2024ApJ...961....1W

Carbon is an essential element for life on Earth, and the relative abundances of major carbon species (CO₂, CO, and CH₄) in the atmosphere exert fundamental controls on planetary climate and biogeochemistry. 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. We focused on the conditions for the formation of a CO-rich atmosphere, which would be favorable for the origin of life. Results demonstrated 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. Higher volcanic outgassing fluxes of reduced C (CO and CH₄) also tend to initiate CO runaway. Our systematic examinations revealed that anoxic atmospheres of Earth-like lifeless planets 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. Our findings 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.