Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals

Rock and soil samples from the planet Mars are due to be returned to Earth within a decade. Martian samples initially will be tested for evidence of l ife and biological hazard under strict biological containment. Wider distri bution of samples for organic and inorganic analysis may occur only if neit her evidence of life nor hazard is detected, or if the samples ate first st erilized. We subjected a range of Mars analog rocks and minerals to high do ses of gamma radiation in order to determine the effects of gamma steriliza tion on the samples' isotopic, chemical, and physical properties. Gamma pho tons from Co-60 (1.17 and 1.33 MeV) in doses as high as 3 x 10(7) rads did not induce radioactivity in the samples and produced no measurable changes in their isotopic and chemical compositions. This level of irradiation also produced no measurable changes in the crystallographic structure of any sa mple, the surface areas of soil analogs, or the fluid inclusion homogenizat ion temperature of quartz. The only detectable effects of irradiation were dose-dependent changes in the visible and near-infrared spectral region (e. g., discoloration and darkening of quartz and halite and an increase in alb edo of carbonates) and increases in the thermoluminescence of quartz and pl agioclase. If samples returned from Mars require biological sterilization, gamma irradiation provides a feasible option.