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

TL;DR

This study models the atmospheres of sub-Neptunes with reduced magma oceans, showing that SiH4 can persist in detectable amounts, which could reveal the presence of magma oceans and rocky cores in these planets.

Contribution

It introduces a new atmospheric model considering H2O dissolution effects, demonstrating the potential for SiH4 to be a detectable atmospheric signature of magma oceans.

Findings

SiH4 can reach 0.1-10% molar fraction in atmospheres overlying reduced magma oceans.

SiH4-rich atmospheres are possible at temperatures 2000-6000 K and pressures 10^2-10^5 bar.

Detection of SiH4 would indicate a reduced magma ocean and rocky core in sub-Neptunes.

Abstract

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 (SiH$_4$) 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 SiH$_4$ can persist throughout the atmospheres of sub-Neptunes with FeO-free reduced magma oceans by considering the dissolution of H$_2$O 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 H$_2$O enhances the SiH$_4$ molar fraction to levels of 0.1--10~\%, preventing it from reverting to silicates in the upper atmospheric layers. We find that SiH$_4$-rich atmospheres can exist across a broad parameter space with ground temperatures of 2000--6000~K and hydrogen pressures of 10$^2$--10$^5$~bar. We discuss that SiH$_4$-rich atmospheres could contain the other silanes but lack C-/N-/O-bearing species. The detection of SiH$_4$ 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 SiH$_4$, highlighting the need for further numerical and laboratory investigations.