2023ApJ...944..209L

Here we constrain the speciation of carbon that may outgas in ocean exoplanets. Ocean exoplanets likely have at least a few percent by mass of water, which is sufficient to build a high-pressure ice layer between a rocky interior and the outer hydrosphere. We study the possible formation of a filled ice in the ternary system H₂O-CO₂-CH₄. The incorporation of CH₄ and CO₂ in filled ice would be an important mechanism for transporting carbon across a high-pressure ice mantle into the atmosphere. The CH₄-CO₂ pair is also important as a potential biosignature. We find that a filled ice in the system H₂O-CO₂-CH₄ is possible though enriched in CH₄. CO₂ cannot account for more than 15% by mole of the carbon content of the filled ice. Such a filled ice is less dense than an overlying ocean and would therefore discharge into the ocean, depressurize, and outgas its carbon content into the atmosphere. A high-pressure, water-rich mantle in ocean worlds may therefore support the transport of carbon from the interior into the atmosphere. More than 75% by mole of this carbon would be reduced. As long as CH₄ exists/is produced in the interior and the ice mantle convects, thus transporting chemical species outward, a flux of carbon enriched in CH₄ would outgas. If this persists over geological time it would negate atmospheric sinks for CH₄, and explain low concentrations of atmospheric CO₂. If the contrary is correct than the interior of the planet may be oxidizing.