Effects of exposure conditions on cosmic-ray records in lunar rocks

Abstract

The records of solar and galactic cosmic rays in lunar rocks in the form of induced radioactivity and heavy-nuclei tracks are examined to evaluate the importance of micrometeorite impacts resulting in erosion, fragmentation, and multiple exposures. These calculations suggest that the activity ratio of the radioisotope pair Al-26/Mn-53 in the top millimeter is sensitive to erosion, whereas the heavy-nuclei track profiles faithfully record the fragmentation and irradiation history of rocks for single or multiple exposures. The surface exposure ages of rocks can be determined from the plateau in the depth-dependent exposure-age curves. Measurements in rocks 12002, 14310, 14321, and 61016 are considered in the light of the above calculations. The data indicate that the observed Al-26/Mn-53 ratios are much lower than expected on the basis of plausible erosion, cosmic-ray energy spectrum, and exposure times, and may indicate time variations in the solar-proton fluxes over the last million to 10 million yr.