Ocean Worlds

CO2 Ocean Bistability On Terrestrial Exoplanets

By Keith Cowing
Press Release
October 3, 2022
Filed under , , , , ,
CO2 Ocean Bistability On Terrestrial Exoplanets
Energetic properties of terrestrial planetary climates at low instellation as a function of surface temperature Tsurf (y-axes) and CO2 partial pressure (x-axes). In each panel, the black line represents the CO2 saturation vapor pressure curve; climates with CO2 greater than that of the vapor pressure curve at a given temperature are super-saturated at their surface (grey region in all three panels). The white dotted line in each panel represents the approximate contour location of the Seff value separating climates that can support CO2 bistability vs. those that may limit cycle between condensing and non-condensing states (see Sections 3.2 and 3.3 for discussion of those climate types). The black dotted line in each panel marks the temperature below which CO2 clathrate hydrates may be stable. In panel A, the contours represent the top-of-atmosphere stellar irradiation required to maintain a climate with a given surface temperature and pCO2, normalized by the instellation received at Earth’s orbit, i.e. S/SEarth = Seff. Panel B shows the global-mean outgoing longwave radiation (OLR). Panel C shows the global-mean planetary albedo.

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.

R.J. Graham, Tim Lichtenberg, Ray Pierrehumbert

Comments: Accepted for publication in JGR-Planets. 19 pages (including references and Tables), 6 figures, corresponding author email: robert.graham@physics.this http URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2210.00149 [astro-ph.EP] (or arXiv:2210.00149v1 [astro-ph.EP] for this version)
Submission history
From: R.J. Graham
[v1] Sat, 1 Oct 2022 00:08:34 UTC (41,626 KB)

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