CONDITIONS FOR OCEANS ON EARTH-LIKE PLANETS ORBITING WITHIN THE HABITABLE ZONE: IMPORTANCE OF VOLCANIC CO2 DEGASSING

Earth-like planets in the habitable zone (HZ) have been considered to have warm climates and liquid water on their surfaces if the carbonate-silicate geochemical cycle is working as on Earth. However, it is known that even the present Earth may be globally ice-covered when the rate of CO2 degassing via volcanism becomes low. Here we discuss the climates of Earth-like planets in which the carbonate-silicate geochemical cycle is working, with focusing particularly on insolation and the CO2 degassing rate. The climate of Earth-like planets within the HZ can be classified into three climate modes (hot, warm, and snowball climate modes). We found that the conditions for the existence of liquid water should be largely restricted even when the planet is orbiting within the HZ and the carbonate-silicate geochemical cycle is working. We show that these conditions should depend strongly on the rate of CO2 degassing via volcanism. It is, therefore, suggested that thermal evolution of the planetary interiors will be a controlling factor for Earth-like planets to have liquid water on their surface.