Response of Atmospheric Biomarkers to NOx-induced Photochemistry Generated by Stellar Cosmic Rays for Earth-like Planets in the Habitable Zone of M-Dwarf Stars

摘要

Understanding whether M-dwarf stars may host habitable planets withEarth-like atmospheres and biospheres is a major goal in exoplanet research. Ifsuch planets exist, the question remains as to whether they could be identifiedvia spectral signatures of biomarkers. Such planets may be exposed to extremeintensities of cosmic rays that could perturb their atmospheric photochemistry.Here, we consider stellar activity of M-dwarfs ranging from quiet up to strongflaring conditions and investigate one particular effect upon biomarkers,namely, the ability of secondary electrons caused by stellar cosmic rays tobreak up atmospheric molecular nitrogen (N2), which leads to production ofnitrogen oxides in the planetary atmosphere, hence affecting biomarkers such asozone. We apply a stationary model, that is, without a time-dependence, hencewe are calculating the limiting case where the atmospheric chemistry responsetime of the biomarkers is assumed to be slow and remains constant compared withrapid forcing by the impinging stellar flares. This point should be furtherexplored in future work with time-dependent models. For the flaring case O3 ismainly destroyed via direct titration with nitrogen oxides and not via thefamiliar catalytic cycle photochemistry, which occurs on Earth. For scenarioswith low O3, Rayleigh scattering by the main atmospheric gases became moreimportant for shielding the planetary surface from ultra-violet radiation. Amajor result of this work is that the biomarker O3 survived all thestellar-activity scenarios considered except for the strong case, whereas thebiomarker nitrous oxide could survive in the planetary atmosphere under allconditions of stellar activity considered here, which clearly has importantimplications for missions that aim to detect spectroscopic biomarkers.