Silane–methane competition in sub-Neptune atmospheres as a diagnostic of metallicity and magma oceans
Year: 2026
Type: article
Abstract: ABSTRACT The James Webb Space Telescope is characterizing 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, $>$10 kbar), gas properties deviate from ideality, yet the effects of real gas behaviour on chemical equilibria remain underexplored. Here, we compute equilibrium between magma–gas and gas–gas reactions using real gas equations of state in the H–He–C–N–O–Si system for TOI-421b, a canonical hot sub-Neptune potentially hosting a magma ocean. We find that H and N are the most soluble in magma, followed by He and C. We fit real gas equations of state to experimental data on SiH$_4$, and show that, for a fully molten mantle, SiH$_4$ dominates at the MEB under accreted gas metallicity of 1$\times$ solar, but is supplanted by CH$_4$ at 100$\times$ solar. Lower mantle melt fractions lower both magma-derived Si abundances in the envelope and the solubility of H and He in magma, yielding H$_2$- 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 SiH$_4$ remains key, especially when a solar gas is accreted. SiH$_4$/CH$_4$ and Si/C ratios increase with mantle melt fraction and decrease with accreted gas metallicity. The competition between SiH$_4$ and CH$_4$ is therefore diagnostic of metallicity and the presence of magma oceans on sub-Neptunes with equilibrium temperatures below 1000 K. The corollary is that H$_2$- and He-rich, SiH$_4$-deficient and CH$_4$-bearing observable atmospheres may indicate a limited role or absence of magma oceans on sub-Neptunes.
Institutions: Royal Observatory of Belgium, KU Leuven, ETH Zurich
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $3,046
Funders: National Science Foundation, Spine Education and Research Institute, European Cooperation in Science and Technology, Department of Science and Technology, Republic of South Africa, Department of Science and Technology, Ministry of Science and Technology, India, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Belgian Federal Science Policy Office, Fonds Wetenschappelijk Onderzoek, Vlaamse regering, Staatssekretariat für Bildung, Forschung und Innovation
OpenAlex ID: W7124909975