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

TL;DR

This study explores how CO2 is stored and exchanged in ocean exoplanets with deep liquid oceans, revealing two stable atmospheric CO2 states influenced by oceanic processes and temperature-dependent feedback mechanisms.

Contribution

It introduces a novel CO2 deposition mechanism via sea-ice formation and analyzes its impact on atmospheric CO2 levels on water-rich super-Earths.

Findings

Atmospheric CO2 can be tens of bars or a few bars depending on dominant exchange mechanisms.

Sea-ice formation acts as a CO2 deposition process affecting atmospheric composition.

A potential negative feedback loop links subpolar temperature reductions to increased atmospheric CO2.

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 CO2; the amount of CO2 dissolved in the ocean for the various saturation conditions encountered, and the ocean-atmosphere exchange flux of CO2. We find that in steady state the abundance of CO2 in the atmosphere has two possible states. When the wind-driven circulation is the dominant CO2 exchange mechanism, an atmosphere of tens of bars of CO2 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 CO2 deposition mechanism, is the dominant exchange mechanism, an atmosphere of a few bars of CO2 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 CO2 into the atmosphere to increase the greenhouse effect.