Cosmic Ray and UV Radiation Models on the Ancient Martian Surface

The atmospheric evolution on Mars is influenced by nonthermal loss processes of heavy atmospheric constituents. Since Mars does not have an appreciable intrinsic magnetic field at present and a comparatively small gravitational acceleration, all known atmospheric loss processes can be active and several important constituents are lost to space. The escape rates of atmospheric constituents including water from Mars indicate that the red planet could have lost an atmosphere of at least 1 bar to space during the past 3.5 Gyr. We investigated with a theoretical model the atmospheric flux of cosmic-ray-induced particles on the martian surface by solving Boltzmann equations governing the propagation of protons, neutrons, muons, and pions in 7-mbar and 1-bar CO 2 atmospheres. We found that, at present, the thin atmosphere is not able to protect the surface of Mars from cosmic radiation and most of the incoming protons and neutrons impact the ground with energy fluxes of about 6000 and 1400 MeV cm −2 s −1, respectively. On the other hand, the much denser ancient atmosphere attenuated the radiation and only secondary particles, mainly muons, were able to reach the surface with energy fluxes of about 100 MeV cm −2 s −1, while the fluxes of protons and neutrons were negligible. We also evaluated the influence of an intrinsic magnetic field on the magnitude of the surface flux of charged cosmic particles and found no effect, even if one assumes an Earth-type magnetic field on ancient Mars, as suggested by recent Mars Global Surveyor magnetometer data. In addition we investigated the shielding of the martian surface from ultraviolet (UV) radiation by a dense early 1-bar CO 2 atmosphere including additional absorption effects of O 2 and O 3 and Rayleigh scattering. This atmosphere gradually becomes transparent to UV radiation above wavelengths of 220 nm.