Cosmic‐ray exposure ages of diogenites and the recent collisional history of the howardite, eucrite and diogenite parent body/bodies

We determined the cosmic‐ray exposure age of 20 diogenites from measured cosmogenic noble gas isotopes and calculated production rates of 3 He, 21 Ne and 38 Ar. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic 22 Ne/ 21 Ne ratio of each sample. The shielding conditions of each sample were also checked on the basis of the measured 10 Be and 26 AI concentrations. The exposure ages range from 6 to 50 Ma but do not form a continuous distribution: ten ages cluster at 21–25 Ma and four at 35–42 Ma. The two diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the exposure age distribution of eucrites and howardites. After the selection from literature data of 32 eucrites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) group meteorites at 20–25 Ma and 10 at 35–42 Ma. The shape of the two peaks is consistent with single impact events, and random number statistics show that they are statistically significant at the 99% level. Altogether, this provides strong evidence for two major impact events 22 Ma and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to explain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 2–4 km in diameter for Vesta and of 60–300 m for the 100× smaller Vesta‐derived “vestoids.” Based on the HED exposure‐age distribution, the size distribution of the main‐belt asteroids and the difference in size between Vesta and the kilometer size vestoids, we favor Vesta as the major source of HED meteorites, although some of the meteorites may have been ejected from the vestoids rather than directly from Vesta.