2020ApJ...890..107M

Water worlds are water-rich (>1 wt% H₂O) exoplanets. The classical models of water worlds considered layered structures determined by the phase boundaries of pure water. However, water worlds are likely to possess comet-like compositions, with between ∼3 and 30 mol% CO₂ relative to water. In this study, we build an interior structure model of habitable (i.e., surface liquid ocean-bearing) water worlds using the latest results from experimental data on the CO₂-H₂O system to explore the CO₂ budget and localize the main CO₂ reservoirs inside of these planets. We show that CO₂ dissolved in the ocean and trapped inside of a clathrate layer cannot accommodate a cometary amount of CO₂ if the planet accretes more than 11 wt% of volatiles (CO₂ + H₂O) during its formation. If the atmosphere holds more than a negligible amount of the CO₂ (>0.01% of the planet mass), the planet will not have a habitable surface temperature. We propose a new, potentially dominant, CO₂ reservoir for water worlds: CO₂ buried inside of the high-pressure water ice mantle as CO₂ ices or (H₂CO₃ * H₂O), the monohydrate of carbonic acid. If insufficient amounts of CO₂ are sequestered in either this reservoir or the planet's iron core, habitable-zone water worlds could generically be stalled in their cooling before liquid oceans have a chance to condense.