Can Water Be Frozen on the Nightside of Tidal-locking Exoplanets?

It had been worried about that very uneven heating on tidal-locking exoplanets in the habitable zone around M dwarfs would cause atmosphere and water frozen on the nightside of such exoplanets. Previous studies have demonstrated that atmospheric and oceanic circulations are able to transport sufficient heat to warm the nightside and prevent atmosphere collapse there. However, it remains a question of whether water could be completely frozen on the nightside. If it happens, it would cause exoplanets in the habitable zone of M dwarfs uninhabitable because liquid water on the dayside is essential for exoplanets' habitability. Here, we apply a coupled atmospheric and oceanic general circulation model and a three-dimensional ice-sheet model to this problem. It is found that for an ocean planet surface winds drive sea ice toward the dayside and the ocean carries heat toward the nightside, both of which keep the nightside sea ice thin. Our results show that the thickness of nightside sea ice is only about 10 m or less. Thus, nightside water trapping on a water-world should not be significant. We also test whether a large ice sheet could grow on a nightside super-continent using an ice sheet model driven by the climate model output. We find that for weak precipitation generated by the climate model the ice-sheet thickness is strongly dependent on the geothermal heat flux, and could reach 1-2 km if the geothermal heat is similar to Earth's or lower. These suggest that tidal-locking exoplanets with low geothermal flux, continents that cover much of their surface area, and relatively shallow oceans may experience extended periods of dayside desiccation as plate tectonics drives continents onto the nightside, and that complete nightside water trapping would not happen if exoplanets have fairly deep and extensive oceans.