Atomic-scale mixing between MgO and H₂O in the deep interiors of water-rich planets

Water-rich planets exist in our Solar System (Uranus and Neptune) and are found to be common in the extrasolar systems (some of the sub-Neptunes). In conventional models of these planets a thick water-rich layer is underlain by a separate rocky interior. Here we report experimental results on two rock-forming minerals, olivine ((Mg,Fe)₂SiO₄) and ferropericlase ((Mg,Fe)O), in water at the pressure and temperature conditions expected for the water-rich planets. Our data indicate a selective leaching of MgO, which peaks between 20 and 40 GPa and above 1,500 K. For water-rich planets with 1–6 Earth masses (>50 wt% H₂O), the chemical reaction at the deep water–rock interface would lead to high concentrations of MgO in the H₂O layer. For Uranus and Neptune, the top ~3% of the H₂O layer would have a large storage capacity for MgO. If an early dynamic process enables the rock–H₂O reaction, the topmost H₂O layer may be rich in MgO, possibly affecting the thermal history of the planet.