Cosmic-ray exposure history at the Apollo 16 and other lunar sites - Lunar surface dynamics

Concordant Kr-81/Kr exposure ages for four station 11 breccias indicate an age of 50.3 plus or minus 0.8 m.y. for North Ray Crater. Ray structures visible from orbital photography suggest that stations 8 and 9 should contain a substantial amount of South Ray ejecta. Concordant Kr-81/Kr exposure ages at these sites indicate an age for South Ray Crater of 2.04 plus or minus 0.08 m.y. Surface effects show good general agreement with this young age, but discrepancies on a sample-by-sample basis seem to indicate that extensive presurface irradiations must have occurred.