Extrasolar Planets

Less Effective Hydrodynamic Escape Of H2-H2O Atmospheres On Terrestrial Planets Orbiting Pre-main Sequence M Dwarfs

By Keith Cowing
July 14, 2022
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Less Effective Hydrodynamic Escape Of H2-H2O Atmospheres On Terrestrial Planets Orbiting Pre-main Sequence M Dwarfs
Atmospheric profiles when the XUV flux is 100 times the present. (a) Mean velocity for six choices of the basal H2O/H2 mole ratio. The dots represent the transonic points. The triangles indicate the exobase. The black line represents the local escape velocity. (b) Temperature for six basal H2O/H2 ratios. (c) Number density of neutral species for H2O/H2 = 0.01. (d) Number density of ion species for H2O/H2 = 0.01. The radial distance is shown in the planet radius ????????.

Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway greenhouse condition because of a slow decline in host-stellar luminosity in its pre-main sequence phase.

Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar XUV irradiation. However, the atmospheric escape rates remain highly uncertain due partly to a lack of understanding of the effect of radiative cooling in the escape outflows.

Here we carry out 1-D hydrodynamic escape simulations for an H2-H2O atmosphere on a planet with mass of 1M⊕ considering radiative and chemical processes to estimate the atmospheric escape rate and follow the atmospheric evolution during the early runaway greenhouse phase.

We find that the atmospheric escape rate decreases with the basal H2O/H2 ratio due to the energy loss by the radiative cooling of H2O and chemical products such as OH and H+3: the escape rate of H2 becomes one order of magnitude smaller when the basal H2O/H2=0.1 than that of the pure hydrogen atmosphere.

The timescale for H2 escape exceeds the duration of the early runaway greenhouse phase, depending on the initial atmospheric amount and composition, indicating that H2 and H2O could be left behind after the end of the runaway greenhouse phase. Our results suggest that temperate and reducing environments with oceans could be formed on some terrestrial planets around M dwarfs.

Tatsuya Yoshida, Naoki Terada, Masahiro Ikoma, Kiyoshi Kuramoto

Comments: 15 pages, 7 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2207.06570 [astro-ph.EP] (or arXiv:2207.06570v1 [astro-ph.EP] for this version)
Submission history
From: Tatsuya Yoshida
[v1] Thu, 14 Jul 2022 00:23:23 UTC (493 KB)

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