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.
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All X-ray diffraction, IR spectra and SEM–energy-dispersive X-ray spectroscopy data supporting this study are available at https://doi.org/10.5281/zenodo.4632916 or by contacting the corresponding authors.
Python scripts used to analyse the X-ray diffraction data supporting this study are available from https://github.com/SHDShim/PeakPo.
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This work was supported by the Leader Researcher programme (NRF-2018R1A3B1052042) of the Korean Ministry of Science and ICT (MSIT). We also acknowledge the support by grant NRF-2019K1A3A7A09033395 of the MSIT. S.-H.S. was supported by NSF grant EAR1338810 and National Aeronautics and Space Administration (NASA) grant 80NSSC18K0353. S.-H.S. also benefited from collaborations and information exchange within the Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate. S.S. acknowledges support by the GFZ German Research Centre for Geosciences. This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge the support of GSECARS (Sector 13), which is supported by the National Science Foundation—Earth Sciences (EAR-1634415), and the Department of Energy, Geosciences (DE-FG02-94ER14466). Parts of this research were carried out at the P02.2 beamline at PETRA III, and we acknowledge Deutsches Elektronen-Synchrotron (DESY, Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. We also acknowledge the scientific exchange and support of the Center for Molecular Water Science (CMWS) at DESY. This research also used beamline 22-IR-1 of the National Synchrotron Light Source II, a US DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract DE-SC0012704 and COMPRES under NSF cooperative agreement EAR 11-57758 and CDAC (DE-FC03-03N00144).
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Stéphane Mazevet and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Kim, T., Chariton, S., Prakapenka, V. et al. Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets. Nat Astron 5, 815–821 (2021). https://doi.org/10.1038/s41550-021-01368-2
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