Keeping M-Earths Habitable in the Face of Atmospheric Loss from a 1-D Atmosphere by Sequestering Water in the Mantle

M dwarfs are the most common stars in the Galaxy, and many are host to temperate Earth-like planets, which we refer to as “M-Earths”. On Earth, planetary water is partitioned between surface and interior reservoirs — the oceans and mantle, respectively. Water is exchanged between reservoirs on geological timescales through plate-tectonics-driven regassing from surface to mantle by subduction of hydrated oceanic crust, and degassing from mantle to surface by mid-ocean ridge volcanism. However, water on the surface of M-Earths can additionally be lost to space due to the substantial XUV irradiation from their host M dwarfs, which is most significant early in the system’s lifetime. The XUV radiation can photodissociate water molecules high in the atmosphere and drive substantial loss to space, critically impacting the planetary climate and potentially leading to a desiccated surface absent of oceans. We couple a model of terrestrial water cycling with a 1-D radiative-convective atmosphere to determine surface water content over time. These results complement our previous model — in which we found that water sequestered within an M-Earth’s mantle can be degassed to rehydrate a desiccated surface when loss to space is parameterized as a decreasing exponential — and compare our new model results.