Solubility of Magnesium in Water at High Pressures and its Implications for Uranus

Uranus has a lower luminosity than Neptune. However, the origin has been unclear. Recent modeling studies have shown that such a low luminosity could be explained by compositional gradients which can be as a thermal barrier. However, there has been no known process which can explain the formation of such compositional gradients in the interior of Uranus [1]. We have conducted a series of experiments to explore the reaction between minerals and water at high pressures and high temperatures [2,3]. (Mg,Fe)₂SiO₄ olivine, (Mg,Fe)O ferropericlase, and SiO₂ were laser heated separately to 700-2000 K in a H₂O medium at 7-110 GPa in diamond-anvil cells. X-ray diffraction and infrared spectroscopy were conducted at Advanced Photon Source and National Synchrotron Light Source, respectively. The recovered samples were analyzed at Arizona State University and Yonsei University. We also have conducted density functional theory calculations. We found large solubility of Mg in H₂O at 20-40 GPa. Although SiO₂ remains solid, our experiments showed that SiO₂ can contain a large amount of H₂O in the crystal structure at high pressure: x = 0.2 in (Si_1-xH_4x)O₂. Therefore, for Uranus, Mg can form a compositional gradient at the topmost part of the water-rich layer. The reaction could also change the composition of the rocky core, making it much richer in silica and water than the mantle of rocky planets. It will be important to explore how such chemical changes affect the physical properties, such as electrical conductivity and thermal conductivity, of the deep interior in future studies.

References: [1] Nettelmann N. (2021) Nature Astronomy 10.1038/s41550-021-01421-0. [2] Nisr et al. (2020) PNAS 10.1073/pnas.1917448117. [3] Kim et al. (2021) Nature Astronomy 10.1038/s41550-021-01368-2.