Exposure histories of the lunar meteorites: MAC88104, MAC88105, Y791197, and Y86032

Abstract

Four lunar meteorites, MacAlpine Hills (MAC) 88104, MacAlpine Hills 88105, Yamato (Y) 791197, and Yamato 86032 were analyzed for the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca. From these and published data, histories of exposure to cosmic rays were modelled in terms of two-stage irradiations each with a long first stage on the Moon lasting a time T 2 π > 5 Ma at a burial depth d 2π[ g cm 2 ] followed by a second stage in space, i.e., the transit time between the Moon and the Earth, lasting a time T 4π [Ma] in a body of typical meteoroidal size. The terrestrial age T t [Ma]gives the time elapsed between meteorite fall and recovery in Antarctica. The following sets of parameters were obtained: MAC88104/5, 390 ≤ d 2 π ≤ 500, 0.04 ≤ T 4 π ≤ 0.11, 0.10 ≤ T t ≤ 0.19; Y791197, d 2 π < 80, T 4π < 0.1, T t < 0.1; Y86032, d 2 π > 1000, T 4 π = 10 ± 2, 0.08 < T t < 0.12. From the number and exposure histories of lunar meteorites we infer a production rate on the order of 5 Ma −1 and an arrival rate worldwide of about 3 × 10 6 meteorites Ma −1. These results suggest that each impact event large enough to produce lunar meteorites sends a large number of them to the Earth.