14:15 〜 14:30
[PCG19-13] Reduced water loss by photochemical reactions on terrestrial planets in runaway greenhouse conditions around pre-main-sequence M dwarfs
キーワード：系外惑星、暴走温室、水、大気進化、大気散逸
It is an important question in planetary science whether terrestrial planets currently in the habitable zone around M dwarfs, whose detection and atmospheric spectroscopic observation are easier than those around Sun-like G dwarfs, could have habitable environments. When these planets formed with sufficient water, they are estimated to have been in runaway greenhouse conditions for up to ~ 1 Gyr due to the long-term M dwarfs’ pre-main sequence phase (e.g., Baraffe et al. 1998; Kopparapu et al. 2014). A planet under this condition forms an H2O-dominated atmosphere due to a higher surface temperature than the critical temperature of H2O. Then, the UV irradiation from the host star promotes the significant H2O photolysis, followed by an escape of hydrogen to space due to its efficient absorption of the stellar XUV irradiation (Kasting 1988). The significant water loss by this mechanism in runaway greenhouse conditions encumbers the formation of habitable environments on terrestrial planets currently in the habitable zone around M dwarfs.
Previous studies assumed a sufficient H2O photolysis in the H2O-dominated atmosphere to estimate the amount of water loss (e.g., Luger & Barnes 2015; Tian & Ida 2015; Bolmont et al. 2017). They supposed that the escape rate of hydrogen is limited by the stellar XUV flux inducing hydrodynamic escape or the diffusion through an oxygen-rich layer formed by the photolysis of H2O. However, the efficiency of the H2O photolysis and resulting water loss are likely to be reduced by photochemical reactions in the H2O-dominated atmosphere, e.g., photochemical H2O reproduction and UV shielding due to chemical products of the H2O photolysis. In this study, we evaluate the effect of photochemical reactions in the H2O-dominated atmosphere on the water loss on terrestrial planets in runaway greenhouse conditions around pre-main-sequence M dwarfs.
We apply a 1-D photochemical model based on PROTEUS (Nakamura et al. 2023), which solves continuity-transport equations including chemical production and loss, to an Earth-like planet in a runaway greenhouse condition orbiting around an M dwarf. The initial atmospheric condition is a pure H2O atmosphere with the surface pressure equivalent to a mass of the terrestrial ocean (TO: 1.39×1021 kg). 40 chemical reactions are considered for 10 H- and O-bearing species, i.e., H2O, H, OH, H2, O(1D), O3, O2, O, HO2, and H2O2, following Chaffin et al. (2017). To consider the photolysis rate profiles of the chemical species, we use the stellar UV spectrum estimated for TRAPPIST-1 in the wavelength range from 115 nm to 1000 nm (Wilson et al. 2021). For the upper boundary conditions, we assume a diffusion-limited escape for H and the same effusion velocity for H2 as H. To obtain a quasi-steady state, the numerical integration time is set to over 1 Myr, which is longer than the diffusion and chemical timescales.
We find that, in an Earth-like terrestrial planet in a runaway greenhouse condition around a pre-main-sequence M dwarf, the efficiency of H2O photolysis and the water loss are reduced by the efficient H2O reproduction from OH and HO2, and by the UV shielding of O2. Then, only about 15% of the total photon flux involved in the H2O photolysis is used for the water loss; the water loss rate is reduced to ~ 7 (TO/Gyr). Compared to the estimation from Luger and Barnes (2015), this value is several times lower than the diffusion limited value, and several to several hundred times lower than the energy-limited escape rate. This result implies that terrestrial planets currently in the habitable zone around M dwarfs are more likely to retain surface water than previously estimated.
Previous studies assumed a sufficient H2O photolysis in the H2O-dominated atmosphere to estimate the amount of water loss (e.g., Luger & Barnes 2015; Tian & Ida 2015; Bolmont et al. 2017). They supposed that the escape rate of hydrogen is limited by the stellar XUV flux inducing hydrodynamic escape or the diffusion through an oxygen-rich layer formed by the photolysis of H2O. However, the efficiency of the H2O photolysis and resulting water loss are likely to be reduced by photochemical reactions in the H2O-dominated atmosphere, e.g., photochemical H2O reproduction and UV shielding due to chemical products of the H2O photolysis. In this study, we evaluate the effect of photochemical reactions in the H2O-dominated atmosphere on the water loss on terrestrial planets in runaway greenhouse conditions around pre-main-sequence M dwarfs.
We apply a 1-D photochemical model based on PROTEUS (Nakamura et al. 2023), which solves continuity-transport equations including chemical production and loss, to an Earth-like planet in a runaway greenhouse condition orbiting around an M dwarf. The initial atmospheric condition is a pure H2O atmosphere with the surface pressure equivalent to a mass of the terrestrial ocean (TO: 1.39×1021 kg). 40 chemical reactions are considered for 10 H- and O-bearing species, i.e., H2O, H, OH, H2, O(1D), O3, O2, O, HO2, and H2O2, following Chaffin et al. (2017). To consider the photolysis rate profiles of the chemical species, we use the stellar UV spectrum estimated for TRAPPIST-1 in the wavelength range from 115 nm to 1000 nm (Wilson et al. 2021). For the upper boundary conditions, we assume a diffusion-limited escape for H and the same effusion velocity for H2 as H. To obtain a quasi-steady state, the numerical integration time is set to over 1 Myr, which is longer than the diffusion and chemical timescales.
We find that, in an Earth-like terrestrial planet in a runaway greenhouse condition around a pre-main-sequence M dwarf, the efficiency of H2O photolysis and the water loss are reduced by the efficient H2O reproduction from OH and HO2, and by the UV shielding of O2. Then, only about 15% of the total photon flux involved in the H2O photolysis is used for the water loss; the water loss rate is reduced to ~ 7 (TO/Gyr). Compared to the estimation from Luger and Barnes (2015), this value is several times lower than the diffusion limited value, and several to several hundred times lower than the energy-limited escape rate. This result implies that terrestrial planets currently in the habitable zone around M dwarfs are more likely to retain surface water than previously estimated.