Runaway climate cooling of ocean planets in the habitable zone: a consequence of seafloor weathering enhanced by melting of high-pressure ice

Abstract

Terrestrial planets covered globally with thick oceans (termed ocean planets) in the habitable zone were previously inferred to have extremely hot climates in most cases. This is because H 2 O high-pressure (HP) ice on the seafloor prevents chemical weathering and, thus, removal of atmospheric CO 2 . Previous studies, however, ignored melting of the HP ice and horizontal variation in heat flux from oceanic crusts. Here we examine whether high heat fluxes near the mid-ocean ridge melt the HP ice and thereby remove atmospheric CO 2 . We develop integrated climate models of an Earth-size ocean planet with plate tectonics for different ocean masses, which include the effects of HP ice melting, seafloor weathering, and the carbonate-silicate geochemical carbon cycle. We find that the heat flux near the mid-ocean ridge is high enough to melt the ice, enabling seafloor weathering. In contrast to the previous theoretical prediction, we show that climates of terrestrial planets with massive oceans lapse into extremely cold ones (or snowball states) with CO 2 -poor atmospheres. Such extremely cold climates are achieved mainly because the HP ice melting fixes seafloor temperature at the melting temperature, thereby keeping a high weathering flux regardless of surface temperature. We estimate that ocean planets with oceans several tens of the Earth's ocean mass no longer maintain temperate climates. These results suggest that terrestrial planets with extremely cold climates exist even in the habitable zone beyond the solar system, given the frequency of water-rich planets predicted by planet formation theories.

Early volatile depletion on planetesimals inferred from C–S systematics of iron meteorite parent bodies

“…Planetary loss processes occur on large (thousands of kilometers in diameter) bodies ( 14 , 15 ) in which gravity plays an appreciable role—including loss from impacts ( 16 ). The sum of these is an important determinant for whether terrestrial planets form with volatiles sufficient for habitability but not so great as to become ocean worlds ( 17 ) or greenhouse hothouses ( 18 ).…”

Waterworlds Probably Do Not Experience Magmatic Outgassing

“…The solubility limits described in this paper seem to make the runaway glaciations described in Nakayama et al (2019) even more likely. It should be noted, however, that seafloor carbonatization rates will, to some degree, depend on the dissolved inorganic carbon content and pH of pore waters (Krissansen-Totton & Catling 2017 and references therein), as well as the carbon content of the undegassed oceanic crust (see Section 4).…”

“…Tajika & Matsui (1992) incorporated the pressure-dependence of CO 2 outgassing within an Earth system model, and this approach was adopted by Nakayama et al (2019) to explore the atmospheric evolution of waterworlds. However, this outgassing formulation assumed that both melt and gas phase CO 2 degasses upon crystallization, and so even for 200 Earth ocean surface inventories (∼5 GPa seafloor), significant CO 2 outgassing was predicted to occur.…”

“…So long as there is new crustal production, serpentinization reactions may oxidize the crust and produce molecular hydrogen, although the efficiency of such reactions will depend on dissolved silica concentrations (Tutolo et al 2020). While the possible tectonic regimes on waterworlds are uncertain, hydration and carbonatization of newly produced crust may, in principle, return volatiles to the mantle via subduction (Nakayama et al 2019) or slab delamination (Foley & Smye 2018). Nakayama et al (2019) developed this idea and hypothesized that if the water-rock interface is held at a constant temperature by the melting point of high-pressure ices, and if seafloor weathering reactions are purely temperature-dependent, then there will be no weathering thermostat to regulate waterworld climates.…”

“…While the possible tectonic regimes on waterworlds are uncertain, hydration and carbonatization of newly produced crust may, in principle, return volatiles to the mantle via subduction (Nakayama et al 2019) or slab delamination (Foley & Smye 2018). Nakayama et al (2019) developed this idea and hypothesized that if the water-rock interface is held at a constant temperature by the melting point of high-pressure ices, and if seafloor weathering reactions are purely temperature-dependent, then there will be no weathering thermostat to regulate waterworld climates. In this case, either outgassing fluxes exceed seafloor weathering rates and CO 2 accumulates in the atmosphere without bound, or-more likely if outgassing is suppressed by solubility limits-seafloor weathering sinks exceed CO 2 outgassing sources, and a runaway surface glaciation is inevitable.…”