Internal Structure and CO₂ 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, waterworlds are likely to possess comet-like compositions, with between ∼ 3 mol% to 30 mol% CO 2 relative to water. In this study, we build an interior structure model of habitable (i.e. surface-liquid-oceanbearing) water-worlds using the latest results from experimental data on the CO 2 -H 2 O system, to explore the CO 2 budget and to 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 can not 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. 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), monohydrate of carbonic acid. If insufficient amounts of CO 2 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.

A Metamorphic Origin for Europa's Ocean

“…The oxidized ocean may have been reduced in time with hydrogen generated by serpentinization enabled by thermal cracking (Vance et al., 2016), but better constraints on the conditions of fracture formation and propagation are required (Klimczak et al., 2019). Further improvements to the thermodynamic data of high pressure ‐ phases (Abramson et al., 2018) and their integration with thermodynamic models (e.g., Perple_X) are also needed to assess the build‐up of the ocean: the stability of such phases can be the factor dictating whether an ocean world will be habitable (Marounina & Rogers, 2020). Finally, we have also made the simplifying assumption that fluid percolation from depth was efficient.…”

“…H O E -2 CO E phases (Abramson et al, 2018) and their integration with thermodynamic models (e.g., Perple_X) are also needed to assess the build-up of the ocean: the stability of such phases can be the factor dictating whether an ocean world will be habitable (Marounina & Rogers, 2020). Finally, we have also made the simplifying assumption that fluid percolation from depth was efficient.…”

Holistic Approach for Studying Planetary Hydrospheres: Gibbs Representation of Ices Thermodynamics, Elasticity, and the Water Phase Diagram to 2,300 MPa

“…Prior studies of the interiors and geodynamics of icy moons and water‐rich planets (e.g., Noack et al, ; Choblet et al, ; Kalousová et al, ; Marounina & Rogers, ) relied on ad hoc thermodynamic parameterizations with tenuous physical bases, built from limited data. The present framework, using accurate and self‐consistent descriptions of the candidate constituents, will significantly aid investigations of the icy moon interiors and their potential habitability.…”

Atmospheres of Rocky Exoplanets

“…Various proposals have been put forward to allow CO2 levels to remain at the right level in the atmospheres of waterworlds to maintain habitable temperatures (Levi et al 2017;Kite & Ford 2018;Ramirez & Levi 2018). However, for many volatile-rich planets, it is plausible that enough CO2 could be acquired during formation to prevent the surface condensation of water to form oceans in the first place (Marounina & Rogers 2020). The question of waterworld habitability is discussed further in Section 7.…”

“…Aqueous chemistry may also be used to constrain surface liquid water abundances indirectly, for example via a planet's sulfur cycle (Loftus et al 2019), or possibly its ammonia inventory (Hu et al 2021). While liquid water is essential to Earth-like life, too much water may be detrimental to habitability, because of how it affects volatile sequestration (Kitzmann et al 2015;Marounina & Rogers 2020), outgassing (Krissansen-Totton et al 2021b) and near-surface nutrient availability (Wordsworth & Pierrehumbert 2013b;Glaser et al 2020). Techniques to determine surface pressure and temperature, and ultimately to detect surface land masses directly, will therefore also be important (Cowan et al 2009;Benneke & Seager 2012).…”