Rock-Ice Mixtures in the Interiors of Massive Water Worlds*

A water world is an exoplanet that ranges in size between Earth and Neptune that is predicted to be rich in water, lacking a massive atmosphere. The interior structure of a water world is assumed to have three layers: (1) iron core, (2) rocky mantle, and (3) water. This 3-layer model may work in smaller planets where water and rock form distinct layers with limited incorporation of water into silicates. However, in larger planets water and silicates may interact differently due to greater interior pressure and temperatures found at the rock-ice boundary. Determining the dynamics between these two materials at extreme conditions is necessary for understanding a water world's growth and evolution. In this work, we use density functional molecular dynamics (DFT-MD) simulations to investigate the miscibility and dynamics a major end-member silicate phase bridgmanite (MgSiO3), and water (H2O) at extreme conditions pertinent to the rock-ice boundary layer within water worlds. We use a heat-until-it-mixes method to explore pressures ranging from 30–120 GPa and temperatures from 500–8000 K. When the temperatures exceed the melting point of bridgmanite, we show that MgSiO3 and H2O mix in all proportions. To provide proof of concept that these conditions are met during the collisional growth of these water-rich bodies, we performed smoothed particle hydrodynamics simulations. We simulated giant impacts between water worlds of 0.7–4.7 Earth masses. This work provides theoretical evidence that many massive water worlds have mixed mantles during their collisional growth.