The onset of the lunar cataclysm as recorded in its ancient crater populations

The earliest bombardment history of the Moon potentially provides powerful constraints for solar system evolution models. A major uncertainty, however, is how much of this history is actually recorded in lunar craters. For example, some argue that most ancient lunar craters and basins were produced by a declining bombardment of leftover planetesimals produced by terrestrial planet formation processes. Others believe that most lunar craters and large basins were formed in a narrow time interval between 3.8 and 4.0 Ga, the so-called lunar cataclysm. In the light of recent improvements in our understanding of early solar system evolution, it is possible that the contributions from both scenarios could be represented in the lunar crater record. If so, when did the declining bombardment end and the lunar cataclysm begin? Here we show, using new counts of 15-150 km diameter craters on the most ancient lunar terrains, that the craters found on or near Nectaris basin appear to have been created by projectiles hitting twice as fast as those that made the oldest craters on various Pre-Nectarian-era terrains. This dramatic velocity increase is consistent with the existence of a lunar cataclysm and potentially with a late reconfiguration of giant planet orbits, which may have strongly modified the source of lunar impactors. This work also suggests that the lunar cataclysm may have started near the formation time of Nectaris basin. This possibility implies that South Pole-Aitken basin (SPA), the largest lunar basin and one of the oldest by superposition, was not created during the cataclysm. This view is strengthened by our interpretation that a substantial fraction of ancient craters on SPA were made by low velocity impactors. Finally, we believe these results shed new light on the impact history of the primordial Earth. (C) 2012 Elsevier B.V. All rights reserved.