  Atmospheric collapse is likely to be of fundamental importance to tidally
locked rocky exoplanets but remains understudied. Here, general results on the
heat transport and stability of tidally locked terrestrial-type atmospheres are
reported. First, the problem is modeled with an idealized 3D general
circulation model (GCM) with gray gas radiative transfer. It is shown that over
a wide range of parameters the atmospheric boundary layer, rather than the
large-scale circulation, is the key to understanding the planetary energy
balance. Through a scaling analysis of the interhemispheric energy transfer,
theoretical expressions for the day-night temperature difference and surface
wind speed are created that reproduce the GCM results without tuning. Next, the
GCM is used with correlated-k radiative transfer to study heat transport for
two real gases (CO2 and CO). For CO2, empirical formulae for the collapse
pressure as a function of planetary mass and stellar flux are produced, and
critical pressures for atmospheric collapse at Earth's stellar flux are
obtained that are around five times higher (0.14 bar) than previous gray gas
estimates. These results provide constraints on atmospheric stability that will
aid in future interpretation of observations and exoplanet habitability
modeling.
