| abstract: |
Many exoplanets have been detected in the habitable zone where the liquid ocean is stable on the planetary surface so far. The habitable zone implicitly assumes the Earth-like planets and its carbon cycle which plays negative feedback, regulating the partial pressure of CO2 and, thus, the surface temperature. However, planet formation theories predicted that many of terrestrial planets in the habitable zone are covered globally with thick oceans (termed ocean planets), instead of Earth-like water-poor planets. Thus, we must investigate the habitability on ocean planets in the conventional habitable zone. Ocean planets are inferred to have extremely hot climates in most cases by previous studies. This is because H2O high-pressure ice on the seafloor prevents chemical weathering and, thus, removal of atmospheric CO2. Those studies, however, ignored melting of the high-pressure ice and horizontal variation of the heat flux from the oceanic crust. In this study, we explore the possibility that high heat fluxes near the mid-ocean ridge lead to melting the high-pressure ice and, thus, to removing atmospheric CO2. To do so, we develop integrated climate models of an Earth-size ocean planet with plate tectonics for different ocean masses, which include the effects of melting of the high-pressure ice, seafloor weathering, and the carbonate-silicate geochemical carbon cycle. We find that the heat fluxes near the mid-ocean ridge are high enough to melt the ice even for sufficiently large ocean masses, which enables seafloor weathering. In contrast to the prediction by the previous studies, climates of terrestrial planets with massive oceans lapse into extremely cold ones (i.e., snowball states) with CO2-poor atmospheres. The reason why such extremely cold climates come out is that melting of HP ice fixes seafloor temperature at the melting temperature, thereby keeping a high weathering flux regardless of surface temperature. The critical ocean mass beyond which ocean planets no longer maintain temperate climates is estimated to several tens of the Earth’s ocean mass. These results suggest that temperate climates ocean planets with Earth-like plate tectonics are uncommon beyond the solar system, given the diversity in ocean mass predicted by planet formation theories.
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