TERRESTRIAL MANTLE SIDEROPHILES AND THE LUNAR IMPACT RECORD

A simple analytical fit to the lunar cratering record, scaled from final to transient crater diameters, then to impactor masses, implies a total mass ∼1.0 × 1020kg incident on the Moon subsequent to the solidification of the lunar crust ∼4.4 Gyr ago. About half this mass would be retained, and a comparable lunar mass would be eroded. These results are in good agreement with geochemical estimates of the meteoritic component mixed into the lunar crust, which give (0.4-1.5) × 1020 kg. Gravitationally scaling to Earth, and taking account of the statistical probability that the largest impactors incident on Earth were more massive than the largest incident on the Moon, gives an estimate of 1.5 × 1022 kg of material accumulated by Earth subsequent to 4.4 Gyr ago. This result is in excellent accord with geochemical estimates of post-core formation meteoritic input. These estimates, based on abundances of highly siderophile elements in the terrestrial mantle, lie in the range (1-4) × 1022 kg. The significant result is the approximate agreement of the lunar cratering record scaling with both lunar and terrestrial geochemical constraints, numerous uncertainties render exact comparisons pointless. Nevertheless, the close agreement suggests the model developed here may credibly be used to estimate exogenous volatile and prebiotic organic delivery. © 1991.