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 is susceptible to 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 the terrestrial deep-water cycle with a 1-D atmosphere to calculate the loss rate of water to space, and to determine surface water content over time. The results of this new model 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. Since our previous model was a simple two-box model with water being lost directly from the surface, we expect that our new model — in which water is now present throughout the vertical 1-D atmosphere and can be lost above the exobase — should limit the total water lost to space, and be less likely to lead to complete surface desiccation.