Photochemistry in Terrestrial Exoplanet Atmospheres I: Photochemistry Model and Benchmark Cases

Abstract: We present a comprehensive photochemistry model for exploration of the chemical composition of terrestrial exoplanet atmospheres. The photochemistry model is designed from the ground up to have the capacity to treat all types of terrestrial planet atmospheres, ranging from oxidizing through reducing, which makes the code suitable for applications for the wide range of anticipated terrestrial exoplanet compositions. The one-dimensional chemical transport model treats up to 800 chemical reactions, photochemical processes, dry and wet deposition, surface emission and thermal escape of O, H, C, N and S bearing species, as well as formation and deposition of elemental sulfur and sulfuric acid aerosols. We validate the model by computing the atmospheric composition of current Earth and Mars and find agreement with observations of major trace gases in Earth's and Mars' atmospheres. We simulate several plausible atmospheric scenarios of terrestrial exoplanets, and choose three benchmark cases for atmospheres from reducing to oxidizing. The most interesting finding is that atomic hydrogen is always a more abundant reactive radical than the hydroxyl radical in anoxic atmospheres. Whether atomic hydrogen is the most important removal path for a molecule of interest also depends on the relevant reaction rates. We also find that volcanic carbon compounds (i.e., CH4 and CO2) are chemically long-lived and tend to be well mixed in both reducing and oxidizing atmospheres, and their dry deposition velocities to the surface control the atmospheric oxidation states. Furthermore, we revisit whether photochemically produced oxygen can cause false positives for detecting oxygenic photosynthesis, and find that in 1-bar CO2-rich atmospheres oxygen and ozone may build up to levels that have been previously considered unique signatures of life, if there is no surface emission of reducing gases...

Photochemistry in Terrestrial Exoplanet Atmospheres

This paper provides a detailed exposition of a photochemical model designed to analyze the atmospheric composition of terrestrial exoplanets. The model has been created with flexibility and comprehensiveness to cover a broad spectrum of atmospheric types, ranging from reducing to oxidizing environments. The authors use a one-dimensional chemical transport model to evaluate up to 800 chemical reactions, including photochemical processes, dry and wet deposition, surface emission, and thermal escape of various chemical species (O, H, C, N, S), alongside the formation and deposition of elemental sulfur and sulfuric acid aerosols.

Model Validation

The robustness of the model is demonstrated through its application to current Earth and Mars atmospheres, showing agreement with observational data of key trace gases in these planetary environments. This validation is critical, as it confirms the model’s applicability to known planetary atmospheres before extending its use to less understood exoplanetary conditions.

Atmospheric Scenarios

The authors simulate several hypothetical exoplanet atmospheres, selecting three benchmark cases representing a transition from reducing to oxidizing conditions:

• Reducing Atmosphere: Predominantly composed of \ce{H2} and \ce{N2}, the paper highlights that atomic hydrogen (\ce{H}) is a dominant reactive radical compared to hydroxyl radicals (\ce{OH}), which is significant in such anoxic conditions.
• Weakly Oxidizing Atmosphere: Primarily \ce{N2}, the study notes a higher abundance of both reducing and oxidizing radicals compared to the reducing scenario, influencing the oxidation processes and atmospheric stability.
• Highly Oxidizing Atmosphere: Comprised mostly of \ce{CO2}, here, atomic oxygen becomes the most prevalent reactive radical, impacting the atmospheric chemical network profoundly.

In these cases, the model illustrates critical chemical interactions, suggesting that volcanic carbon compounds like \ce{CH4} and \ce{CO2} are chemically stable across atmospheres of varying oxidation states, while sulfur compounds are less stable.

Abiotic Oxygen Production

One of the standout evaluations in the paper revolves around abiotic production of \ce{O2} and \ce{O3}. The authors challenge previous assertions by showing that photochemically produced oxygen can accumulate to levels sufficient to mimic potential biosignatures under certain atmospheric conditions, specifically in 1-bar \ce{CO2}-rich atmospheres without significant emission of reducing gases.

Implications and Speculations

The comprehensive nature of the model offers profound implications for astrobiology and planetary science. With its applicability across different scenarios, the model provides a tool for understanding atmospheric compositions that could potentially harbor biosignatures. It could aid in detecting life or assessing planet habitability in future exoplanet studies, especially in emissions or spectra characterization missions.

The authors encourage further exploration using this model to assess trace gases’ longevity and potential biosignatures in diverse planetary atmospheres. This opens doors for more refined interpretations of exoplanet data in identifying habitable worlds, offering a valuable framework in the pursuit of understanding atmospheres beyond the Solar System.