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

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 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 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 CO₂ prescriptions for temperate, terrestrial planets with Archean-like atmospheres. We generally find that the observational consequences of CO₂ dissociation beyond 200 nm is 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 showing the consequential photochemical impact of extended H₂O cross sections.

Transmission spectra comparison between the recommended, most conservative, and least conservative CO₂ prescriptions, for three different scenarios. The top panel shows transmission spectra for the a planet orbiting the Sun, with a CH₄-flux = 2 × 10¹⁰ molecules cm⁻² s ⁻¹ , with a surface CO₂ volume mixing ratio of 3%. The middle and bottom panel show the transmission spectra for a planet orbiting TRAPPIST-1 with a CH₄-flux = 1 × 10⁹ molecules cm⁻² s ⁻¹ ; the middle also has a surface CO₂ volume mixing ratio of 3%, while the bottom panel has a surface CO₂ volume mixing ratio of 10%. Features that are shared between spectra are labeled in black, features that differ depending on the cross-section prescription are labeled in red. — astro-ph.EP

Wynter Broussard, Edward W. Schwieterman, Clara Sousa-Silva, Grace Sanger-Johnson, Sukrit Ranjan, Olivia Venot

Comments: 24 pages, 21 figures, 3 tables. Accepted for publication in the Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.08434 [astro-ph.EP] (or arXiv:2501.08434v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.08434
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Submission history
From: Wynter Broussard
[v1] Tue, 14 Jan 2025 20:57:16 UTC (17,668 KB)
https://arxiv.org/abs/2501.08434

Astrobiology