Is Ozone A Reliable Proxy For Molecular Oxygen? III. The Impact Of CH4 On The O2-O3 Relationship For Earth-like Atmospheres

In the search for life in the Universe, molecular oxygen (O₂) combined with a reducing species, such as methane (CH₄), is considered a promising disequilibrium biosignature.

In cases where it would be difficult or impossible to detect O₂ (e.g., mid-IR or low O₂ levels), it has been suggested that ozone (O₃), the photochemical product of O₂, could be used as a proxy for determining the abundance of O₂. As the O₂-O₃ relationship is nonlinear, the goal of this series of papers is to explore how it would change for different host stars and atmospheric compositions and learning how to use O3 to infer O₂.

We used photochemistry and climate modeling to further explore the O₂-O₃ relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O₂ between 0.01% to 150% along with high and low CH₄ abundances of 1000% and 10% PAL, respectively. Methane is of interest not only because it is a biosignature, but also the source of hydrogen atoms for hydrogen oxide (HOx) which destroys O₃ through catalytic cycles and acts as a catalyst for the smog mechanism of O₃ formation in the lower atmosphere.

We find varying CH₄ causes changes to the O₂-O₃ relationship in ways that are highly dependent on both host star and O₂ abundance. A striking result for high CH₄ models in high O₂ atmospheres around hotter hosts is that enough CH4 is efficiently converted into H₂O to significantly impact stratospheric temperatures, and therefore the formation/destruction rates of O₃.

Changes in HOx also influenced the HOx catalytic cycle and smog O3, causing variations in harmful UV reaching the surface as well as changes in the 9.6~μm O₃ feature in emission spectra. This demonstrates the need to explore the O₂-O₃ relationship in order to use O₃ as a reliable proxy for O₂ in future observations.

Thea Kozakis, João M. Mendonça, Lars A. Buchhave, Luisa M. Lara

Comments: In production with A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.19062 [astro-ph.EP] (or arXiv:2508.19062v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.19062
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Related DOI:
https://doi.org/10.1051/0004-6361/202556015
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Submission history
From: Thea Kozakis
[v1] Tue, 26 Aug 2025 14:21:28 UTC (682 KB)
https://arxiv.org/abs/2508.19062

Astrobiology