Presentation #107.03 in the session Terrestrial Planets in our Solar System and beyond.
Water-rich planets are found to be common in our galaxy1. The internal structure of these planets has been modeled assuming no mixing between H2O and rock-forming components (such as MgO) and therefore it has been believed that a separate layer of water-rich layer exists on top of a rocky and/or metallic core2. Uranus, likely a water-rich planet with an H2-rich envelope, has a lower luminosity3 than other planets in our solar system. Recent models have suggested that a compositional gradient in the water-rich layer could result in the low luminosity4, but origin of the compositional gradient is not well known. We recently reported experimental results5 on olivine ((Mg,Fe)2SiO4) and ferropericlase ((Mg,Fe)O), two rock-forming minerals, in a H2O medium at the pressure and temperature conditions expected for the interiors of up to ~6 Earth-mass water-rich planets. We have also extended the experimental investigation to a range of H2O + H2 mixtures, which can be applicable for water-rich planets with a H2 atmosphere where some H2 may dissolve in H2O at the interface where pressure would be high. Based on the in-situ X-ray diffraction patterns and electron microscopy analysis, we found that Mg2+ is leached out from the rock-forming minerals between 20–80 GPa and above 1500 K, and dissolved in H2O. With a presence of H2 in H2O, we still observed solubility of Mg2+ in H2O, suggesting that the process is not affected by H2 up to H2/H2O = 3. Unlike the case of pure H2O, H2 in H2O reduces Fe2+ in the rock-forming minerals and form iron-rich metal alloy. Such Fe metal alloy formation will reduce Fe2+ content in a H2O layer as Fe metal alloy would sink toward the center of a planet. If a H2O layer of a water-rich planet is warm enough, the layer would be rich in Mg2+. The large dissolution of Mg2+ could explain the hypothesized compositional gradient in the water-rich layer of Uranus and possibly other massive water-rich planets (e.g., sub-Neptunes). In this talk, we will also discuss how the mixing behavior changes with H2/H2O ratio and its implications for the structure and dynamics of water-rich planets.
Acknowledgements: This work has been supported by NASA and NSF. The results reported herein benefit from NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network. The work was also supported by the Korean Ministry of Science and ICT.
References:  Thompson, S. E. et al. Astrophys. J. Suppl. Ser. 235:38 (2018).  Guillot, T. Annu. Rev. Earth Planet. Sci. 33, 493–530 (2005).  Pearl, J. C. et al. Icarus 84, 12–28 (1990).  Helled, R. et al. Space Sci. Rev. 216:38 (2020).  Kim, T. et al. Nat. Astron. 5, 815–821 (2021).