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 H _2 O–CO _2 –CH _4 . The incorporation of CH _4 and CO _2 in filled ice would be an important mechanism for transporting carbon across a high-pressure ice mantle into the atmosphere. The CH _4 –CO _2 pair is also important as a potential biosignature. We find that a filled ice in the system H _2 O–CO _2 –CH _4 is possible though enriched in CH _4 . CO _2 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 CH _4 exists/is produced in the interior and the ice mantle convects, thus transporting chemical species outward, a flux of carbon enriched in CH _4 would outgas. If this persists over geological time it would negate atmospheric sinks for CH _4 , and explain low concentrations of atmospheric CO _2 . If the contrary is correct than the interior of the planet may be oxidizing.

Keywords

• Planetary interior
• Exoplanet atmospheres
• Biosignatures