Silicate weathering and the habitability of temperate, rocky planets
Keywords: silicate weathering
exoplanets
planetary atmospheres
Earth-like planets
planetary habitability
paleoclimate
exoplanetary astronomy
climate
habitability
climate feedbacks

Through the climate-dependence of silicate weathering, the carbonate-silicate cycle is thought to provide a powerful negative feedback on atmospheric CO2 that has maintained clement surface conditions on Earth almost continuously for 4 billion years in the face of profound environmental change. The fundamental role played by silicate weathering in sustaining long-term habitability on Earth has led to the hypothesis that sufficiently “Earth-like” exoplanets may have their climates regulated by the same process. This allows the definition of a “circumstellar habitable zone” around a star, where planets with carbonate-silicate cycling could in principle adjust CO2 to levels that prevent runaway glaciation or extreme surface warmth. This definition of the habitable zone has become a major organizing principle in exoplanetary astronomy, guiding telescope design and target prioritization. This thesis will deploy coupled models of exoplanetary climate, ocean chemistry, and silicate weathering to examine the implications and validity of the assumption that Earth-like planets can regulate their climates with silicate weathering throughout the putative habitable zone.
In the Introduction (Chapter 1), I review the habitable zone concept, qualitatively describe the carbonate-silicate cycle and the stabilizing negative feedback on climate that it provides, go over the evidence for the importance of the cycle and feedback in Earth history, and describe how these concepts have been applied to evaluate exoplanet carbon cycling in various modeling studies. In Chapter 2, I apply a global-mean climate model and a global-mean ocean chemistry model to show that climate and ocean pH on exoplanets throughout most of the habitable zone will be insensitive to CO2 perturbations of sizes that have caused catastrophes in Earth’s past if we assume the carbonate-silicate cycle adjusts CO2 to high levels on exoplanets receiving low instellation. In Chapter 3, I conduct the first simulations of exoplanet weathering to account for the crucial impact of weathering zone clay precipitation, which leads to large changes in predicted climate behavior compared to previous models that neglected this effect. In particular, I find a much larger climate sensitivity to parameters like land fraction and outgassing in the more complex weathering model, and large sensitivity to parameters like lithology and soil thickness that previous models were too crude to evaluate. This suggests climate control by silicate weathering on exoplanets (and, perhaps, the Earth) may be less efficient than previously assumed. In Chapter 4, I model weathering and climate at various instellations under extremely high CO2 conditions to demonstrate that the carbonate-silicate cycle can behave as a positive feedback on climate when CO2’s Rayleigh scattering effect begins to outweigh its greenhouse effect, converting the molecule into a planetary coolant at high pressures. This may lead to the accumulation of oceans of liquid CO2 and CO2 clathrate hydrates under outgassing and instellation conditions that also support stable, temperate, low-CO2 climate equilibria, implying a novel form of climate hysteresis and bistability for Earth-like planets in the outer reaches of the HZ. Finally, in Chapter 5, I drive weathering models with precipitation and temperature fields from a 3D global climate model to demonstrate that surface energy budget strongly constrains hydrology and therefore weathering behavior for planets with high CO2 at low instellations, if the impact of clay formation in the weathering zone is accounted for. Contrary to expectations from the modern Earth, precipitation rates can actually fall with increasing CO2 and surface temperature when planets begin to bump up against these surface energy budget constraints. This allows silicate weathering to become a destabilizing feedback at lower CO2 levels and higher instellations than those required for the scattering-based destabilization mechanism described in Chapter 4, and may mean that low-instellation planets are very susceptible to runaway CO2 accumulation and attendant catastrophic warming or surface CO2 condensation.
Overall, the work presented in this thesis suggests that the efficacy of the silicate weathering feedback at maintaining clement environments on exoplanets throughout the classical circumstellar habitable zone remains an open question, which may have broad implications for our current strategies for remotely detecting life beyond Earth.