Interaction of cosmic and solar flare radiations with the Martian atmosphere and their biological implications

Abstract

Assuming a constant interplanetary flux of galactic cosmic radiation and a model planetary atmosphere, it is possible to evaluate the magnitude of secondary ionization phenomena therein from parameters measured on Earth. The Martian atmosphere is of particular interest as its total air mass, estimated between 354 and 109 g cm-2, is in the vicinity of the Pfotzer cosmic ray maximum. Assuming the absence of a magnetic field on Mars the maximum neutron production would occur at an atmospheric depth of 75 +/- 5 g cm-2. With the lower air mass limit the surface flux of neutrons reaching the Martian surface could be about 240 times greater than observed at Earth's sea level. Surface minerals containing nuclei with large capture cross sections for slow neutrons, such as Li6, B10 and U235, could thus serve as valuable indicators for the age of the Martian crust. In general, the tenuous Martian atmosphere would result in greater surface radiation dose rates, particularly during times of relativistic solar flares. If the surface air mass is as low as 109 g cm-2 then the rate of nuclear disintegrations due to galactic cosmic radiation would exceed that on Earth's sea level approximately 1000-fold. The tenuous Martian atmosphere would not be a complete shield for heavy primary nuclei and about 1 percent of the incident flux could reach the surface.