Ab Initio Simulations of Water in the Interiors of Ice Giant Planets

Abstract:

Water is one of the most prevalent substances in our solar system. Large quantities are assumed to be stored in the interiors of ice giant planets. Water has an unusually rich phase diagram with 15 solid phases that were determined experimentally and 5 additional ones that were predicted theoretically. At megabar pressures and elevated temperatures, water is predicted to assume a superionic state where the oxygen ions remain confined to lattice sites while the hydrogen ions move through the crystal like a fluid. In our recent article [Physical Review Letters 110 (2013) 151102], we predicted the oxygen sub-lattice to assume a face-centered cubic structure at pressures above 1 Mbar. In this presentation, we present results from additional density functional molecular dynamics simulations and predict the existence of new, not close packed phase. We employed a thermodynamics integration technique to derive the entropy and the Gibbs free energy. We discuss how a novel superionic state could be identified in high pressure experiments and talk about the implications for the interiors of Uranus and Neptune.