Keeping M-Earths Habitable in the Face of Atmospheric Loss by Sequestering Water in the Mantle

Abstract:

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 these 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 radiation from their host M dwarfs, which is most significant early in the system's lifetime. The XUV radiation can photodissociate water molecules in the atmosphere and drive loss to space, critically impacting the planetary climate and potentially leading to complete removal of a planet's oceans. We create a coupled model of water cycling and atmospheric loss to determine surface water content over time, and whether water sequestered within the mantle of an M-Earth can be degassed to rehydrate a desiccated surface once loss to space diminishes. We account for the diversity of tectonic modes of exoplanets, from Earth-like plate-tectonic planets to Venus-like stagnant-lid planets.