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

Thanks to recent advances in infrared spectroscopy, we have entered a new era of detailed atmospheric characterization of sub-Neptunes, potentially providing constraints on their hidden interiors. One possible structure of sub-Neptunes is a rocky core surrounded by a thick hydrogen-dominated atmosphere. The strong blanketing effect of these thick hydrogen-dominated atmospheres can keep the underlying rocky surfaces hot enough to melt and vaporize, leading to gas-magma interactions that may alter the atmospheric composition. Detecting such magma-derived atmospheric species could therefore provide evidence for a rocky interior. Motivated by this, atmospheric models have been developed to explore chemical interactions between hydrogen-dominated atmospheres and 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 have 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. In this study, we investigate the atmospheric composition of sub-Neptunes with reduced FeO-free magma oceans using a one-dimensional atmospheric model based on the chemical equilibrium of H-/O-/Si-bearing species. This model incorporates the vaporization of SiO₂, silicate condensation, and the dissolution of H₂O into the magma ocean. Our results demonstrate that SiH₄ is produced through the reaction between SiO vaporized from magma and H₂, and that the reduction of H₂O due to dissolution shifts the equilibrium further toward SiH₄ production. We find that the effect of water dissolution enhances the atmospheric SiH₄ molar fraction to 0.1-10%, preventing its reversion to silicates in the upper atmospheric layers. Finally, we present the conditions under which large amounts of SiH₄ are produced and discuss the atmospheric spectra of sub-Neptune in such cases. Our results suggest that 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.