The Role of Magma Oceans in Maintaining Surface Water on Rocky Planets Orbiting M-Dwarfs

By Keith Cowing
Status Report
August 3, 2023
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The Role of Magma Oceans in Maintaining Surface Water on Rocky Planets Orbiting M-Dwarfs
Flowchart illustrating the three possible stages in our box model of M-Earth evolution. (a) Surface magma ocean (MO). We assume bottom–up solidification of the MO lasting as long as the runaway greenhouse (𝜏MO = 𝜏RG). As it solidifies from the bottom–up, the magma ocean eventually becomes saturated with water, and excess water is degassed into a steam atmosphere, from which it may be lost to space through energy-limited escape. (b) Plate-tectonicsdriven deep-water cycling including a pure water vapour atmosphere. Water is photodissociated into hydrogen and oxygen high in the atmosphere. Hydrogen may then be lost to space. Water is degassed from mantle to surface through mid-ocean ridge volcanism and regassed from the surface to the mantle through subduction of hydrated oceanic crust. (c) Water cycling in the presence of a basal magma ocean (BMO). After MO solidification, a residual BMO remains below the solid mantle (Labrosse et al. 2007). While the BMO is present, water may be degassed/regassed, following our deep-water cycling parameterization, or lost to space. Additionally, water is slowly injected into the solid mantle at a constant rate until the BMO completely solidifies. Once the basal magma ocean solidifies at 𝜏BMO, the M-Earth evolves from the basal magma ocean model to the deep-water cycling model for the remainder of the simulation. Hence, the two evolutionary pathways are (a)-(b) and (a)-(c)-(b). — astro-ph.EP

Earth-like planets orbiting M-dwarf stars, M-Earths, are currently the best targets to search for signatures of life. Life as we know it requires water. The habitability of M-Earths is jeopardized by water loss to space: high flux from young M-dwarf stars can drive the loss of 3–20 Earth oceans from otherwise habitable planets.

We develop a 0-D box model for Earth-mass terrestrial exoplanets, orbiting within the habitable zone, which tracks water loss to space and exchange between reservoirs during an early surface magma ocean phase and the longer deep-water cycling phase. A key feature is the duration of the surface magma ocean, assumed concurrent with the runaway greenhouse. This timescale can discriminate between desiccated planets, planets with desiccated mantles but substantial surface water, and planets with significant water sequestered in the mantle.

A longer-lived surface magma ocean helps M-Earths retain water: dissolution of water in the magma provides a barrier against significant loss to space during the earliest, most active stage of the host M-dwarf, depending on the water saturation limit of the magma. Although a short-lived basal magma ocean can be beneficial to surface habitability, a long-lived basal magma ocean may sequester significant water in the mantle at the detriment of surface habitability.

We find that magma oceans and deep-water cycling can maintain or recover habitable surface conditions on Earth-like planets 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.

Keavin Moore, Nicolas B. Cowan, Charles-Édouard Boukaré

Comments: 14 pages, 5 figures, re-submitted to MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2308.00585 [astro-ph.EP]
(or arXiv:2308.00585v1 [astro-ph.EP] for this version)
Submission history
From: Keavin Moore
[v1] Tue, 1 Aug 2023 15:02:27 UTC (957 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻