A High-pressure Filled Ice in the H₂O–CO₂–CH₄ System, with Possible Consequences for the CO₂–CH₄ Biosignature Pair

Abstract

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.