Chemical Interactions Between Hydrogen-Rich Envelopes and Silicate Magma Oceans

Sub-Neptunesplanets of radius 2.7-3 REarthare 4-10x more abundant than planets just 20% larger (3.3-3.7 REarth), a phenomenon known as the radius cliff [1]. Studies have shown that sub-Neptune interiors have silicates and metal overlain by thick hydrogen-dominated atmospheres [2]. A modeling study suggested the radius cliff is caused by the increasing solubility of H2 from the atmosphere in the magma ocean at pressures greater than 109 Pa [3], which is readily achieved at the atmosphere-mantle interface of 3MEarth planets accreting nebular gas [4]. Current models rely on extrapolations from lower pressure data [5]; therefore, it is important to understand H-solubility/reactions with magma at the pressure-temperature conditions in sub-Neptunes. Diamond anvil cells were loaded with either samples of San Carlos Olivine mixed with metallic iron or samples of fayalite, followed by pure H2 gas. We performed pulsed laser heating at temperatures of 2600-4500 K to ensure melting at pressures of 5-42 GPa with in-situ XRD and Raman spectroscopy at GSECARS. In all cases, we observe the formation of FeH both by the hydrogenation of any preexisting metallic iron, and by the reduction of oxidized iron from the silicate phase after laser heating. When Fe is removed from silicates, MgO and SiO2 remain. In some cases, the oxide component reacts further: SiO2 reduces to Si, which alloys with iron, and O is released to the hydrogen medium to form H2O. The observed reactions provide a pathway for chemical dissolution of a high-pressure hydrogen envelope into a silicate magma ocean. The hydrogen could be sequestered in the core of a planet as FeHx, or in the mantle as H2O after reducing Fe or Si to metal. These redox reactions involving hydrogen at high pressure magma-ocean envelope interfaces may result in large hydrogen ingassing and therefore explain the radius cliff whereby additional hydrogen accreted is partitioned to the interior, thus disrupting the radius distribution. Additionally, the reduction of silicon from Si4+ to Si0 may have important implications for the light element budget in cores of terrestrial planets if nebular hydrogen is ingassed into differentiating planetesimals. [1] Fulton & Petigura, 2018, AJ [2] Rogers et al., 2011, ApJ [3] Kite et al., 2019, ApJL [4] Pollack et al., 1996, icarus [5] Hirschmann et al., 2012, EPSL