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 nonideal behavior and radiative properties of SiH₄, highlighting the need for further numerical and laboratory investigations.