Is ozone a reliable proxy for molecular oxygen?

Abstract

Molecular oxygen (O 2 ) paired with a reducing gas is regarded as a promising biosignature pair for the atmospheric characterization of terrestrial exoplanets. In circumstances when O 2 may not be detectable in a planetary atmosphere (e.g., at mid-IR wavelengths) it has been suggested that ozone (O 3 ), the photochemical product of O 2 , could be used as a proxy to infer the presence of O 2 . However, O 3 production has a nonlinear dependence on O 2 and is strongly influenced by the UV spectrum of the host star. To evaluate the reliability of O 3 as a proxy for O 2 , we used Atmos, a 1D coupled climate and photochemistry code, to study the O 2 -O 3 relationship for "Earth-like" habitable zone planets around a variety of stellar hosts (G0V-M5V) and O 2 abundances. Overall, we found that the O 2 -O 3 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 O 3 when O 2 levels are initially decreased from 100% Earth's present atmospheric level (PAL), with a maximum O 3 abundance occurring at 25-55% PAL O 2 . As O 2 abundance initially decreases, larger amounts of UV photons capable of O 2 photolysis reach the lower (denser) regions of the atmosphere where O 3 production is more efficient, thus resulting in these increased O 3 levels. This effect does not occur for cooler host stars (K5V-M5V), since the weaker incident UV flux does not allow O 3 formation to occur at dense enough regions of the atmosphere where the faster O 3 production can outweigh a smaller source of O 2 from which to create O 3 . Thus, planets experiencing higher amounts of incident UV possessed larger stratospheric temperature inversions, leading to shallower O 3 features in planetary emission spectra. Overall it will be extremely difficult (or impossible) to infer precise O 2 levels from an O 3 measurement, however, with information about the UV spectrum of the host star and context clues, O 3 will provide valuable information about potential surface habitability of an exoplanet.

Bayesian Analysis for Remote Biosignature Identification on exoEarths (BARBIE). II. Using Grid-based Nested Sampling in Coronagraphy Observation Simulations for O2 and O3

“…This O 2 /O 3 relationship means that we can constrain the overall molecular oxygen abundance by detecting either of the species; O 3 is in fact a highly sensitive tracer of lower abundances of O 2 . In order to explore the impact of this, we examined several scenarios where we varied our parameter values as coupled parameters, i.e., 10% PAL O2/105% PAL O3, etc., consistent with the results for a Sun-like star from Kozakis et al (2022; as seen in Table 3). We present our detectability results for these abundance pairs in Figure 9.…”

“…However, due to the photochemical relationship between the O 2 abundance and the production of O 3 , the abundances of both molecules would actually be linked in any atmospheric scenario. As found in Kozakis et al (2022), the relationship between O 2 and O 3 varies for model atmospheres depending on the type of host star, and the linearity of the relationship also appears to vary. When looking at hotter host stars (e.g., a G2 star), peak O 3 abundance occurs at lower than modern-Earth O 2 abundances due to the O 3 layer shifting in the atmosphere downward to O 2 levels due to O 2 photochemical shielding.…”

Metal-rich stars are less suitable for the evolution of life on their planets

“…Smaller O 2 amounts cause the ozone layer to form in lower and denser atmospheric layers where more O 2 is available for O 3 production (Supplementary Fig. 3a, b ), leaving the total O 3 column density only weakly affected 36 .…”

Lightning-induced chemistry on tidally-locked Earth-like exoplanets

“…Furthermore, 1-D photochemical models have been used to quantify the amount of ozone resulting from different UV distributions in SEDs (e.g. Segura et al 2005Segura et al , 2010Domagal-Goldman et al 2014;O'Malley-James & Kaltenegger 2017;Teal et al 2022) and alongside varying molecular oxygen abundances (Kozakis et al 2022). To illustrate the dependence of assuming different SEDs on ozone photochemistry we compare in Appendix A results from the BT-Settl spectrum and the improved bin distribution, with the results we report here using the MUSCLES spectrum.…”

“…Ozone is the photochemical byproduct of molecular oxygen and thus depends on oxygen levels, as is shown by the 3-D CCM simulations of Cooke et al (2022b) for various epochs in Earth's history. Furthermore, the nonlinear oxygen-ozone relationship depends on the host star's UV spectrum (Kozakis et al 2022). Since oxygen on Earth is largely produced by sources of biological origin (Schwieterman et al 2018), its photochemical product ozone can be seen as a potential biosignature.…”