Temperate Venuses: Does CO₂ affect the habitability of terrestrial planets outside the conventional habitable zone?

Consensus suggests that the outer boundary of the habitable zone is set by the maximum greenhouse limit, where increasing atmospheric CO₂ no longer warms the planet but through enhanced Rayleigh scattering drives declining temperatures (Kopparapu et al. 2013). Beyond this threshold, other greenhouse gasses are needed to maintain clement surface conditions. For example, atmospheres with large molecular hydrogen abundances can extend the outer edge of the habitable zone well past the Kopparapu et al. values (Pierrehumbert et al. 2011). However, unlike the carbonate-silicate cycle which provides the long term negative climate feedback to maintain temperate surface temperatures, there is no known analogous process for hydrogen-dominated atmospheres.

Here, we present simulations of terrestrial atmospheres using a 1-D coupled photochemistry and climate model, Atmos (Arney et al., 2016), to study the effect that varying levels of CO₂ and H₂ have on surface temperature. Outside of the conventional habitable zone, planets with substantial H₂ and CO₂ atmospheres could be buffered at modest surface temperatures by the condensation of CO₂, which acts as a negative feedback on climate over a wide range of planetary conditions. These ”temperate Venuses’ may be amenable to surface habitability, and we determine if these states of high CO₂ and H₂ can be maintained and whether the liquid CO₂ pools persist over long time scales in the face of potential biological and geochemical sinks.

References:

Arney et al. (2016) Astrobiology 16.11 (2016): 873-899.

Kopparapu et al. (2013) The Astrophysical Journal 765.2 (2013): 131.

Pierrehumbert et al. (2011) The Astrophysical Journal Letters 734.1 (2011): L13.