The sulfur species in hot rocky exoplanet atmospheres

The first JWST observations of hot Jupiters showed an unexpected detection of SO in their hydrogen-rich atmospheres. We investigate how much sulfur can be expected in the atmospheres of rocky exoplanets and which sulfur molecules can be expected to be most abundant and detectable by transmission spectroscopy. We run thermochemical equilibrium models at the crust–atmosphere interface, considering surface temperatures 500–5000 K, surface pressures 1–100 bar, and various sets of element abundances based on common rock compositions. Between 1000 and 2000 K, we find gaseous sulfur concentrations of up to 25% above the rock in our models. SO₂, SO, H₂S, and S₂ are by far the most abundant sulfur molecules. SO2 shows potentially detectable features in transmission spectra at about 4 μm, between 7 and 8 μm, and beyond 15 μm. In contrast, the sometimes abundant H₂S molecule is difficult to detect in these spectra, which are mostly dominated by H₂O and CO₂. Although the molecule PS only occurs with concentrations  < 300 ppm, it can cause a strong absorption feature between 0.3 and 0.65 μm in some of our models for high surface pressures. The detection of sulfur molecules would enable a better characterization of the planetary surface.