Atomic Scale Mixing between MgO and H2O in the Deep Interiors of Water-rich Planets

Water-rich planets are common in our galaxy: Uranus and Neptune in our solar system and some of the sub-Neptunes in extrasolar systems1. These planets may have a thick water-rich layer above the rocky interior. Recent models2-4 have invoked possible existence of heavier elements in the water-rich layer of Uranus based on need for explaining the low luminosity5-6. However, it is not well understood how heavy elements can exist and be stabilized in the low-density layer for a sufficiently long period of time. Here we report the results of laser-heated diamond-anvil cell (LH-DAC) experiments on two rock-forming minerals, olivine ((Mg,Fe)2SiO4) and ferropericlase ((Mg,Fe)O), in a H2O medium at the pressure and temperature conditions expected for the interiors of approximately 1-6 Earth-mass water-rich planets. The X-ray diffraction patterns and the morphology of the recovered samples in electron microscopy indicate a selective leaching of MgO from starting olivine during heating and large solubility of MgO in H2O peaking between 20-40 GPa and above 1500 K. The observed reaction can chemically stabilize heavier elements (such as Mg and O) in water-rich layer, providing a key chemical process for explaining the hypothesized heavier element presence at shallow depths in Uranus and possibly other water-rich planets. In addition, the H2O layer of hot and smaller water-rich planets7 (Earth-size) is more likely to have a wider MgO-rich layer than cold and larger water-rich planets where the depth range of the MgO-rich layer is likely to be narrower, enabling a range of geochemical evolution among water-rich planets.