Early bombardment of the moon: Connecting the lunar crater record to the terrestrial planet formation

Abstract

The lunar crater record features ∼50 basins. The radiometric dating of Apollo samples indicates that the Imbrium basin formed relatively late — from the planet formation perspective — some ≃3.9 Ga. Here we develop a dynamical model for impactors in the inner solar system to provide context for the interpretation of the lunar crater record. The contribution of cometary impactors is found to be insignificant. Asteroids produced most large impacts on the terrestrial worlds in the last ≃3 Gyr. The great majority of early impactors were rocky planetesimals left behind at ∼0.5–1.5 au after the terrestrial planet accretion. The population of terrestrial planetesimals was reduced by disruptive collisions in the first t∼20 Myr after the gas disk dispersal. We estimate that there were ∼4×105 diameter d>10 km bodies when the Moon formed (total planetesimal mass ∼0.015MEarth at t∼50 Myr). The early bombardment of the Moon was intense. To accommodate ∼50 known basins, the lunar basins that formed before ≃4.35–4.41 Ga must have been erased. The late formation of Imbrium occurs with a ∼15–35% probability in our model. About 20 d>10-km bodies were expected to hit the Earth between 2.5 and 3.5 Ga, which is comparable to the number of known spherule beds in the late Archean. We discuss implications of our model for the lunar/Martian crater chronologies, Late Veneer, and noble gases in the Earth atmosphere.