The extent of lunar regolith mixing

The activity of solar cosmic-ray-produced 53Mn has been measured as a function of depth in the upper 100 g/cm2 (approximately 55 cm) of lunar cores 60009-60010 and 12025-12028. Additional samples which supplement earlier work were analyzed from the Apollo 15 and 16 drill stems. These data, taken in conjunction with previously published results and the 22Na and 26Al data of the Battelle Northwest group, indicate that in at least three of the four cases studied the regolith has been measureably disturbed within the last 10 m.y. In one case gardening to > 19 g/cm2 is required. Activities measured in the uppermost 2 g/cm2 indicate frequent mixing within this depth range. No undisturbed profiles were observed nor were any major discontinuities observed in the profiles. The Monte Carlo gardening model of Arnold has been used to derive profiles for the gardened moon-wide average of 53Mn and 26Al as a function of depth. The 53Mn and 26Al experimental results are compared with these theoretical predictions. Agreement is good in several respects, but the calculated depth of disturbance appears to be too low. (Auth.)