Atmospheric collapse and maintenance of habitable environment on tidally-locked exoplanets using a 3D-GCM

Habitable planets around M dwarfs are remarkable targets as they are among the least difficult terrestrial-size planets to detect and characterize. However, the climate characteristics on these planets are thought to be different from solar system terrestrial planets in particular because these planets are expected to be in a tidally-locked state, with a permanent irradiated hemisphere (dayside) and opposite one (nightside). If a planet orbits far from its host star, nightside temperatures can become extremely low, which leads volatile species such as CO2 and CH4 in the atmosphere to condense onto the surface. This phenomenon, known as atmospheric collapse, is thought to prevent habitable environment since removal from the atmosphere (especially CO2) decreases the stabilizing greenhouse effect.We investigated the relationship between atmospheric collapse and habitability using the Generic Planetary Climate Model (Generic PCM), a 3-D global climate model historically developed at the Laboratoire de Météorologie Dynamique (LMD), by changing stellar insolation and CO2 partial pressure. The onset of atmospheric collapse for each case is decided by the surface amount of condensed CO2. As a result, we found cases where locally habitable environment on the dayside remains during/after the atmospheric collapse event in spite of the decrease in greenhouse effect. This is because the decrease in atmospheric mass and thus in atmospheric pressure makes day-night atmospheric heat transport less efficient, resulting in less of energy by insolation on the dayside. In addition, lower background gas cases were more habitable (i.e., more likely to have surface liquid water) over a wide range of CO2 partial pressure. While high background gas pressure is usually considered to enhance the greenhouse effect due to pressure broadening, on tidally-locked planets less background gas contributes to an increase in dayside temperatures.To conclude, our results provide a new picture of the relationship between atmospheric collapse and habitability on tidally-locked, cool planets.