Atmospheres & Climate

Narrow Loophole for H2-dominated Atmospheres On Habitable Rocky Planets Around M Dwarfs

By Keith Cowing
April 27, 2023
Filed under , , , , ,
Narrow Loophole for H2-dominated Atmospheres On Habitable Rocky Planets Around M Dwarfs
Comparison between sources and sinks of H2 on rocky planets around M dwarfs. The grey areas show the range of escape rates depending on the type of the host star. The dark grey area is for a planet that locates at the 1-AU equivalent distance, while the light grey area is for a planet that locates 2.6 times farther away (i.e., receiving 7 times less irradiation). The outgassing rates encapsulate the plausible range from a highly reduced mantle (informed by Figure 1), with the lower bound corresponding to xH = 0.005 wt % and t βˆ’1 scaling, and the higher bound corresponding to xH = 0.02 wt % and t βˆ’2 scaling. The rate of serpentinization is a very generous upper limit (Section 4). — astro-ph.EP

Habitable rocky planets around M dwarfs that have H2-dominated atmospheres, if they exist, would permit characterizing habitable exoplanets with detailed spectroscopy using JWST, owing to their extended atmospheres and small stars.

However, the H2-dominated atmospheres that are consistent with habitable conditions cannot be too massive, and a moderate-size H2-dominated atmosphere will lose mass to irradiation-driven atmospheric escape on rocky planets around M dwarfs. We evaluate volcanic outgassing and serpentinization as two potential ways to supply H2 and form a steady-state H2-dominated atmosphere.

For rocky planets of 1-7 Earth mass and early, mid, and late M dwarfs, the expected volcanic outgassing rates from a reduced mantle fall short of the escape rates by >~1 order of magnitude, and a generous upper limit of the serpentinization rate is still less than the escape rate by a factor of a few.

Special mechanisms that may sustain the steady-state H2-dominated atmosphere include direct interaction between liquid water and mantle, heat-pipe volcanism from a reduced mantle, and hydrodynamic escape slowed down by efficient upper-atmospheric cooling. It is thus unlikely to find moderate-size, H2-dominated atmospheres on rocky planets of M dwarfs that would support habitable environments.

Renyu Hu, Fabrice Gaillard, Edwin Kite

Comments: Accepted for publication in ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2304.13659 [astro-ph.EP] (or arXiv:2304.13659v1 [astro-ph.EP] for this version)
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Submission history
From: Renyu Hu
[v1] Wed, 26 Apr 2023 16:34:12 UTC (299 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) πŸ––πŸ»