Atmospheric Collapse and Habitability on Tidally-Locked Exoplanets

Abstract

The habitability of terrestrial exoplanets orbiting M dwarfs is a key topic in the search for extraterrestrial life. The climates of these planets differ significantly from the Earth's due to their likely tidal locking, resulting in a hotter dayside and a colder nightside caused by uneven stellar irradiation. On tidally-locked planets around the outer edge of the habitable zone (HZ), although the definition of the classical HZ requires thick CO2 atmosphere, CO2 can condense onto the surface, leading to the reduction of greenhouse effect. However, the dayside permanent stellar irradiation could maintain a surface liquid water area. The onset of atmospheric collapse and the persistence of surface liquid water are governed by global heat redistribution which is influenced by factors such as atmospheric mass, stellar irradiation, and greenhouse effects. In this study, we used a three-dimensional global climate model to investigate the impact of atmospheric collapse on the presence of dayside surface liquid water. Our results indicate that surface liquid water could counter-intuitively persist despite atmospheric collapse. This is because the loss of atmospheric CO2 weakens not only the greenhouse effect but also daynight heat transport, leading to less redistribution of the energy of dayside insolation to the nightside. While atmospheric collapse is typically seen as an obstacle to maintaining a habitable climate, our findings suggest that it could play a positive role in sustaining surface liquid water on tidally-locked planets. Our work provides new light into the relationship between atmospheric collapse and planetary habitability.

Figures (4)

• Figure 1: Atmospheric conditions in the case with $S_{\mathrm{p}} = 0.4 \, S_{0}$, $p_{\ce{N2}} = 1.0bar$, $p_{\ce{CO2}} = 0.1bar$. Each case shows (a) horizontal distribution of surface temperature, (b) horizontal distribution of CO2 ice amount (c) horizontal distribution of H2O ice amount, and (d) mass streamfunction between the substellar point (SS) and antistellar point (AS) in tidally-locekd coordinate. In (a), (b), and (c), 0° of latitude and of longitude corresponds to the substellar point (SS). Note that horizontal axis in (d) represents the latitude in tidally-locked coordinate and the mass flux where the value is positive is anticlockwise circulation.
• Figure 2: Atmospheric conditions in the case with $S_{\mathrm{p}} = 0.4 \, S_{0}$, $p_{\ce{N2}} = 1.0bar$, $p_{\ce{CO2}} = 10^{-4} \, bar$ as in Figure \ref{['fig:tv_C1e-01']}; (a) horizontal distribution of surface temperature, (b) horizontal distribution of CO2 ice, (c) horizontal distribution of H2O ice, and (d) mass streamfunction between the substellar point and antistellar point.
• Figure 3: Maximum (red dots) and minimum (blue dots) surface temperatures and the onset of atmospheric collapse of each case. The red dots shows that the maximum surface temperature is higher than the melting point of H2O (273K), and on the other hand, red circles show that the surface is completely freezing. The black curve indicates the condensation temperature of CO2 as a function of $p_{\ce{CO2}}$. The blue dots indicate that the minimum surface temperature is higher than the condensation temperature (black curve). The blue circles show that the atmospheric collapse is occurring and thus the circles locate on the black curve of condensation temperature. The red/blue shaded areas are corresponding to the cases where minimum surface temperature is above/equal to CO2 condensation temperature.
• Figure 4: Thermal redistribution efficiency calculated by the day/nightside outgoing longwave radiation (OLR) $\eta = OLR_{\mathrm{nightside}}/OLR_{\mathrm{dayside}}$ in case of (a) $S_{\mathrm{p}} = 0.4 \, S_{0}$, , $p_{\ce{N2}} = 0.1bar$, (b) $S_{\mathrm{p}} = 0.4 \, S_{0}$, , $p_{\ce{N2}} = 1.0bar$.