Evidence Supporting an Early as Well as Late Heavy Bombardment on the Moon

Evidence supporting an intense early bombardment on the Moon in addition to the traditional Late Heavy Bombardment at ~ 4 BY ago include the distribution of N(50) Crater Retention Ages (CRAs) for candidate basins, a variety of absolute age scenarios for both a “young” and an “old” Nectaris age, and the decreasing contrasts in both topographic relief and Bouguer gravity with increasing CRA. Crater Retention Ages: N(50) Crater Retention Ages (CRAs) for an expanded inventory of large lunar basins [1-3] based on Quasi-Circular Depressions (QCDs) in LOLA data [2,3] and Circular Thin Areas (CTAs) from model crustal thickness [4,5] show two peaks, even when weaker candidates are eliminated [6] (Figure 1). The break between older and younger impact basins is pre-Nectarian [6], as others suggested based on a smaller number of basins [7], shown in Figure 2. This two peak distribution suggests the possibility of both an Early Heavy Bombardment [6] as well as the generally recognized Late Heavy Bombardment [8-10].