Internal Structure and CO2 Reservoirs of Habitable Water-Worlds
Abstract
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 mol% to 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 to 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. 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), monohydrate of carbonic acid. If insufficient amounts of CO2 are sequestered either in 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.
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