The Abundance of Atmospheric CO₂ in Ocean Exoplanets: a Novel CO₂ Deposition Mechanism

Abstract

We consider super-Earth sized planets which have a water mass fraction that is large enough to form an external mantle composed of high pressure water ice polymorphs and that lack a substantial H/He atmosphere. We consider such planets in their habitable zone so that their outermost condensed mantle is a global deep liquid ocean. For these ocean planets we investigate potential internal reservoirs of CO 2 ; the amount of CO 2 dissolved in the ocean for the various saturation conditions encountered, and the ocean-atmosphere exchange flux of CO 2 . We find that in steady state the abundance of CO 2 in the atmosphere has two possible states. When the wind-driven circulation is the dominant CO 2 exchange mechanism, an atmosphere of tens of bars of CO 2 results, where the exact value depends on the subtropical ocean surface temperature and the deep ocean temperature. When sea-ice formation, acting on these planets as a CO 2 deposition mechanism, is the dominant exchange mechanism, an atmosphere of a few bars of CO 2 is established. The exact value depends on the subpolar surface temperature. Our results suggest the possibility of a negative feedback mechanism, unique to water planets, where a reduction in the subpolar temperature drives more CO 2 into the atmosphere to increase the greenhouse effect.

A More Comprehensive Habitable Zone for Finding Life on Other Planets

“…Warm (> 265 K) tropical and subtropical regions in these ocean worlds are necessary to prevent global glaciation and to bolster the circulation. Cold subpolar regions are also necessary to establish freeze-thaw cycles that help life evolve in diluted ocean worlds by concentrating the nutrients that life needs [214]. Once sea ice enriched in clathrates becomes denser than water (occurs once ice thickness exceeds ~ 1 -2m), the ices sink, which acts to hinder global glaciation [214].…”

“…The Levi et al [214] mechanism was recently modeled using coupled energy balance and singlecolumn radiative convective climate models [216]. They find that high rotation rates (> ~8 hours) are necessary to generate the warm subtropical and cold subtropical temperatures required to sustain the mechanism.…”

The Ice Cap Zone: A Unique Habitable Zone for Ocean Worlds

“…Although the work in Levi et al (2017) suggests that ocean worlds may be habitable without the carbonate-silicate cycle, the ocean model was not coupled to an atmospheric model to assess the plausibility of such solutions. Here, we couple the model in Levi et al (2017) with a single-column radiative-convective climate model and an energy balance climate model developed by one of us (Ramirez) to determine the location of the ice cap zone for G-M-stars. In Section 2 we give a brief description of the CO2 ocean-atmospheric exchange modelled in Levi et al (2017) and describe the inner and outer boundaries of the ice cap zone from a geophysical perspective.…”

“…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.…”

Planetesimal formation starts at the snow line

“…As a consequence, planetary cores formed from these planetesimals would also be water-rich. For small mass planets (without a massive gas envelope), the presence of large amounts of water may be detrimental for habitability (see Alibert et al (2013); Kitzmann et al (2015), see however Levi et al (2017) for another view). Planetesimal formation mechanism we describe here could therefore imply that the majority of low mass planets are not habitable.…”