Abstract

Ozone is a potential exoplanet biosignature due to its association with photosynthetically produced oxygen and strong absorption in the mid-infrared. However, the existence of ozone in Venus’s observable atmosphere, a planet with no known life, raises the possibility of ozone biosignature false-positives on Venus-like exoplanets. We use a photochemical model of Venus’s atmosphere to investigate the origin of its mesospheric ozone layer, and how similar ozone layers would manifest for Venus-like exoplanets. Our model shows that the hypothesis that Venus’s ozone forms on the nightside due to a flux of O radicals from the dayside, cannot generate enough ozone to match observed levels. Furthermore, we show that sufficient ozone cannot be produced by varying the surface chemistry, atmospheric thermal structure, or stellar flux in our model of Venus’s atmosphere, implying that a presently unknown chemical pathway is responsible for the ozone in Venus’s mesosphere. Until the origin of Venus’s ozone is understood, we cannot rule out that ozone production will be common on abiotic Venus-like worlds, a possibility that limits the usefulness of ozone as a biosignature.