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 CO 2 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 CO 2 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 CO 2 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.

Constraining the surface environment of the early Earth is essential for understanding the origin and evolution of life. The release of cations from silicate weathering depends on climatic temperature and pCO 2 , and such cations sequester CO 2 into carbonate minerals in or on the seafloor, providing a stabilizing feedback on climate. Previous studies have suggested that this carbonate-silicate cycle can keep the early Earth's surface temperature moderate by increasing pCO 2 to compensate for the faint young Sun. However, the Hadean Earth experienced a high meteorite impactor flux, which produced ejecta that is easily weathered by carbonic acid. In this study, we estimated the histories of surface temperature and ocean pH during the Hadean and early Archean using a new model that includes the weathering of impact ejecta, empirically justified seafloor weathering, and ocean carbonate chemistry. We find that relatively low pCO 2 and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, log 10 (pCO 2 ) (in bar) is −2.21 +3.01 −2.54 [2 ] and temperature is 259.2 +84.1 −14.4 [2 ] K. Such a low pCO 2 would result in a circumneutral to basic pH of seawater, for example, 7.90 +1.21 −1.69 [2 ] at 4.3 Ga.A probably cold and alkaline marine environment is associated with a high impact flux. Hence, if there was an interval of an enhanced impact flux, that is, Late Heavy Bombardment, similar conditions may have existed in the early Archean. Therefore, if the origin of life occurred in the Hadean, life likely emerged in a cold global environment and probably spread into an alkaline ocean. Plain Language SummaryThe Earth's environment during the Hadean eon, 4.5 to 4 billion years ago, is obscured by a lack of geological evidence. However, life likely arose then, so improving our knowledge of the early environment is essential for understanding the origin and evolution of life. Here, we build a geological carbon cycle model that simulates the early surface environment and generates probability distributions for the level of atmospheric carbon dioxide (CO 2 ), average surface temperature, and ocean pH over time. During the Hadean, CO 2 dissolved in water is consumed by reacting with material ejected from meteorite impacts, so CO 2 levels tend to be low and the greenhouse effect weak. The consequences are low surface temperature and alkaline seawater. The probability that the surface temperature was lower than the freezing point of water and that seawater pH exceeded 7 is 70% at 4.3 billion years ago. Thus, if life began in the Hadean, it likely emerged in a cold global environment, and early life may have spread into an alkaline ocean. Key Points:• We estimate the range of pCO 2 , surface temperature, and ocean pH in the Hadean and Archean • A cold, Hadean surface environment and circumneutral to basic ocean pH is likely because of impact ejecta weathering • The origin of life may have occurred in a cold environment, and early life may have spread into a cold alkaline ocean Supporting Information:• Supporti...

We investigate the climate modes and climate evolution of Earth-like planets—specifically planets that receive lower insolation than the present Earth—to discuss the outer limits of the habitable zones (HZs) associated with main-sequence stars. The HZ outer limit is discussed in terms of the insolation above which a planet maintains liquid water on its surface (e.g., warm climate). We call this a “warm start limit.” However, an alternative outer limit exists above which a planet characterized by a snowball climate transitions to a warm climate, which we call a “cold start limit.” The cold start limit is determined by two different mechanisms: condensation of CO2 and maximum CO2 greenhouse effect. The threshold insolation is always higher than the warm start limit. We find that for the initial few billion years following the formation of a planetary system, the location of its outer HZ limit does not evolve away from the initial warm start limit. It subsequently moves in accordance with the evolution of the cold start limit. We also find that when the luminosity increase of the central star is slow relative to the decrease in the CO2 degassing rate, the location of the outer limit of the HZ may even move inward. We conclude that the cold start limit should be treated as the outer limit of the HZ given the prevailing uncertainties, including the evolution of the CO2 degassing rate and continental growth.