Deep-Water Cycling & Loss to Space on M-Earths: From Magma Ocean Through Plate Tectonics

Rocky planets orbiting M-dwarf stars are more abundant – and easier to detect – than those orbiting Sun-like stars. However, Earth-like planets orbiting M dwarfs (hereafter, "M-Earths") are susceptible to significant water loss to space, due to their tighter orbits and the enhanced stellar activity of their host star which may lead to complete surface desiccation. Specifically, emission in the X-ray and extreme ultraviolet, collectively known as the "XUV", photodissociates water molecules and drives its loss to space. Since surface liquid water is the definition of planetary habitability, we aim to predict surface water inventories over time for a variety of potential M-Earths, using a coupled model of water cycling and atmospheric loss to space. We consider a pure water vapour atmosphere and account for the early magma ocean, within which water is highly soluble, and which is likely concurrent with a runaway greenhouse phase. Our simulations then continue through a deep water-cycle mediated by active plate tectonics, akin to the modern-day Earth. Since water is highly soluble within the silicate melts of the magma, the duration of the magma ocean and the runaway greenhouse phases are crucial in determining the amount of water lost to space during the earliest stages of the M-Earth's lifetime. Once the magma ocean solidifies, we assume the M-Earth shifts into a plate-tectonics-driven deep-water cycling mode, with degassing from interior to surface at mid-ocean ridges and regassing from surface to mantle through subduction of hydrated oceanic crust. Atmospheric loss during this stage occurs at a much lower rate and decreases with time, roughly tracking the decreasing XUV output of the host M dwarf; the steam atmosphere mostly condenses into a surface ocean, so less water is available in the upper atmosphere to be photodissociated and lost to space. We also test a model accounting for a potentially long-lived basal magma ocean, an additional water reservoir that could exist below the solid mantle following solidification of the global magma ocean and slowly inject water into the overlying mantle for billions of years.