Atmospheric effects of stellar cosmic rays on Earth-like exoplanets orbiting M-dwarfs

Abstract

M-dwarf stars are generally considered favourable for rocky planet detection. However, such planets may be subject to extreme conditions due to possible high stellar activity. The goal of this work is to determine the potential effect of stellar cosmic rays on key atmospheric species of Earth-like planets orbiting in the habitable zone of M-dwarf stars and show corresponding changes in the planetary spectra. We build upon the cosmic rays model scheme of Grenfell et al. (2012), who considered cosmic ray induced NOx production, by adding further cosmic ray induced production mechanisms (e.g. for HOx) and introducing primary protons of a wider energy range (16 MeV - 0.5 TeV). Previous studies suggested that planets in the habitable zone that are subject to strong flaring conditions have high atmospheric methane concentrations, while their ozone biosignature is completely destroyed. Our current study shows, however, that adding cosmic ray induced HOx production can cause a decrease in atmospheric methane abundance of up to 80\%. Furthermore, the cosmic ray induced HOx molecules react with NOx to produce HNO$_3$, which produces strong HNO$_3$ signals in the theoretical spectra and reduces NOx-induced catalytic destruction of ozone so that more than 25\% of the ozone column remains. Hence, an ozone signal remains visible in the theoretical spectrum (albeit with a weaker intensity) when incorporating the new cosmic ray induced NOx and HOx schemes, even for a constantly flaring M-star case. We also find that HNO$_3$ levels may be high enough to be potentially detectable. Since ozone concentrations, which act as the key shield against harmful UV radiation, are affected by cosmic rays via NOx-induced catalytic destruction of ozone, the impact of stellar cosmic rays on surface UV fluxes is also studied.