Is ozone a reliable proxy for molecular oxygen? III. The impact of CH${}_4$ on the O${}_2$-O${}_3$ relationship for Earth-like atmospheres

Abstract

In the search for life in the Universe, molecular oxygen (O${}_2$) combined with a reducing species, such as methane (CH${}_4$), is considered a promising disequilibrium biosignature. In cases where it would be difficult or impossible to detect O${}_2$ (e.g., mid-IR or low O${}_2$ levels), it has been suggested that ozone (O${}_3$), the photochemical product of O${}_2$, could be used as a proxy for determining the abundance of O${}_2$. As the O${}_2$-O${}_3$ 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 O${}_3$ to infer O${}_2$. We used photochemistry and climate modeling to further explore the O${}_2$-O${}_3$ relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O${}_2$ between 0.01% to 150% along with high and low CH${}_4$ 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 (HO${}_x$) which destroys O${}_3$ through catalytic cycles and acts as a catalyst for the smog mechanism of O${}_3$ formation in the lower atmosphere. We find varying CH${}_4$ causes changes to the O${}_2$-O${}_3$ relationship in ways that are highly dependent on both host star and O${}_2$ abundance. A striking result for high CH${}_4$ models in high O${}_2$ atmospheres around hotter hosts is that enough CH${}_4$ is efficiently converted into H${}_2$O to significantly impact stratospheric temperatures, and therefore the formation/destruction rates of O${}_3$. Changes in HO${}_x$ also influenced the HO${}_x$ catalytic cycle and smog O${}_3$, causing variations in harmful UV reaching the surface as well as changes in the 9.6~$\mu$m O${}_3$ feature in emission spectra. This demonstrates the need to explore the O${}_2$-O${}_3$ relationship in order to use O${}_3$ as a reliable proxy for O${}_2$ in future observations.

In production with A&A

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