Hydrogen Chemistry for understanding Sub-Neptunes

Hydrogen is abundant in planetary systems, and the main constituent of gas giants and some exoplanets (such as sub-Neptunes). The gravity data from the Juno mission suggest that the hydrogen envelope of Jupiter may contain significant amounts of heavier elements, suggesting mixing between hydrogen and heavier elements at high pressures and high temperatures. Sub-Neptunes are believed to have a thick hydrogen-rich envelope covering silicates and metals (likely molten based on estimated temperatures). Therefore, they would have a deep hydrogen-magma interface at high pressures. Hydrogen gas is well known to be a strong reducing agent. However, chemical behaviors of dense hydrogen fluid, which is more relevant for the planetary interiors, are largely unknown because of the paucity of data. By taking advantage of some recent technical developments in laser-heated diamond-anvil cell, we have conducted a series of experiments to study chemical reaction between hydrogen and silicates at high pressures and high temperatures (5-50 GPa and over 2000 K).

The experiments found that hydrogen reduces Fe²⁺ and Fe³⁺ in molten (Mg,Fe)O and Fe₂O₃ to Fe metal which subsequently reacts with H to form FeH_x alloy. Molten silicate, (Mg,Fe)₂SiO₄, reacts with hydrogen and produces FeH_x alloy liquid as well. In this case, Si⁴⁺ in molten silicate is also reduced to metal and alloys with Fe to form FeSi liquid. The redox reaction results in a magma deficient in Si, making it essentially MgO melt rather than silicate melt. We also detected OH vibrational modes from the quenched oxide melt, suggesting that the oxide magma contains much water after reacting with hydrogen. Some oxygen from the reduction of Fe²⁺ and Si⁴⁺ should be released to hydrogen medium. Therefore, the reaction opens possibilities to convert nebular hydrogen into water and then store it in both the interior and the atmosphere of sub-Neptunes. If some sub-Neptunes convert to super-Earths by gas loss, as suggested by the observed radius gap, the water stored in the interior could allow for the formation of secondary atmosphere and hydrosphere on the surface, potentially impacting surface habitability of the large rocky planets. The redox reaction with hydrogen can remove Si from the oxide layer and produce Fe-Si alloy which forms the metallic core. Therefore, the sub-Neptunes and super-Earths converted from sub-Neptunes may have the Si-poor mantle and the light element rich (such as Si and H) core, differing significantly from smaller rocky planets, such as Earth and Venus.