Heavily cratered surfaces on the Moon, Mars, and Mercury show the terrestrial planets were battered by an intense bombardment during their first billion years. This Late Heavy Bombardment (LHB) is well documented by the number of impact craters visible on the ancient feldspathic highlands crust of the Moon compared to the younger mare basalts. Lunar basins are the most obvious and unique geological expression of the LHB, but their ages and implications for solar system dynamics are controversial. An endmember class of models proposes that the LHB occurred as a cataclysmic 'spike' in the impact flux at ~ 3.9 +/- 0.1 Ga. Through geochemical and isotopic studies of lunar samples, we have documented the formation of a basin-scale impact on the Moon at 4.22 +/- 0.01 Ga, thereby disproving the strong version of the Cataclysm hypothesis. The petrology and trace element geochemistry of a melt rock produced by this impact shows that it is a cumulate that formed by differentiation of the melt sheet, in contrast to previous views developed from terrestrial craters.
The most parsimonious dynamical solution to account for the LHB seems to involve both an early phase of post-accretion and a later population of impactors generated by orbital instabilities of the outer planets.
Bottke W. F. and Norman M. D. (2017) The Late Heavy Bombardment. Annual Reviews of Earth and Planetary Sciences (invited review), in press.
Norman M. D., Taylor L. A., Shih C.-Y., and Nyquist L. E. (2016) Crystal accumulation in a 4.2 Ga lunar impact melt. Geochimica et Cosmochimica Acta 172, 410-429. DOI: 10.1016/j.gca.2015.09.021.
Norman M. D. and Nemchin A. A. (2014) A 4.2 Billion Year Old Impact Basin on the Moon: U-Pb Dating of Zirconolite and Apatite in Lunar Melt Rock 67955. Earth and Planetary Science Letters 388, 387-398.