M-Earth Habitability Enhanced by Basal Magma Ocean

 Authors: K. Moore, N. Cowan, C.-E. Boukaré, C. Goldblatt

Earth-like planets orbiting M-dwarf stars, M-Earths, are likely the best targets to search for signatures of life in the foreseeable future. Life as we know it requires liquid surface water. The habitability of M-Earths is jeopardized by the loss of water to space: high bolometric and extreme ultraviolet flux from young M dwarfs can drive the loss of 1--10 Earth Oceans. 

To investigate the fate of surface water on M-Earths, we develop a 0-D "box model" for Earth-mass terrestrial exoplanets tracking water loss to space and exchange between reservoirs during an early global magma ocean phase, and during the longer deep-water cycling phase, for planets orbiting different host stars at various distances within the habitable zone. In our coupled simulations including a magma ocean, deep-water cycling, and water loss to space, a key feature is the relative duration of the magma ocean and runaway greenhouse. These timescales can inform the distinction between completely desiccated planets, planets with desiccated mantles but substantial surface water, and planets with significant water inventories in the mantle. A longer magma ocean phase --- or a long-lived basal magma ocean --- helps M-Earths retain water. 

The high solubility of water within a silicate magma ocean provides a barrier against significant loss to space during the earliest, most active stage of the host M dwarf.  We find that the presence of a long-lived basal magma ocean can maintain and recover habitable conditions on M-Earths at the inner edge of the habitable zone around late M-dwarf stars --- these planets would otherwise be desiccated if they form with less than 10 terrestrial oceans of water.