Internal Structure and CO 2 Reservoirs of Habitable Water Worlds

Abstract

Water worlds are water-rich (>1 wt% H 2 O) 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% CO 2 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 CO 2 –H 2 O system to explore the CO 2 budget and localize the main CO 2 reservoirs inside of these planets. We show that CO 2 dissolved in the ocean and trapped inside of a clathrate layer cannot accommodate a cometary amount of CO 2 if the planet accretes more than 11 wt% of volatiles (CO 2 + H 2 O) during its formation. If the atmosphere holds more than a negligible amount of the CO 2 (>0.01% of the planet mass), the planet will not have a habitable surface temperature. We propose a new, potentially dominant, CO 2 reservoir for water worlds: CO 2 buried inside of the high-pressure water ice mantle as CO 2 ices or (H 2 CO 3 · H 2 O), the monohydrate of carbonic acid. If insufficient amounts of CO 2 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.