CO-rich atmospheres on terrestrial planets

抄録

  Understanding the environmental conditions that enable life to emerge remains a central challenge in Earth and planetary sciences. On terrestrial planets, the relative abundances of carbon-bearing species—carbon dioxide (CO₂), carbon monoxide (CO), and methane (CH₄)—in the atmosphere exert fundamental control on climate, ocean chemistry, and (bio)geochemical cycles, thereby linking atmospheric composition to planetary habitability and the origin of life. Among these gases, CO is of particular interest because it is relevant to both prebiotic chemistry and to the interpretation of atmospheric biosignatures, motivating a deeper investigation into how CO-rich atmospheres form, how they may support prebiotic chemistry, and how such atmospheres can be detected with the next generation of telescopes. Here, we summarize the potential importance of CO in both origin of life research and biosignature assessment, focusing on theoretical modeling studies that examine conditions for the formation of CO-rich atmospheres on prebiotic Earth (and potentially habitable exoplanets more broadly). These studies have explored how key planetary parameters—including atmospheric CO₂ levels, volcanic outgassing, and stellar spectral types—shape atmospheric CO abundance, with particular attention to conditions that can trigger photochemical instability of the CO budget—i.e., CO runaway. We further discuss implications for prebiotic organic synthesis, with formaldehyde as a key intermediate. Our results indicate that CO-rich atmospheres are most likely under elevated CO₂ levels, high volcanic outgassing fluxes of reducing gases, cool climates, and irradiation from low-mass stars. These findings have important ramifications for future spectroscopic observations with next-generation observatories, offering new constraints on planetary environments that may support porebiotic chemistry.