Biosignature false positives in potentially habitable planets around M dwarfs: the effect of UV radiation from one flare

Abstract

Many past studies have predicted the steady-state production and maintenance of abiotic O$_2$ and O$_3$ in the atmospheres of CO$_2$-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 O$_2$/O$_3$ production in prebiotic, CO$_2$-dominated atmospheres of M dwarf planets with CO$_2$ levels ranging from 10% to 90% of 1 bar. We find that a single flare generally destroys O$_2$ while modestly enhancing O$_3$ column densities. 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 O$_3$ UV Hartley band is modestly enhanced by a maximum of 6 ppm while the CO$_2$ molecular transit depths modestly decline by 7 ppm. In both emitted and transmitted light spectra, the 9.65 $\mu$m O$_3$ band is hidden by the overlapping 9.4 $\mu$m CO$_2$ band for all scenarios considered. Overall, we find that the possible enhancements of abiotic O$_3$ due to a single flare are small compared to O$_3$'s sensitivity to other parameters such as CO$_2$ and H$_2$O abundances or the availability of reducing gases such as H$_2$.