  The James Webb Space Telescope (JWST) will open up the possibility of
comprehensively measuring the emission spectra of rocky exoplanets orbiting M
dwarfs to detect and characterize their atmospheres. In preparation for this
opportunity, we present model atmospheres for three M-dwarf planets
particularly amenable to secondary eclipse spectroscopy -- TRAPPIST-1b, GJ
1132b, and LHS 3844b. Using three limiting cases of candidate atmospheric
compositions (pure H2O, pure CO2 and solar abundances) we calculate
temperature-pressure profiles and emission and reflection spectra in
radiative-convective equilibrium, including the effects of a solid surface. We
find that the atmospheric radiative transfer is significantly influenced by the
cool M-star irradiation; H2O and CO2 absorption bands in the near-infrared are
strong enough to absorb a sizeable fraction of the incoming stellar light at
low pressures, which leads to temperature inversions in the upper atmosphere.
The non-gray band structure of gaseous opacities in the infrared is hereby an
important factor. Opacity windows are muted at higher atmospheric temperatures,
so we expect temperature inversions to be common only for sufficiently cool
planets. We also find that pure CO2 atmospheres exhibit lower overall
temperatures and stronger reflection spectra compared to models of the other
two compositions. We estimate that for GJ 1132b and LHS 3844b we should be able
to distinguish between different atmospheric compositions with JWST. The
emission lines from the predicted temperature inversions are currently hard to
measure, but high resolution spectroscopy with future Extremely Large
Telescopes may be able to detect them.
