[PAE20-P08] Planetary climate on terrestrial exoplanets with excess water within the habitable zone
Keywords:Exoplanet, Habitable, Terrestrial planets, Carbon cycle, High-pressure ice
In this study, we consider an Earth-sized ocean planets with plate tectonics and explore the possibility that the high heat flux at the mid-ocean ridge leads to melting the HP ice and thus making seafloor weathering efficiently works. To that end, we develop integrated climate model for ocean planets that includes melting of HP ice, seafloor weathering, and the carbon cycle. As a result, we have found that the high-pressure ice is entirely molten when the high-pressure ice is thin (e.g., < 130 Earth's ocean masses for the current mean heat flow in the Earth's mantle) and then the weathering occurs in the seafloor. We have also found that the planetary climate for large ocean masses (e.g., > 60 Earth's ocean masses for Earth-like degassing flux) lapses into two extreme climates; quite hot ones with a CO2 rich atmosphere, in which the surface temperature is larger than 350 K, or cold ones with a CO2 poor atmosphere, even a snowball state. The abrupt transitions from warm to extreme climates are triggered by two runaway processes; CO2 accumulation induced by the upper limit to the seafloor weathering flux and CO2 clearing induced by melting of the high-pressure ice. This result suggests a low probability of exoplanets with clement climates, like the present Earth, and necessity that we consider water amounts and their thermo-chemical structure to discuss planetary climate, even if the planet in the habitable zone.