The Late Heavy Bombardment: Cataclysm or Continuum?

The impact history of the Moon has significant implications beyond simply counting craters on a dry and lifeless world. The idea of a Late Heavy Bombardment (LHB) at 3.9 Ga has motivated new models of planetary dynamics, invoking either late formation or migration of the outer planets, or destabilization of Planet V between Mars and the asteroid belt. Absolute ages applied to Mars stratigraphy are linked genetically to lunar cratering curves, so our understanding of rates and timescales on other planets depends fundamentally on the Moon. On Earth, the oldest preserved continental crust and earliest (albeit contentious) signs of life are strikingly similar in age to the lunar impact melt breccias. Whether this is coincidence or has more fundamental significance, better constraints on impact rates would provide essential information about early Earth and Mars environments.

Relating lunar surface deposits to specific basins is critical for assessing the reality (or otherwise) of the lunar cataclysm. Only Apollo 17 sampled a geologically well-defined impact-melt unit that can be linked with confidence to a major basin (Serenitatis). The concept of a LHB is critically tied to the age of Nectaris, one of the older nearside basins. Since the early 1980's consensus favored an origin of the Apollo 16 Descartes Formation as Nectaris ejecta, such that the ~3.9 Ga ages of Descartes breccia clasts were considered as strong support for a LHB. Recent high-precision dating of these breccias, however, established an age that is identical with Imbrium and younger than Serenitatis. As Nectaris is stratigraphically older than Serenitatis, this pulls the pin on the absolute age of Nectaris and removes one of the key arguments supporting a lunar cataclysm. Future lunar explorers face the difficult but necessary task of establishing the absolute ages of lunar basins in order to test the late cataclysm hypothesis.