No Snowball Cycles at the Outer Edge of the Habitable Zone for Habitable Tidally Locked Planets

Planets orbiting within the habitable zones of M stars are prime targets for future observations, which motivates a greater understanding of how tidal locking can affect planetary habitability. In this Letter we will consider the effect of tidal locking on limit cycling between snowball and warm climate states, which has been suggested could occur for rapidly rotating planets in the outer regions of the habitable zone with low CO2 outgassing rates. Here, we use a 3D Global Climate Model that calculates silicate-weathering to show that tidally locked planets with an active carbon cycle will not experience limit cycling between warm and snowball states. Instead, they smoothly settle into "Eyeball" states with a small unglaciated substellar region. The size of this unglaciated region depends on the stellar irradiation, the CO2 outgassing rate, and the continental configuration. Furthermore, we argue that a tidally locked habitable zone planet cannot stay in a snowball state for a geologically significant time. This may be beneficial to the survival of complex life on tidally locked planets orbiting the outer edge of their stars, but might also make it less likely for complex life to arise.