Is ozone a reliable proxy for molecular oxygen?

III. The impact of CH₄ on the O₂–O₃ relationship for Earth-like atmospheres

¹ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
² National Space Institute, Technical University of Denmark, Elektrovej, 2800 Kgs. Lyngby, Denmark
³ School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
⁴ School of Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK

^★ Corresponding author.

Received: 18 June 2025
Accepted: 7 August 2025

Abstract

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 (such as in the 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 known to be 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% and 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 it is also the source of hydrogen atoms for hydrogen oxide (HO_x), which destroys O₃ through catalytic cycles, and acts as a catalyst for the smog mechanism of O₃ formation in the lower atmosphere. We find that varying CH₄ causes changes to the O₂–O₃ relationship in ways that are highly dependent on both the 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 and destruction rates of O₃. Changes in HO_x have also been shown to influence both the HO_x catalytic cycle and production of 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 study further demonstrates the need to explore the O₂–O₃ relationship in different atmospheric compositions in order to use O₃ as a reliable proxy for O₂ in future observations.