On the impact of nebular H2 capture on the internal geochemistry of Sub-Neptune class exoplanets

In the Exoplanetary zoo revealed by astrophysical observations, the Super-Earth to Sub-Neptune planets constitute an intriguing array as they lie at the boundary between rocky and gaseous bodies. In order to build Sub-Neptune planets, gravitational capture of the nebular gas must occur; this forms a dense gaseous atmosphere which encapsulates the growing planet. This dense gas is expected to interact with the planetary interior causing some elemental redistributions, modifying planetary redox state, and potentially changing the internal structure. At some points, this dense nebular atmosphere can be lost, leaving behind a rocky world with an internal geochemistry that has been profoundly affected by its nebular history. Here, we simulate the geochemical consequences of an H₂-rich atmosphere effectively accreting around a young and molten rocky planet. We compute a series of reactions involving three phases: the gaseous fluid reacts with a molten silicate at the pressure prevailing at the base of the atmosphere, and the molten silicate reacts with a molten metal alloys at the pressure of the core-magma ocean boundary.

The nebular gas, mostly composed of molecular H₂, has a great reducing capacity, causing internal redox redistribution. The magnitude of such redox redistribution depends on the availability of oxygen in the planetary interior, which is, itself, governed by reactions between the molten silicate and the molten core. In particular, molecular H₂ can produce massive amounts of water if oxygen is available in the molten silicate. This occurs if the magma ocean is rich in iron oxides (FeO & Fe₂O₃) or if silicon is incorporated in the metallic core, a reaction which produces oxygen. In contrast, if oxygen is itself incorporated in to the metallic core, this diminishes the amount of molecular hydrogen that can be converted into water. Finally, a redox neutral process is also identified with the direct solubilization of H₂ into the molten metallic core. The simultaneous resolution of these equilibria indicates that the system tends to self-regulate with diverse initial scenarios converging to a unique final configuration in which abundant water is produced.