Science-driven performance requirements for UV coronagraph design: implications for O3 detection in Earth-like atmospheres

Detecting biosignatures on Earth-like exoplanets from the Proterozoic era requires instrumentation capable of probing near-ultraviolet (NUV) spectral features, particularly ozone (O3), which serves as an indirect proxy for molecular oxygen (O2) when direct detection is challenging. Using a combination of photochemical-climate modeling and atmospheric retrieval simulations, we quantify fiducial instrument performance requirements for robust O3 detection and assess the risk of spectral false positives from sulfur dioxide (SO2). Our analysis demonstrates the O3 abundances for Proterozoic Earth-like planets with O2 as low as 0.1% the present atmospheric level can be constrained to within an order of magnitude at 1σ confidence using three adjacent photometric bandpasses at a signal-to-noise ratio of 10 at 0.366 μm, each with a 15% width, and a short wavelength cutoff for those observations near 0.26 μm. The self-consistent steady-state atmospheric modeling results show that SO2 buildup is difficult in a habitable environment with appreciable amounts of water vapor in its atmosphere and would require volcanic outgassing rates at least two orders of magnitude above present-day Earth to reach surface abundance levels comparable to Proterozoic Earth-like abundances of O3. Even under these high SO2 flux scenarios, SO2 remains largely unconstrained with the recommended observational configuration, reinforcing the need for NUV coverage in coronagraphic instruments. Our findings support the inclusion of UV-optimized optics and detectors on the Habitable Worlds Observatory (HWO) to enable biosignature detection strategies sensitive to atmospheric conditions across key phases of Earth’s geological history.