Observability of silicates in volatile atmospheres of super-Earths and sub-Neptunes

Exploring the edge of the evaporation desert

¹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333CA Leiden, The Netherlands
e-mail: zilinskas@strw.leidenuniv.nl
² SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands

Received: 21 November 2022
Accepted: 12 January 2023

Abstract

Many of the confirmed short-period super-Earths and smaller sub-Neptunes are sufficiently irradiated for the surface silicates to be sustained in a long-lasting molten state. While there is no direct evidence of magma ocean influence on exoplanets, theory suggests that, due to outgassing and diverse evolution paths, a wide range of resulting atmospheric compositions should be possible. Atmospheric contamination caused by the outgassing of the underlying magma ocean is potentially detectable using low-resolution spectroscopy. The James Webb Space Telescope provides the necessary spectral coverage and sensitivity to characterise smaller planets, including lava worlds. In light of this, we assess the observability of outgassed silicates submerged in volatile atmospheres on the edge of the evaporation valley. By placing a hypothetical 2 R_⊕ planet around a Sun-like star, we self-consistently model in 1D a wide range of potential atmospheric compositions, including thermal structure and outgassing. We focus on atmospheres rich in H, C, and N. We assess the diverse chemistry of silicates and volatiles, and what features of outgassed species could be detected via emission spectroscopy using MIRI LRS. Results indicate that even for substantial volatile envelopes, strong in infrared opacity, the presence of silicates causes deep thermal inversions that affect emission. Similar to pure lava worlds, SiO remains the only outgassed species with major infrared bands at 5 and 9 µm. However, even a small amount of volatiles, especially of H₂O and H⁻, may hinder its observability. We also find that the C/O ratio plays a large role in determining the abundance of SiO. Detecting SiO on a strongly irradiated planet could indicate an atmosphere with high metallicity and a low C/O ratio, which may be a result of efficient interaction between the atmosphere and the underlying melt.