  The first JWST observations of hot Jupiters showed an unexpected detection of
SO2 in their hydrogen-rich atmospheres. We investigate how much sulphur can be
expected in the atmospheres of rocky exoplanets and which sulphur molecules can
be expected to be most abundant and detectable by transmission spectroscopy. We
run thermo-chemical equilibrium models at the crust-atmosphere interface,
considering surface temperatures 500 to 5000 K, surface pressures 1 to 100 bar,
and various sets of element abundances based on common rock compositions.
Between 1000 K and 2000 K, we find gaseous sulphur concentrations of up to 25
percent above the rock in our models. SO2, SO, H2S and S2 are by far the most
abundant sulphur molecules. SO2 shows potentially detectable features in
transmission spectra at about 4 micron, between 7 and 8 micron, and beyond 15
micron. 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 below 300 ppm, it can cause a
strong absorption feature between 0.3 and 0.65 micron in some of our models for
high surface pressures. The detection of sulphur molecules would enable a
better characterisation of the planetary surface.
