THE GREAT ARCHEAN BOMBARDMENT, OR THE LATE LATE HEAVY BOMBARDMENT

The Late Heavy Bombardment (LHB) is often described as an impact spike on the Moon and terrestrial planets that was triggered by giant planet migration ~4 Gy ago (Ga). The comets and main belt asteroids scattered by this event, however, were probably too limited in number to reproduce ancient lunar crater populations. Here we show that many LHB projectiles came from the E-belt, a hypothesized extension of the main belt between 1.7-2.1 AU. Destabilized by giant planet migration, this belt produced ~10 of the Moon's youngest basins between 3.7-4.1 Ga. Assuming 2-3 additional basins come from the main belt population, we predict the LHB consisted of 12-13 lunar basins. This places the twelfth youngest basin Nectaris near the start of the LHB at ~4,1 Ga, a result consistent with both crater and (limited) sample constraints.

An unexpected attribute of the E-belt is that it makes a long-lived tail of impactors; we predict it formed ~60 and ~4 Chicxulub-sized or larger craters on the Earth and Moon between 1.7-3.7 Ga, respectively. This matches the number of large lunar craters found on Late Imbrian terrains (3 from 3.2-3.7 Ga) and Eratostenian terrains (1 on 1.5-3.2 Ga). Interestingly, our work also reproduces Earth's distribution of Archean-era impact spherule beds. with 9-10, 4, 1, and 0 spherule beds identified in 3.23-3.47, 2.49-2.63, 1.7-2.1, and 0.6-1.7 Ga outcrops. We conclude that major LHB impacts ended on Earth much later than on the Moon, and that the ~1.85 and ~2 Gy old terrestrial craters Sudbury and Vredefort probably represent the last gasp of the LHB on Earth.