2018MNRAS.477.4627R

Traditional definitions of the habitable zone assume that habitable planets contain a carbonate-silicate cycle that regulates CO₂ between the atmosphere, surface, and the interior. Such theories have been used to cast doubt on the habitability of ocean worlds. However, Levi et al. have recently proposed a mechanism by which CO₂ is mobilized between the atmosphere and the interior of an ocean world. At high enough CO₂ pressures, sea ice can become enriched in CO₂ clathrates and sink after a threshold density is achieved. The presence of subpolar sea ice is of great importance for habitability in ocean worlds. It may moderate the climate and is fundamental in current theories of life formation in diluted environments. Here, we model the Levi et al. mechanism and use latitudinally dependent non-grey energy balance and single-column radiative-convective climate models and find that this mechanism may be sustained on ocean worlds that rotate at least 3 times faster than the Earth. We calculate the circumstellar region in which this cycle may operate for G-M stars (T_eff = 2600-5800 K), extending from ∼1.23-1.65, 0.69-0.954, 0.38-0.528, 0.219-0.308 , 0.146-0.206, and 0.0428-0.0617 au for G2, K2, M0, M3, M5, and M8 stars, respectively. However, unless planets are very young and not tidally locked, our mechanism would be unlikely to apply to stars cooler than a ∼M3. We predict C/O ratios for our atmospheres (∼0.5) that can be verified by the James Webb Space Telescope mission.