Planet Mixology: Stirring the Mantle of Water Worlds

Water worlds are exoplanets ranging in size between Earth and Neptune that are predicted to be rich in water. The interior structure of a water world is assumed to have distinct layers: (1) iron core, (2) rocky mantle, and (3) water. This 3-layer model may work in smaller planets where water and rock form differentiated layers with limited incorporation of water into silicates. However, in larger planets water and silicates may interact differently due to greater interior pressures and temperatures found at the rock-ice boundary. Determining the dynamics of 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 the conditions pertinent to the rock-ice boundary layer within water worlds. We use a heat-until-it-mixes approach to explore pressures ranging from 30 - 120 GPa and temperatures from 500 - 8000 K. When 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 ran smoothed particle hydrodynamics simulations. We simulated the collisional growth of water worlds via giant impacts between water rich planetesimals of 0.7 - 4.7 Earth masses. This work provides theoretical evidence that many massive water worlds have mixed mantles.

Speaker: Tanja Kovacevic, UC Berkeley