CO₂ Ocean Bistability on Terrestrial Exoplanets

Abstract

Cycling of carbon dioxide between the atmosphere and interior of rocky planets can stabilize global climate and enable planetary surface temperatures above freezing over geologic time. However, variations in global carbon budget and unstable feedback cycles between planetary sub‐systems may destabilize the climate of rocky exoplanets toward regimes unknown in the Solar System. Here, we perform clear‐sky atmospheric radiative transfer and surface weathering simulations to probe the stability of climate equilibria for rocky, ocean‐bearing exoplanets at instellations relevant for planetary systems in the outer regions of the circumstellar habitable zone. Our simulations suggest that planets orbiting G‐ and F‐type stars (but not M‐type stars) may display bistability between an Earth‐like climate state with efficient carbon sequestration and an alternative stable climate equilibrium where CO2 condenses at the surface and forms a blanket of either clathrate hydrate or liquid CO2. At increasing instellation and with ineffective weathering, the latter state oscillates between cool, surface CO2‐condensing and hot, non‐condensing climates. CO2 bistable climates may emerge early in planetary history and remain stable for billions of years. The carbon dioxide‐condensing climates follow an opposite trend in pCO2 versus instellation compared to the weathering‐stabilized planet population, suggesting the possibility of observational discrimination between these distinct climate categories.

Carbon Cycle Instability for High-CO2 Exoplanets: Implications for Habitability

“…Radiative transfer calculations coupled to Isca's dynamics are carried out with the SOCRATES code (Edwards & Slingo 1996), using the correlated-k method to solve the planeparallel, two-stream approximated radiative transfer equation with scattering for atmospheres irradiated by a solar (G-star) spectrum. In high-instellation, low-pCO 2 simulations, we used Graham et al (2022) and found the output from the GCM stayed within 5 W m −2 of the higherresolution calculations). Rayleigh scattering is interactively calculated with coefficients included in SOCRATES for the relevant gases.…”

“…At somewhat lower instellations than S = 1000 W m −2 , CO 2 condensation at the surface would become a possibility at high enough pCO 2 , meaning the energetic limit on precipitation presents a pathway to reach the stable CO 2 ocean states proposed in Graham et al (2022). The mechanism of carbon cycle destabilization suggested in that study was based on the fact that CO 2 becomes a coolant at extremely high partial pressures, which also leads to a slowdown of weathering with pCO 2 accumulation but requires much higher CO 2 to initiate than the mechanism discussed here.…”

“…If the system starts somewhat to the right of P 2 , then CO 2 will accumulate until something arrests the process at higher pCO 2 . This could terminate at complete degassing of the interior CO 2 reservoir, leading to a hot Venus-like state with a thick gaseous atmosphere or a more temperate state where CO 2 accumulates in the form of a liquid ocean, depending on instellation as in Graham et al (2022). Alternatively, it is possible that the weathering curve turns around at sufficiently high pCO 2 , allowing a new stable high-pCO 2 equilibrium to exist, though possibly at an uninhabitably hot temperature.…”

“…This means that energetically limited weathering may increase the probability that low-instellation planets within the HZ end up with CO 2 oceans, with uncertain implications for their habitability. Although CO 2 ocean worlds are restricted to relatively clement climates below 304 K (the critical temperature of CO 2 ), they would still sport extreme surface pressures (up to 72 bars), and in the process of accumulating that much CO 2 surface temperatures can peak at extreme levels (>400 K) before Rayleigh scattering begins to outweigh CO 2 ʼs greenhouse effect and further accumulation begins to cool the planet (Graham et al 2022). Intriguingly, some work has suggested that liquid and supercritical CO 2 may be conducive to prebiotic chemistry (Shibuya & Takai 2022), and supercritical CO 2 may drive rapid carbonate formation (see Section 5 and McGrail et al 2017), suggesting these worlds may not be as hostile to life as they appear at first.…”

Bioverse: The Habitable Zone Inner Edge Discontinuity as an Imprint of Runaway Greenhouse Climates on Exoplanet Demographics

“…Consequently, the instellation at which the runaway greenhouse transition occurs is aptly considered to be the inner boundary of the habitable zone (e.g., Ramirez 2018;. Its prevalent definition refers to the possibility of a sustained liquid water body on the surface of an Earth-like planet with an oxidized CO 2 /H 2 O/N 2 -rich atmosphere (Kasting et al 1993;Kopparapu et al 2013Kopparapu et al , 2014; its exact spatial location and extent may be strongly influenced by the interior and atmosphere oxidation state and resulting atmosphere composition (Pierrehumbert & Gaidos 2011;Ramirez & Kaltenegger 2017, 2018Katyal et al 2019;Graham & Pierrehumbert 2020;Graham et al 2022;Hakim et al 2023). The fundamental concept of a habitable zone dates back centuries (Newton 1846;Whewell 1858;Shapley 1953;Huang 1959), and its modern form has proven popular in the planetary literature.…”