Monosilane Worlds: Sub-Neptunes with Atmospheres Shaped by Reduced Magma Oceans

High-precision infrared spectroscopic measurements now enable detailed characterization of sub-Neptune atmospheres, potentially providing constraints on their interiors. Motivated by this, atmospheric models have been developed to explore chemical interactions between hydrogen-dominated atmospheres and possibly underlying magma oceans with various redox states.

Recent models have predicted monosilane (SiH₄) as a potential atmospheric species derived from magma oceans in sub-Neptunes, but suggested that it is highly depleted in the observable atmospheric layers.

Here, we propose that SiH₄ can persist throughout the atmospheres of sub-Neptunes with FeO-free reduced magma oceans by considering the dissolution of H₂O into the magma oceans, a factor not accounted for in previous models. We construct a one-dimensional atmospheric model to simulate the chemical equilibrium composition of hydrogen-dominated atmospheres overlying FeO-free magma oceans, incorporating H-O-Si chemistry.

Our results show that the dissolution of H₂O enhances the SiH₄ molar fraction to levels of 0.1–10~%, preventing it from reverting to silicates in the upper atmospheric layers. We find that SiH₄-rich atmospheres can exist across a broad parameter space with ground temperatures of 2000–6000~K and hydrogen pressures of 10²–10⁵~bar. We discuss that SiH₄-rich atmospheres could contain the other silanes but lack C-/N-/O-bearing species.

The detection of SiH₄ in future observations of sub-Neptunes would provide compelling evidence for the presence of a rocky core with a reduced magma ocean. However, the accuracy of our model is limited by the lack of data on the non-ideal behavior and radiative properties of SiH₄, highlighting the need for further numerical and laboratory investigations.

Yuichi Ito, Tadahiro Kimura, Kazumasa Ohno, Yuka Fujii, Masahiro Ikoma

Comments: 27 pages, 15 figures, accepted in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2505.03200 [astro-ph.EP] (or arXiv:2505.03200v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.03200
Focus to learn more
Submission history
From: Yuichi Ito
[v1] Tue, 6 May 2025 05:36:12 UTC (9,593 KB)
https://arxiv.org/abs/2505.03200
Astrobiology