Lethal Surface Ozone Concentrations Are Possible on Habitable Zone Exoplanets

Abstract

Ozone (O _3 ) is important for the survival of life on Earth because it shields the surface from ionizing ultraviolet radiation. However, the existence of O _3 in Earth’s atmosphere is not always beneficial. Resulting from anthropogenic activity, O _3 exists as a biologically harmful pollutant at the surface when it forms in the presence of sunlight and other pollutants. As a strong oxidizer, O _3 can be lethal to several different organisms; thus, when assessing the potential habitability of an exoplanet, a key part is determining whether toxic gases could be present at its surface. Using the Whole Atmosphere Community Climate Model version 6 (WACCM6; a three-dimensional chemistry-climate model), 12 atmospheric simulations of the terrestrial exoplanet TRAPPIST-1 e are performed with a variety of O _2 concentrations and assuming two different stellar spectra proposed in the literature. Four atmospheric simulations of the exoplanet Proxima Centauri b are also included. Some scenarios for both exoplanets exhibit time-averaged surface O _3 mixing ratios exceeding harmful levels of 40 ppbv, with 2120 ppbv the maximum concentration found in the cases simulated. These concentrations are toxic and can be fatal to most life on Earth. In other scenarios O _3 remains under harmful limits over a significant fraction of the surface, despite there being present regions that may prove inhospitable. In the case in which O _3 is detected in a terrestrial exoplanet’s atmosphere, determining the surface concentration is an important step when evaluating a planet’s habitability.

Keywords

• Exoplanets
• Habitable planets
• Exoplanet atmospheres
• Exoplanet atmospheric composition