ 745 (2012) 54]. The intermediate pressure regime that is of interest for giant planets in our solar system and sub-Neptune exoplanets has not been explored. We present results from ab initio computer simulations in combination with Gibbs free-energy calculations and different thermodynamic integration methods. We determine the pressure-temperature conditions for hydrogen-water phase separation and discuss the consequences for the giant planet interiors. Our findings may have implication for the heat transfer and the distribution of other compounds like ammonia in giant planets envelopes.