Retention of Surface Water on Tidally Locked Rocky Planets in the Venus Zone around M Dwarfs

Terrestrial planets within the Venus zone surrounding M-dwarf stars can retain surface ice caps on the perpetual dark side if atmospheric heat transport is inefficient, as suggested by previous global climate simulations. This condition is proposed to play a role in the potential regional habitability of these planets. However, the amount of surface ice may be limited by considering the water condensed from the steam atmosphere in a runaway greenhouse state, and the physical mechanism for triggering the condensation process is not clear. Here, we use a two-column moist radiative–convective–subsiding model to investigate the water condensation process on tidally locked planets from the runaway greenhouse state. We find that the water condensation process is characterized by two distinct equilibrium states under the same incoming stellar flux. The initiation of condensation corresponds to a warm, unstable state exhibiting positive Planck feedback, whereas the termination phase corresponds to a cold, stable state exhibiting negative Planck feedback. We further show that the surface water mass in the collapsed state decreases with the incoming stellar flux, background surface pressure, and optical thickness of noncondensable greenhouse gases, with a global equivalent depth of less than ∼20 cm. Our two-column approach provides a straightforward way to understand the water evolution on Venus zone planets around M dwarfs.