[U-11] Biogeochemistry of CO world

[U11-01] Conditions for CO runaway in atmospheres of the Earth and Earth-like exoplanets and implications for habitability

★Invited Papers

*Yasuto Watanabe¹, Kazumi Ozaki² (1.Meteorological Research Institute/University of Tokyo, 2.Tokyo Institute of Technology)

Keywords:Early Atmosphere

With the progress of exoplanetary science, the primary focus of this field is gradually shifting from exoplanet detection to identification of signs of habitability. The concept of habitability has centered around the presence of liquid water on the planetary surface, given its vital role in supporting life on Earth (e.g., Kasting et al. 1993). For a further constraint on habitability, considerations of additional factors, such as atmospheric composition and climatic conditions would be required. One of the most critical factors would be the behaviors of carbon. Carbon is an element essential for life on Earth, and it exerts fundamental controls on planetary climate because both CO₂ and CH₄ are potent greenhouse gases. In addition, CO is considered to be crucial for the early evolution of life, serving as an important source of carbon and energy for microorganisms. More importantly, it has been recognized that there is an existence of a state called CO runaway on early Earth (Kasting et al. 1983), under which the rate of CO production increases to a level higher than the rate of water vapor photolysis, leading to CO accumulation over time, which may have facilitated the emergence of life on Earth (e.g., Zang et al. 2022). Therefore, understanding the factors that control the relative abundances of CO₂, CO, and CH₄ in planetary atmospheres has important implications in the search for habitable planets beyond our solar system.
Here we employed a theoretical model of atmospheric chemistry to investigate diversity in the atmospheric abundances of CO₂, CO, and CH₄ on Earth-like lifeless planets orbiting Sun-like (F-, G-, and K-type) stars, with a specific focus on the conditions for the occurrence of CO runaway behaviors. We demonstrate that elevated atmospheric CO₂ levels trigger photochemical instability of the CO budget in the atmosphere (i.e., CO runaway) owing to enhanced CO₂ photolysis relative to H₂O photolysis on Earth and Earth-like exoplanets around Sun-like stars. We also found that the conditions for CO runaway in exoplanets orbiting K-type stars extend over a wide range of atmospheric pCO₂ and reducing gas outgassing flux, suggesting that Earth-like planets orbiting K-type stars might be a feasible candidate for searching exoplanets characterized by CO runaway. By contrast, for the case of exoplanets orbiting F-type stars, CO runaway does not occur in the range of atmospheric pCO₂ and reducing gas outgassing flux expected for early Earth. Our systematic examinations revealed that anoxic atmospheres of Earth-like lifeless exoplanets could be classified in the phase space of CH₄/CO₂ versus CO/CO₂, where a distinct gap in atmospheric carbon chemistry is expected to be observed. This CO-runaway gap structure would be a general feature of Earth-like planets orbiting Sun-like stars. These results would indicate that the gap structure is a general feature of Earth-like lifeless planets with reducing atmospheres orbiting Sun-like (F-, G-, and K-type) stars. We expect that the most preferable condition for CO runaway is near the outer edge of the habitable zone because atmospheric pCO₂ is expected to be high in this region owing to the operation of carbonate–silicate geochemical cycles. These findings would advance our understanding of the intricate relationship between the spectral types of central stars, atmospheric composition, climate, tectonic activity, and the origin of life, contributing to the ongoing exploration of habitable worlds beyond our solar system.