Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets

Abstract

Models of thermal evolution, crustal production, and CO cycling are used to constrain the prospects for habitability of rocky planets, with Earth-like size and composition, in the stagnant lid regime. Specifically, we determine the conditions under which such planets can maintain rates of CO degassing large enough to prevent global surface glaciation but small enough so as not to exceed the upper limit on weathering rates provided by the supply of fresh rock, a situation which would lead to runaway atmospheric CO accumulation and an inhospitably hot climate. The models show that stagnant lid planets with initial radiogenic heating rates of 100-250 TW, and with total CO budgets ranging from ∼10 to 1 times Earth's estimated CO budget, can maintain volcanic outgassing rates suitable for habitability for ≈1-5 Gyr; larger CO budgets result in uninhabitably hot climates, while smaller budgets result in global glaciation. High radiogenic heat production rates favor habitability by sustaining volcanism and CO outgassing longer. Thus, the results suggest that plate tectonics may not be required for establishing a long-term carbon cycle and maintaining a stable, habitable climate. The model is necessarily highly simplified, as the uncertainties with exoplanet thermal evolution and outgassing are large. Nevertheless, the results provide some first-order guidance for future exoplanet missions, by predicting the age at which habitability becomes unlikely for a stagnant lid planet as a function of initial radiogenic heat budget. This prediction is powerful because both planet heat budget and age can potentially be constrained from stellar observations. Key Words: Exoplanets-Habitability-Stagnant lid tectonics-Carbon cycle-Volcanism. Astrobiology 18, 873-896.

Carbon cycling and interior evolution of water-covered plate tectonics and stagnant-lid planets

“…The release of CO 2 back into the atmosphere caused by the increase of temperature with depth is known as metamorphic decarbonation (e.g., Bickle 1996). Accounting for burial and decarbonation of the crust, Foley & Smye (2018) showed that for stagnant lid planets with an Earthlike total CO 2 budget, carbon cycling could provide a negative feedback avoiding a supply-limited regime, which would occur through a complete carbonation of the crust. It remains unclear, however, whether this negative feedback is sufficiently strong to regulate the climate.…”

“…However, the mantle temperature of a planet that is not tidally heated substantially evolves with time. This is particularly important for the stability of carbonates, which can enter the mantle in substantial quantities only if the temperature-depth gradient is shallow (Foley & Smye 2018). Foley (2019) explored the timespan a stagnant-lid planet can sustain active volcanism and therefore a temperate climate, depending on the planetary CO 2 budget and on the internal heating rate.…”

Venusian Habitable Climate Scenarios: Modeling Venus through time and applications to slowly rotating Venus-Like Exoplanets

“…Without a mechanism to efficiently cycle volatiles in a stagnant lid mode (e.g., Tosi et al, 2017;Höning et al, 2019), outgassing would have continued without the major weathering and subduction surface sinks that operate on Earth, hence CO 2 and N 2 would build up over time to reach the levels we see on Venus today. Some studies have also shown that even in a stagnant lid mode it is possible to cycle volatiles, possibly up to gigayears in time (e.g., Foley & Smye, 2018;Godolt et al, 2019), but these mechanisms depend on the initial CO 2 budget and the retention of at least some water after cooldown. 8.)…”

Uncertainty Quantification in Planetary Thermal History Models: Implications for Hypotheses Discrimination and Habitability Modeling

“…The question above relates to the extension of thermal history studies from Earth to planetary application, habitability in particular [Kite et al, 2009, Schaefer and Sasselov, 2015, Komacek and Abbot, 2016, Foley, 2015, Foley and Driscoll, 2016, Tosi et al, 2017, Foley and Smye, 2018, Barnes et al, 2019. Thermal history models applied to the Earth are postdictive: they set out to match historical data.…”

“…That connection to elemental cycling, along with the discovery of life that can tap into the Earth's internal energy and an expanding search for life beyond Earth, has rejuvenated interest in the cooling history of the Earth and, by association, thermal history models. This renaissance has moved thermal history modeling from the realm of geosciences into the realm of astronomy and astrophysics [Kite et al, 2009, Schaefer and Sasselov, 2015, Komacek and Abbot, 2016, Foley, 2015, Foley and Driscoll, 2016, Tosi et al, 2017, Foley and Smye, 2018, Rushby et al, 2018, Barnes et al, 2019.…”