Day-night Temperature Gradients and Atmospheric Collapse on Synchronously Rotating Terrestrial Planets

Abstract:

Terrestrial exoplanets orbiting small host stars are abundant and are also the most promising observational targets for finding life outside our Solar system. Due to their close-in orbits, these planets experience significant tidal interactions with their host stars and will tend to evolve towards spin-orbit resonances or synchronous rotation (=tidally locked). Synchronous rotation has a number of interesting implications for habitability, including the potential for atmospheric collapse on the night side if the surface temperature drops below the condensation point of the gases in the atmosphere. To understand the habitability of synchronously rotating planets, it is therefore important to work out a theory of their temperature and wind structure. Many of these planets will be rotating slowly enough that the well-known weak-temperature-gradient theory holds in the free atmosphere, but even for these planets this theory does not constrain the maximum surface temperature gradient, the planets' thermal phase curve signature, or the threshold for atmospheric collapse. Here we study tidally locked planets using theory and a large array of simulations in a global climate model (GCM) with grey radiative transfer and a full boundary layer scheme. We derive a theory for surface temperatures and atmospheric circulation on synchronously rotating planets that allows us to predict the night-side surface temperature and determine whether atmospheric collapse will occur. We find that atmospheric collapse is sensitive to both the ratio of the Rossby radius to the planetary radius and the ratio of the surface drag timescale to the radiative cooling timescale.