16:00 〜 16:15
[PAE18-21] Simulation of the day/night water cycle on Trappist-1e using a global climate and hydrological model
キーワード：系外惑星、Trappist-1e、全球気候モデル、水循環、境界領域
To date, more than 5,800 exoplanets have been discovered in our cosmic neighborhood (see NASA Exoplanet Archive), many of which orbit M-dwarf stars. Due to their lower luminosity compared to the Sun, the habitable zones around these stars are located at approximately 0.1 AU. Over the past decade, several promising candidate systems for habitability have been identified, including TRAPPIST-1, Proxima Centauri, and TOI-700. In general, exoplanets around M-dwarfs undergo rapid tidal locking, resulting in synchronous rotation that produces a permanent day side and a permanent night side – with the night side potentially accumulating significant amounts of volatiles. In the TRAPPIST-1system, TRAPPIST-1e could represent a habitable environment even in the absence of greenhouse gases, as it receives a stellar flux of about 900 W m-2 (roughly 66 % of Earth's insolation). For tidally locked, water-limited planets, a temperate climate may be maintained in the terminator region; however, the mechanism by which water is transported from the night side back to the day side remains not fully understood.
In this study, we updated our previous global climate models and developed a self-consistent model of the water cycle model on exoplanets. In our model, we achieve a complete coupling between atmosphere, land, and ocean including rivers and ice sheets. Our river model implements river temperature, evaporation from the river surface, and freezing of the river surface. We also investigate the effects of geothermal heating due to internal tidal heating - associated with the eccentric orbit of TRAPPIST-1e, which supports subglacial melting, and water supply from the night side to the day side via the terminator region. We ran climate simulations with a horizontal resolution of about 5.6° in both longitude and latitude, using 15 σ-level layers extending to an altitude of about 50 km. For the radiative transfer scheme, we used the TRAPPIST-1 radiation spectrum and performed calculations under atmospheric conditions composed of either N2 or CO2, with surface pressures of 1 bar.
We found that the evening side of the terminator region was consistently warmer than the morning side due to the inflow of warm air from the subsolar point. This temperature difference promoted active subglacial melting in the evening side, leading to the formation of rivers that flow toward the day side, especially in the CO2 dominated atmosphere. As these day side rivers gradually warmed, evaporation ensued. Portions of ocean and river water evaporated, with the resulting water vapor eventually precipitating as rain or snow and replenishing the terminator and night side regions. In addition, ice sheets on the night side flow towards the day side, and as they approach the terminator region, melting occurs - further contributing to the dynamics of the global hydrological cycle. The pronounced supraglacial and subglacial melting, together with the river flow patterns, suggest that TRAPPIST-1e hosts a complex climate system in which the terminator acts as a transition zone, facilitating significant water exchange between the night and day sides. In addition, libration of TRAPPIST-1e was found to cause temporal variations in the stability of the terminator region.
In this study, we updated our previous global climate models and developed a self-consistent model of the water cycle model on exoplanets. In our model, we achieve a complete coupling between atmosphere, land, and ocean including rivers and ice sheets. Our river model implements river temperature, evaporation from the river surface, and freezing of the river surface. We also investigate the effects of geothermal heating due to internal tidal heating - associated with the eccentric orbit of TRAPPIST-1e, which supports subglacial melting, and water supply from the night side to the day side via the terminator region. We ran climate simulations with a horizontal resolution of about 5.6° in both longitude and latitude, using 15 σ-level layers extending to an altitude of about 50 km. For the radiative transfer scheme, we used the TRAPPIST-1 radiation spectrum and performed calculations under atmospheric conditions composed of either N2 or CO2, with surface pressures of 1 bar.
We found that the evening side of the terminator region was consistently warmer than the morning side due to the inflow of warm air from the subsolar point. This temperature difference promoted active subglacial melting in the evening side, leading to the formation of rivers that flow toward the day side, especially in the CO2 dominated atmosphere. As these day side rivers gradually warmed, evaporation ensued. Portions of ocean and river water evaporated, with the resulting water vapor eventually precipitating as rain or snow and replenishing the terminator and night side regions. In addition, ice sheets on the night side flow towards the day side, and as they approach the terminator region, melting occurs - further contributing to the dynamics of the global hydrological cycle. The pronounced supraglacial and subglacial melting, together with the river flow patterns, suggest that TRAPPIST-1e hosts a complex climate system in which the terminator acts as a transition zone, facilitating significant water exchange between the night and day sides. In addition, libration of TRAPPIST-1e was found to cause temporal variations in the stability of the terminator region.