Silane-Methane Competition in Sub-Neptune Atmospheres As A Diagnostic Of Metallicity And Magma Oceans

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 (100 bar-100 kbar) and atmospheres (1 mbar-100 bar).

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-421 b, 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 atmosphere (1 mbar-100 bar), we find that condensation of β-quartz ‘clouds’ depletes Si-bearing gases, although SiH₄ remains abundant at solar metallicity. SiH₄/CH₄ ratios increase with mantle melt fraction and decrease with metallicity. These effects also deplete H₂ via CH₄ and SiH₄ formation and the dissolution of H₂ into magma.

The competition between SiH₄ and CH₄ presents a diagnostic of metallicity and magma oceans. The corollary is that H₂– and He-rich, SiH₄– and CH₄-poor (<5%) atmospheres may indicate a limited role or absence of magma oceans on sub-Neptunes.

Kaustubh Hakim, Dan J. Bower, Fabian Seidler, Paolo A. Sossi

Comments: 15 pages, 13 figures, submitted
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.19235 [astro-ph.EP] (or arXiv:2508.19235v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.19235
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Submission history
From: Kaustubh Hakim
[v1] Tue, 26 Aug 2025 17:55:13 UTC (643 KB)
https://arxiv.org/abs/2508.19235

Astrobiology