The Impact of Extended CO2 Cross Sections on Temperate Anoxic Planet Atmospheres

Abstract

Our interpretation of terrestrial exoplanet atmospheric spectra will always be limited by the accuracy of the data we use as input in our forward and retrieval models. Ultraviolet molecular absorption cross sections are one category of these essential model inputs; however, they are often poorly characterized at the longest wavelengths relevant to photodissociation. Photolysis reactions dominate the chemical kinetics of temperate terrestrial planet atmospheres. One molecule of particular importance is CO₂, which is likely present in all terrestrial planet atmospheres. The photolysis of CO₂ can introduce CO and O, as well as shield tropospheric water vapor from undergoing photolysis. This is important because H₂O photolysis produces OH, which serves as a major reactive sink to many atmospheric trace gases. Here, we construct CO₂ cross-section prescriptions at 195 K and 300 K extrapolated beyond 200 nm from measured cross sections. We compare results from the implementation of these new cross sections to the most commonly used CO₂ prescriptions for temperate terrestrial planets with Archean-like atmospheres. We generally find that the observational consequences of CO₂ dissociation beyond 200 nm are minimal so long as our least conservative (highest opacity) prescription can be ruled out. Moreover, implementing our recommended extended CO₂ cross sections does not substantially alter previous results that show the consequential photochemical impact of extended H₂O cross sections.