Long-term characterization of the Martian atmosphere and soil from cosmic ray effects in returned samples

Abstract

Analyses of cosmic ray effects in returned Martian soil and rock samples may fundamentally increase our understanding of: (a) past climatic variations, (b) the characteristic erosion rate of rocks, and (c) the average redistribution rate for soils. The reason is that the Martian atmosphere is a selective mass filter for galactic cosmic rays with the specific nuclear effects produced in surface materials being quite dependent on the atmospheric shielding. Cosmic ray VH nuclei produce particle tracks in crystalline materials, while the protons produce spallation products that are easily detected as isotopic anomalies in noble gases in both crystalline materials and weathering products. The ratio of tracks/spallation products changes by five orders of magnitude for a three order of the atmospheric shielding averaged over the exposure age of crystalline fragments within a soil sample. In addition, the absolute abundance of spallation gases in windblown soils can be used to infer the globally averaged ratio of exposure age to soil deposit thickness. A layered soil sample may provide data on temporal variations in atmospheric shielding. Spallation products in rock samples that have been in a chemically or mechanically erosive environment can be used to infer the average erosion rate if enough time for secular equilibrium has elapsed. Finally, cores from rocks (∼10 cm deep) in an erosive environment can be used to infer the mean atmospheric shielding and the erosion rate over a time scale roughly two orders of magnitude less than the time scale for secular equilibrium of spallation gases, based on the decrease in nuclear particle track abundances with depth.