The Fate of Ice Sheets on Near-Synchronously Rotating Land Planets

Abstract

Rocky exoplanets orbiting M dwarf stars are often assumed to be synchronously rotating, such that water-trapping in ice sheets on the permanent nightside may pose a threat to habitability. However, in compact M dwarf systems, gravitational interactions between planets may cause the substellar point to librate or circulate on millennial timescales and sporadically change direction or speed. Here, we use a 3D global climate model to quantify, for the first time, the water-trapping capabilities of these "near"-synchronously rotating exoplanets, with TRAPPIST-1e as our test case. Across a range of synodic day lengths, we find that there is a permanent polar ice reservoir and an increasingly large build-up of lower-latitude transient nightside ice for longer days. A persistent ice-free region develops on the dayside where surface conditions can sustain liquid water. However, this steady-state ice distribution may never be realized since the accumulation timescale may be slower than changes in the spin regime. Additionally, we revise the nightside ice build-up on a synchronously rotating exoplanet using a coupled atmosphere+ice numerical model that accounts for atmospheric lapse rate and isostatic adjustment feedbacks. We find that accounting for this ice-elevation feedback reduces the predicted ice accumulation by a factor of ~3. Taken together, our study suggests that water-trapping in ice sheets is less severe than previously thought for land planets orbiting M dwarf stars.