Is ozone a reliable proxy for molecular oxygen?

I. The O₂–O₃ relationship for Earth-like atmospheres

National Space Institute, Technical University of Denmark, Elektrovej, 2800 Kgs. Lyngby, Denmark
e-mail: theakozakis@space.dtu.dk

Received: 1 June 2022
Accepted: 7 August 2022

Abstract

Molecular oxygen (O₂) paired with a reducing gas is regarded as a promising biosignature pair for the atmospheric characterization of terrestrial exoplanets. In circumstances when O₂ may not be detectable in a planetary atmosphere (e.g., at mid-IR wavelengths) it has been suggested that ozone (O₃), the photochemical product of O₂, could be used as a proxy to infer the presence of O₂. However, O₃ production has a nonlinear dependence on O₂ and is strongly influenced by the UV spectrum of the host star. To evaluate the reliability of O₃ as a proxy for O₂, we used Atmos, a 1D coupled climate and photochemistry code, to study the O₂–O₃ relationship for “Earth-like” habitable zone planets around a variety of stellar hosts (G0V-M5V) and O₂ abundances. Overall, we found that the O₂–O₃ relationship differed significantly with stellar hosts and resulted in different trends for hotter stars (G0V-K2V) versus cooler stars (K5V-M5V). Planets orbiting hotter host stars counter-intuitively experience an increase in O₃ when O₂ levels are initially decreased from 100% Earth’s present atmospheric level (PAL), with a maximum O₃ abundance occurring at 25–55% PAL O₂. As O₂ abundance initially decreases, larger amounts of UV photons capable of O₂ photolysis reach the lower (denser) regions of the atmosphere where O₃ production is more efficient, thus resulting in these increased O₃ levels. This effect does not occur for cooler host stars (K5V-M5V), since the weaker incident UV flux does not allow O₃ formation to occur at dense enough regions of the atmosphere where the faster O₃ production can outweigh a smaller source of O₂ from which to create O₃. Thus, planets experiencing higher amounts of incident UV possessed larger stratospheric temperature inversions, leading to shallower O₃ features in planetary emission spectra. Overall it will be extremely difficult (or impossible) to infer precise O₂ levels from an O₃ measurement, however, with information about the UV spectrum of the host star and context clues, O₃ will provide valuable information about potential surface habitability of an exoplanet.