Abstract The James Webb Space Telescope is characterising the atmospheres of sub-Neptunes. The presence of magma oceans on sub-Neptunes is expected to strongly alter the chemistry of their envelopes and observable atmospheres. At the magma ocean—envelope boundary (MEB, 10 kbar), gas properties deviate from ideality, yet the effects of real gas behaviour on chemical equilibria remain underexplored. Here, we compute equilibrium between magma–gas and gas–gas reactions using real gas equations of state in the H–He–C–N–O–Si system for TOI-421b, a canonical hot sub-Neptune potentially hosting a magma ocean. We find that H and N are the most soluble in magma, followed by He and C. We fit real gas equations of state to experimental data on SiH4, and show that, for a fully molten mantle, SiH4 dominates at the MEB under accreted gas metallicity of 1× solar, but is supplanted by CH4 at 100× solar. Lower mantle melt fractions lower both magma-derived Si abundances in the envelope and the solubility of H and He in magma, yielding H2- and He-rich envelopes. Projecting equilibrium chemistry through the observable atmosphere (1 mbar–100 bar), we find that ‘clouds’ of Si-bearing condensates strongly deplete Si-bearing gases, although SiH4 remains key, especially when a solar gas is accreted. SiH4/CH4 and Si/C ratios increase with mantle melt fraction and decrease with accreted gas metallicity. The competition between SiH4 and CH4 is therefore diagnostic of metallicity and presence of magma oceans on sub-Neptunes with equilibrium temperatures below 1000 K. The corollary is that H2- and He-rich, SiH4-deficient and CH4-bearing observable atmospheres may indicate a limited role or absence of magma oceans on sub-Neptunes.
Hakim et al. (Tue,) studied this question.