Simulations of Rock-Ice Mixtures in the mantles of water-rich planets

Determining the stable phase of the binary rock-ice mixture at extreme conditions is crucial for structure and evolution models. Yet, research into rock-ice interactions at pressure and temperature conditions exceeding those found on Earth remains limited. The discovery of new exoplanets points to some of them having substantial water abundance, implying that the interaction between an overlying layer of ice and the underlying rock would occur under several megabar of pressure. We determine the solvus temperatures for magnesium silicates and water across pressures relevant to the deep interiors of super-Earth to sub-Neptune sized bodies by atomistic simulations. Using density functional theory molecular dynamics simulations, we investigate the miscibility of magnesium silicates (MgSiO₃ and MgO) and water ice (H₂O) up to 8000 K and from 0.3-2 Mbar, using a heat-until-it-mixes approach to determine liquidus conditions. Our findings reveal that MgSiO₃ becomes miscible in H₂O upon melting in the entire pressure range we explored. Surprisingly, we observe a significant depression in the melting point of MgO by over 2000 K in the presence of H₂O, highlighting complex interactions between rock and water at extreme conditions. These findings have implications for structure and evolution models, including those of the deep interiors of the ice giants, as well as for considerations ofplanet habitability.