1:45 PM - 3:15 PM
[PEM11-P08] Limitation of escape of H2O-dominated atmospheres on terrestrial planets in the runaway greenhouse phase around M dwarfs by less effective H2O photodissociation
Keywords:exoplanets, runaway greenhouse, water, evolution, escape, photodissociation
Terrestrial planets currently in the Habitable Zone (HZ) around M dwarfs, which are statically more plentiful and easier to detect than those orbiting around Sun-like G dwarfs, are expected to have temperate surface environments like Earth. On the other hand, such planets are estimated to have been located on the stellar side from the HZ inner edge up to for ~1 Gyr due to a relatively high host-star luminosity during the long-term M dwarfs' pre-main-sequence phase (Baraffe et al., 2015; Kopparapu et al., 2014).
When a planet formed with sufficient surface water is located inside of the HZ inner edge, it should enter a runaway greenhouse condition to form an H2O-dominated atmosphere (Kasting, 1988). Then, the stellar UV irradiation from the host star promotes the photodissociation of water vapor followed by the escape of hydrogen to space. By this mechanism, terrestrial planets currently in the habitable zones around M dwarfs are estimated to have lost significant portions of H2O during the early runaway greenhouse phase (Luger and Barnes, 2015; Tian and Ida, 2015; Bolmont et al., 2017). The previous studies supposed that the escape of hydrogen is limited by the stellar XUV flux inducing hydrodynamic escape of hydrogen or the diffusion of hydrogen through an oxygen-rich layer formed by photodissociation of H2O on the assumption that the photodissociation of H2O in the lower atmosphere occurs sufficiently. However, their assumption could overestimate the amount of water loss if the photodissociation of H2O in the lower atmosphere limits the production of escaping hydrogen. In the present study, we consider whether the photodissociation of H2O in the lower atmosphere limits the water loss.
We evaluate the supply of hydrogen from the lower atmosphere to the upper atmosphere on the planet with an H2O-dominated atmosphere in a runaway greenhouse state by developing a 1-D photochemical model based on PROTEUS (Nakamura et al., 2023). We consider 40 photochemical reactions for 10 chemical species (H2O, H, H2, O, etc.), assuming all of which are produced from the photodissociation of water vapor. The escape velocity of H at the upper boundary is assumed to be diffusion-limited and the deposition of O at the surface is considered. We input the stellar UV spectrum estimated for TRAPPIST1 in the wavelength range of 7 nm - 230 nm affecting the photodissociation of H2O (Wilson et al., 2021). We divide the wavelength range into two specified regions (XUV region: 7 nm - 115 nm, FUV region: 115 nm - 230 nm) and give independent magnification factors for both regions, to clarify the dependence of the H2O photodissociation rate relative to the hydrogen escape rate estimated by previous studies of on the XUV flux.
The H2O-dominated atmospheres in the runaway greenhouse state efficiently absorb stellar UV irradiations at the wavelength range considered by this study. We found that the supply of hydrogen through the photodissociation of H2O is lower than the escape rate of hydrogen estimated by the previous studies in a part of the range of XUV flux. This suggests that the photodissociation of water vapor in the lower atmosphere limits the amount of water loss. Furthermore, we found that this range varies depending on the photon flux in the FUV region. Thus, further study constraining the FUV intensity from young M dwarfs is needed to quantify the amount of water loss from the H2O-dominated atmospheres around M dwarfs.
When a planet formed with sufficient surface water is located inside of the HZ inner edge, it should enter a runaway greenhouse condition to form an H2O-dominated atmosphere (Kasting, 1988). Then, the stellar UV irradiation from the host star promotes the photodissociation of water vapor followed by the escape of hydrogen to space. By this mechanism, terrestrial planets currently in the habitable zones around M dwarfs are estimated to have lost significant portions of H2O during the early runaway greenhouse phase (Luger and Barnes, 2015; Tian and Ida, 2015; Bolmont et al., 2017). The previous studies supposed that the escape of hydrogen is limited by the stellar XUV flux inducing hydrodynamic escape of hydrogen or the diffusion of hydrogen through an oxygen-rich layer formed by photodissociation of H2O on the assumption that the photodissociation of H2O in the lower atmosphere occurs sufficiently. However, their assumption could overestimate the amount of water loss if the photodissociation of H2O in the lower atmosphere limits the production of escaping hydrogen. In the present study, we consider whether the photodissociation of H2O in the lower atmosphere limits the water loss.
We evaluate the supply of hydrogen from the lower atmosphere to the upper atmosphere on the planet with an H2O-dominated atmosphere in a runaway greenhouse state by developing a 1-D photochemical model based on PROTEUS (Nakamura et al., 2023). We consider 40 photochemical reactions for 10 chemical species (H2O, H, H2, O, etc.), assuming all of which are produced from the photodissociation of water vapor. The escape velocity of H at the upper boundary is assumed to be diffusion-limited and the deposition of O at the surface is considered. We input the stellar UV spectrum estimated for TRAPPIST1 in the wavelength range of 7 nm - 230 nm affecting the photodissociation of H2O (Wilson et al., 2021). We divide the wavelength range into two specified regions (XUV region: 7 nm - 115 nm, FUV region: 115 nm - 230 nm) and give independent magnification factors for both regions, to clarify the dependence of the H2O photodissociation rate relative to the hydrogen escape rate estimated by previous studies of on the XUV flux.
The H2O-dominated atmospheres in the runaway greenhouse state efficiently absorb stellar UV irradiations at the wavelength range considered by this study. We found that the supply of hydrogen through the photodissociation of H2O is lower than the escape rate of hydrogen estimated by the previous studies in a part of the range of XUV flux. This suggests that the photodissociation of water vapor in the lower atmosphere limits the amount of water loss. Furthermore, we found that this range varies depending on the photon flux in the FUV region. Thus, further study constraining the FUV intensity from young M dwarfs is needed to quantify the amount of water loss from the H2O-dominated atmospheres around M dwarfs.