Extremely Tenuous, Dusty Terrestrial Atmospheres Remain Stable against Global Collapse: An Argument for Arid Planet Habitability in the Trappist-1 System

Abstract The potential habitability of tidally locked planets orbiting M stars remains uncertain. However, recent observations with JWST and numerous theoretical studies investigating atmospheric evolution over time suggest that thick atmospheres (>1 bar) with abundant surface water are increasingly unlikely. A significant challenge to habitability for tidally locked systems is the prevention of atmospheric collapse. For tenuous atmospheres, this challenge is even more extreme, since low-pressure atmospheres cannot transport heat effectively from the day- to nightside. Previous studies find that collapse is likely for CO 2 atmospheres with surface pressures less than 100 mbar for planets in the habitable zone of Trappist-1, which would significantly reduce the likelihood for long-term habitability in that system. In this study, we find that modest levels of atmospheric mineral dust mitigate this collapse for a tenuous 7.05 mbar atmosphere on a small Mars-sized planet. Dust heating modifies the vertical and zonal thermal gradients and wind sheers. Due to the rapid planetary rotation (1 yr = 6 sols), this change is reflected in the magnitude of rotational components of the global circulation, including the equatorial jet and midlatitude eddies. We find that heat transport by these components is increased in dusty simulations, and atmospheric collapse on the permanent nightside is prevented. This dust-dominated climate is consistent with a planetary atmosphere evolving towards low pressure and high aridity with time. Our findings therefore provide an alternative target to Earth-like climates around M stars and reopens the possibility of habitability in M star systems.