Reduction of Silicates by Hydrogen at High Pressure-Temperature – Experimental Evidence for the Formation of Hot Wet Sub-Neptunes

Sub-Neptunes – planets of radius ~1.75-3.5 R_Earth – with orbital periods of <100 days are the most abundant type of observed exo-planets [1]. Their mass-radius relations can be modeled as either dry, rocky worlds overlain by a thick hydrogen atmosphere, or water-rich planets, with the latter favored due to their orbit within the ice line requiring migration or delivery of water-rich materials from beyond the ice line [2]. We conducted experiments in the pulsed-laser-heated diamond-anvil-cell loaded with silicate samples in a hydrogen medium at pressures of 6-42 GPa and temperatures of 1650-4050 K. Pulsed laser-heating (along with inert gasket coatings) was used to mitigate H-diffusion out of the sample chamber, a challenge that previously made high P-T studies with pure H infeasible. We found that H reduces Si⁴⁺ and Fe²⁺ in silicate melt to metal, releasing O which in turn forms H₂O. This reaction can convert a dry rocky planet with a hydrogen atmosphere into a water rich planet from within, even inside the snow line. Depending on the degree of this reaction, a wide range of H₂ to H₂O ratios are possible, offering a new explanation for the diverse mass-radius relations in sub-Neptune populations.

[1] Fulton, B. J., et al. AJ 154.3 (2017)

[2] Bean, J. L., et al. JGR: Planets 126.1 (2021)