Solar flare exposure ages of lunar rocks and boulders based on Al-26

Abstract

Exposure ages of several lunar rock surfaces have been determined from the solar flare produced Al-26 activity in the top 400 microns of the exposed face of the rock. The fine depth resolution is achieved by using the non-destructive beta-gamma counting technique. The results show a distribution decreasing with exposure age with about 40% of rocks having exposure of less than 0.4 m.y. Minor fragmentations, involving several millimeters of rock surfaces, appear to be responsible for such a distribution. These results are compared with solar flare track, rare gas, and crater ages.