The Impact of Extended CO$_2$ Cross Sections on Temperate Anoxic Planet Atmospheres

TL;DR

This study assesses how extended CO₂ ultraviolet absorption cross sections affect the photochemistry and spectra of temperate, anoxic exoplanet atmospheres, finding minimal impact when using less conservative models.

Contribution

It introduces extrapolated CO₂ cross sections at 195K and 300K beyond 200 nm and evaluates their effects on atmospheric photochemistry models.

Findings

Extended CO₂ cross sections have minimal impact on atmospheric photochemistry.

Using the most conservative (highest opacity) cross sections can significantly alter photolysis predictions.

Previous results on H₂O photochemistry remain valid with the new CO₂ data.

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 photo-dissociation. Photolysis reactions dominate the chemical kinetics of temperate terrestrial planet atmospheres. One molecule of particular importance is CO$_2$, which is likely present in all terrestrial planet atmospheres. The photolysis of CO$_2$ can introduce CO and O, as well as shield tropospheric water vapor from undergoing photolysis. This is important because H$_2$O photolysis produces OH, which serves as a major reactive sink to many atmospheric trace gases. Here, we construct CO$_2$ cross-section prescriptions at 195K and 300K extrapolated beyond 200 nm from measured cross sections. We compare results from the implementation of these new cross sections to the most commonly used CO$_2$ prescriptions for temperate, terrestrial planets with Archean-like atmospheres. We generally find that the observational consequences of CO$_2$ dissociation beyond 200 nm is minimal so long as our least conservative (highest opacity) prescription can be ruled out. Moreover, implementing our recommended extended CO$_2$ cross sections does not substantially alter previous results showing the consequential photochemical impact of extended H$_2$O cross sections.