Is ozone a reliable proxy for molecular oxygen?

II. The impact of N₂O on the O₂-O₃ relationship for Earth-like atmospheres

¹ National Space Institute, Technical University of Denmark, Elektrovej, 2800 Kgs. Lyngby, Denmark
² Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
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: 24 April 2025
Accepted: 27 May 2025

Abstract

Molecular oxygen (O₂) will be an important molecule in the search for biosignatures in terrestrial planetary atmospheres in the coming decades. In particular, O₂ combined with a reducing gas (e.g., methane) is considered strong evidence for disequilibrium caused by surface life. However, there are circumstances where it would be very difficult or impossible to detect O₂, in which case it has been suggested that ozone (O₃), the photochemical product of O₂, could be used instead. Unfortunately, the O₂-O₃ relationship is highly nonlinear and dependent on the host star, as shown in detail in the first paper of this series. This paper further explores the O₂-O₃ relationship around G0V-M5V host stars, using climate and photochemistry modeling to simulate atmospheres while varying abundances of O₂ and nitrous oxide (N₂O). Nitrous oxide is of particular importance to the O₂-O₃ relationship not only because it is produced biologically, but because it is the primary source of nitrogen oxides (NO_x), which fuel the NO_x catalytic cycle, which destroys O₃ and the smog mechanism that produces O₃. In our models we varied the O₂ mixing ratio from 0.01–150% of the present atmospheric level (PAL) and N₂O abundances of 10% and 1000% PAL. We find that varying N₂O impacts the O₂-O₃ relationship differently depending strongly on both the host star and the amount of atmospheric O₂. Planets orbiting hotter hosts with strong UV fluxes efficiently convert N₂O into NO_x, often depleting a significant amount of O₃ via faster NO_x catalytic cycles. However, for cooler hosts and low O₂ levels we find that increasing N₂O can lead to an increase in overall O₃ due to the smog mechanism producing O₃ in the lower atmosphere. Variations in O₃ result in significant changes in the amount of harmful UV reaching the surfaces of the model planets as well as the strength of the 9.6 µm O₃ emission spectral feature, demonstrating potential impacts on habitability and future observations.