  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 (2017) 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 (Teff = 2,600 to 5,800 K), extending from about 1.23 to 1.65, 0.69 to
0.954, 0.38 to 0.528 AU, 0.219 to 0.308 AU, 0.146 to 0.206 AU, and 0.0428 to
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
(about 0.5) that can be verified by the JWST mission.
