Narrow Loophole for H2-Dominated Atmospheres on Habitable Rocky Planets around M Dwarfs

Habitable rocky planets around M dwarfs that have H-2-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 H-2-dominated atmospheres that are consistent with habitable conditions cannot be too massive, and a moderate-sized H-2-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 H-2 and form a steady-state H-2-dominated atmosphere. For rocky planets of 1-7 M (circle plus) and early-, mid-, and late M-type dwarfs, the expected volcanic outgassing rates from a reduced mantle fall short of the escape rates by > similar to 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 H-2-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, H-2-dominated atmospheres on rocky planets of M dwarfs that would support habitable environments.

Automated summary

If habitable rocky planets with H-2-dominated atmospheres exist around M dwarfs, the James Webb Space Telescope (JWST) can be used to study them in detail. However, these atmospheres cannot be too massive and are prone to atmospheric escape due to irradiation. Volcanic outgassing and serpentinization are two potential ways to maintain a steady-state H-2-dominated atmosphere, but they fall short of the necessary rates. It is unlikely to find moderate-size, H-2-dominated atmospheres on rocky planets around M dwarfs that would support habitable environments.