CRATERING IN THE SOLAR SYSTEM

Over the last 4.6 Gy, the terrestrial planets have been repeatedly struck by asteroids and comets wandering the inner Solar System. The craters produced by these events provide critical constraints that can be used to determine how the planets and small body reservoirs (e.g., main belt, Trojans, Kuiper belt) evolved over the last 4.6 Gy. The most well-preserved and best characterized crater population in the Solar System is found on the Moon. According to Apollo data, the lunar impact record can be divided into two broad phases. In Phase 1, the crater production rate has been nearly constant from ~3.9 Ga to today. Numerical modeling work indicates most lunar impactors were main belt asteroids produced and delivered via collisional/dynamical processes. In Phase 2, which is also called the Late Heavy Bombardment (LHB), the impactor flux between 4.6 to 3.9 Ga was >100 times higher than in Phase 1. Note that while all known lunar basins (diameter D > 300 km craters) were formed during this interval, those with well-constrained ages (i.e., Imbrium, Nectaris, Orientale, and Serenitatis) have relatively young ages of ~3.9 Ga (i.e., >0.6 Gy younger than the Moon's formation age). The same can be said for nearly all lunar samples. Some have argued this age “gap” was produced by a spike in the impactor flux ~3.9 Ga. Others have suggested it was naturally created by the tail end of a declining impactor population produced by planet formation processes, with older basins and terrains steadily eliminated over time. We argue here for the former scenario; using new dynamical modeling results, we can show the LHB was produced by a sudden change in the orbital configuration of the Jovian planets ~3.9 Ga. As the Jovian planets migrated to their current orbits, they scattered numerous asteroids and comets into the terrestrial planet region and created a terminal cataclysm on the Moon. The strength of this model is that it not only explains the ages of the lunar basins but also reproduces (i) the current orbits of the Jovian planets, (ii) the orbital distributions of the Kuiper belt and Trojan populations, and (iii) the shape of the crater size-frequency distribution found on the lunar highlands. Accordingly, we find it likely that the pre-3.9 Ga surfaces of Mercury and Mars, like the Moon, were almost entirely obliterated by the LHB.