Relative Abundances of CO<sub>2</sub>, CO, and CH<sub>4</sub> in Atmospheres of Earth-like Lifeless Planets
Yasuto Watanabe, Kazumi Ozaki
Abstract
Abstract Carbon is an essential element for life on Earth, and the relative abundances of major carbon species (CO 2 , CO, and CH 4 ) 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 CO 2 , CO, and CH 4 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 CO 2 levels trigger photochemical instability of the CO budget in the atmosphere (i.e., CO runaway) owing to enhanced CO 2 photolysis relative to H 2 O photolysis. Higher volcanic outgassing fluxes of reduced C (CO and CH 4 ) 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 CH 4 /CO 2 versus CO/CO 2 , 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.