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

We consider super-Earth sized planets which have a water mass fraction large enough to form an external mantle composed of high-pressure water-ice polymorphs and also lack a substantial H/He atmosphere. We consider such planets in their habitable zone, so that their outermost condensed mantle is a g...

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Bibliographic Details
Published in:The Astrophysical journal 2017-03, Vol.838 (1)
Main Authors: Levi, A., Sasselov, D., Podolak, M.
Format: Article
Language:English
Subjects:
Abundance
Astrophysics
Atmosphere
Atmospheric circulation
atmospheric effects
Carbon dioxide
Carbon dioxide atmospheric concentrations
Carbon dioxide exchange
Circumstellar habitable zone
Deposition
Exchanging
External pressure
Extrasolar planets
Greenhouse effect
Ice formation
Mantle
Negative feedback
Ocean surface
Ocean temperature
Oceans
Planet formation
planets and satellites: atmospheres
planets and satellites: composition
planets and satellites: interiors
planets and satellites: oceans
planets and satellites: surfaces
Sea ice
Sea ice formation
Sea ice temperatures
Surface temperature
Water masses
Wind-driven circulation
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Summary:We consider super-Earth sized planets which have a water mass fraction large enough to form an external mantle composed of high-pressure water-ice polymorphs and also 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 a steady state, the abundance of CO2 in the atmosphere has two possible states. When 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.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/aa5cfe