2022ApJ...934..137Y

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-star 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 X-ray and extreme ultraviolet (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 1D 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 OH⁺: 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.