Detecting Biosignatures on Weakly Oxygenated Terrestrial Exoplanets: The Importance of UV Imaging Capabilities of Next Generation Telescopes

The strongest remotely detectable signature of life on our planet today is the photosynthetically produced oxygen (O₂) in our atmosphere. However, recent studies of Earth's geochemical proxy record suggest that for all but the last 500 million years, atmospheric O₂ would have been undetectable to a remote observer—and thus a potential 'false negative' for life. During an extended period in Earth's middle history known as the mid-Proterozoic ( 2.0 to 0.8 billion years ago, Ga), O₂ was likely present but in low concentrations, with some pO₂ estimates of 0.1 - 1% of present-day levels. Although O₂ has a weak spectral impact in reflected light at these low abundances, O₃ in photochemical equilibrium with that O₂ would produce notable spectral features in the UV Hartley-Huggins band ( 0.25 µm), with a weaker impact in the mid-IR band near 9.7 µm. In contrast to O₃, near-modern levels of CH₄ (< 10 ppm) and even high levels of N₂O may elude detection in direct-imaging observations. Thus, taking Earth history as an informative example, there likely exists a category of exoplanets for which conventional biosignatures can only be identified in the UV. In this presentation, we detail the importance of UV planet-imaging capabilities in the design of future space-based direct imaging telescopes such as HabEx and LUVOIR to detect O₃ on planets with low-intermediate oxygenation states. We use a coronagraph instrument model to show that the Hartley-Huggins O₃ UV band is detectable in reflected light for a low-oxygen exoplanet (pO₂ <= 0.1% PAL) orbiting nearby FGK stars with 10-m class mirrors. Additionally, we show that life on a low-O₂ planet like the mid-Proterozoic Earth may be identified by seasonal variations in the O₃ UV band, which would assist in accurately interpreting biogenicity in the face of potential 'false positive' mechanisms for O₂/O₃ build-up such as CO₂ photolysis or H-escape. We predict O₃ seasonality may be observable due to non-saturated levels of O₃ and large relative (rather than absolute) variations in seasonal O₂ production with season, which may require molecular fluxes greater than plausible abiotic O₂ production mechanisms. Together these factors highlight the importance of the UV O₃ biosignature in finding life beyond the solar system.