2025ApJ...989...34M

Many past studies have predicted the steady-state production and maintenance of abiotic O₂ and O₃ in the atmospheres of CO₂-rich terrestrial planets orbiting M dwarf stars. However, the time-dependent responses of these planetary atmospheres to flare events-and the possible temporary production or enhancement of false-positive biosignatures therein-has been comparatively less well studied. Most past works that have modeled the photochemical response to flares have assumed abundant free oxygen, like that of the modern or Proterozoic Earth. Here we examine in detail the photochemical impact of the UV emitted by a single flare on abiotic O₂/O₃ production in prebiotic, CO₂-dominated atmospheres of M dwarf planets with CO₂ levels ranging from 3% to 80% of 1 bar. We find that a single flare generally destroys O₂ and O₃ over short timescales while modestly enhancing their column densities over intermediate timescales. We simulate the spectral observables of both the steady-state atmosphere and time-dependent spectral response over the flare window for both emitted and transmitted light spectra. Over the course of the flare, the O₃ UV Hartley band is decreased by a maximum of 47 ppm. In both emitted and transmitted light spectra, the 9.65 μm O₃ band is hidden by the overlapping 9.4 μm CO₂ band for all scenarios considered. Overall, we find that the possible enhancements of abiotic O₃ due to a single flare are small compared to O₃'s sensitivity to other parameters such as CO₂ and H₂O abundances or the availability of reducing gases such as H₂.