Solar ultraviolet transfer in the Martian atmosphere: biological and geological implications

Abstract

The Martian environment has an exceedingly strong component of damaging solar far-ultraviolet radiation, including most of the UV-C range (190– 280 nm ) because of the lack of an effective ozone shield. Two-stream radiative transfer modelling, including particulate aerosol content and surface albedo, indicates that the present abundance of SO 2 does not provide any surface protection of the UV radiation. However, larger abundances of sulfur dioxide (mixing ratio, q, comprised between 10 −5 and 10 −4) introduced in the present 6 mbar Martian atmosphere can partially protect the surface from the harmful solar UV radiation. Furthermore, Mie backscattering by dust and/or aerosols noticeably reduces the harmful solar UV radiation. Regardless of the ozone concentration, the required dust content for almost blocking this damaging radiation is such that the optical depth at 550 nm is τ=0.8–1.5 (for a given solar zenithal angle (SZA) of 38°), typical of a turbid atmosphere, and τ⩾1.6 more characteristic of dust storms. The required mass of SO 2 (i.e. 10 14– 10 15 gr ) and/or ashes could have been easily provided by volcanic activity on Mars several times along the entire geologic history of the planet. In terms of DNA protection, volcanic ashes and SO 2 considerably reduced levels of UV radiation lead to a biological dose comparable to the existing on the present Earth, together with the possibility of a non-deterioration of the environment due to UV photo-oxidation. Therefore, preserving life forms on Mars surface at any past epoch cannot be completely ruled out.