We have developed a new chronology for early lunar bombardment (Morbidelli et al. 2012; EPSL). Using dynamical models linked to numerous constraints, we find the impactor flux followed a sawtooth profile with two main phases. Phase A can be described as a declining impact flux between ~4.1 and 4.5 Ga likely produced by leftover planetesimals in the terrestrial planet region (see Walsh et al. 2011; Nature). Phase B starts with an uptick in the flux near ~4.1 Ga caused by the destabilization of comet and asteroid reservoirs via late giant planet migration (e.g., Gomes et al. 2005). Often called the Late Heavy Bombardment (LHB), Phase B declined slowly, allowing sizeable lunar impacts to take place over many hundreds of My. (Bottke et al. 2012; Nature). Our model chronology predicts that Nectaris basin formed near the beginning of the LHB, while most Pre-Nectarian basins were likely created during Phase A.
If true, South Pole-Aitken (SPA) basin, allegedly the oldest surviving basin on the Moon, formed during Phase A (see Marchi et al. 2012; EPSL). Moreover, counts of D > 20 km craters on terrains within and near SPA suggest its crater retention age is ~4.36 Ga (Morbidelli et al. 2012; EPSL). This is ~0.1 Gy younger than the oldest lunar crust at ~4.45 Ga. While some might consider this to be shockingly young, a ~4.36 Ga SPA basin age lines up well with an apparent spike of ages found in ancient lunar zircons and rock samples (e.g., multiple abstracts at the 2013 LPSC). This could mean the SPA impact was responsible for a pulse of magmatism that took place on the Moon at the same time. A younger age for SPA basin might also help explain its substantial depth (13 km). It is thought that impacts into a warm, relatively low-viscosity crust, as the Moon may have had shortly after it formed, are more likely to produce craters that undergo substantial relaxation and slumping. This might explain the relatively flat appearance of many large basins on Mercury as well as those like Imbrium that formed in the Procellarum KREEP terrain. The formation of SPA into an older, fairly cool lunar farside crust, however, might have prevented slumping.
Of course, we cannot have it both ways; if SPA is young, there must also be older lunar basins. This is not as controversial as one might think. Wilhelms (1987) stated that many Pre-Nectarian basins cannot be dated by crater counts, and the superposition relationships between the most ancient basins are murky. There is also no modern geologic map of the lunar farside. We speculate that many ancient Pre-Nectarian basins are older than SPA. Similarly, if our age is right, SPA ejecta could not have substantially affected the spatial densities of D > 20 km craters found on the oldest lunar terrains. Note that the shape of the crater size-frequency distributions on these ancient terrains are similar to those found on/near the (younger) SPA terrains (Marchi et al. 2012; EPSL). Intriguingly, our initial tests show no meaningful changes in the spatial densities of D > 20 km craters as we move away from SPA's rim. This may indicate that SPA ejecta was dominated by small fragments. Alternatively, our results could mean that (i) our Phase A modeling is wrong and needs revision, or (ii) SPA is actually much older than its crater retention age. All of these possibilities need to be further investigated.