  As the characterization of exoplanet atmospheres proceeds, providing insights
into atmospheric chemistry and composition, a key question is how much deeper
into the planet we might be able to see from its atmospheric properties alone.
For small planets with modest atmospheres and equilibrium temperatures, the
first layer below the atmosphere will be their rocky surface. For such warm
rocky planets, broadly Venus-like planets, the high temperatures and moderate
pressures at the base of their atmospheres may enable thermochemical
equilibrium between rock and gas. This links the composition of the surface to
that of the observable atmosphere. Using an equilibrium chemistry code, we find
a boundary in surface pressure-temperature space which simultaneously separates
distinct mineralogical regimes and atmospheric regimes, potentially enabling
inference of surface mineralogy from spectroscopic observations of the
atmosphere. Weak constraints on the surface pressure and temperature also
emerge. This regime boundary corresponds to conditions under which SO2 is
oxidized and absorbed by calcium-bearing minerals in the crust, thus the two
regimes reflect the sulphidation of the crust. The existence of these
atmospheric regimes for Venus-like planets is robust to plausible changes in
the elemental composition. Our results pave the way to the prospect of
characterizing exoplanetary surfaces as new data for short period rocky planet
atmospheres emerge.

Earth and Planetary Astrophysics