A plausible cause of the late heavy bombardment

Abstract

Abstract— We show that at the end of the main accretional period of the terrestrial planets, a few percent of the initial planetesimal population in the 1–2 AU zone is left on highly‐inclined orbits in the inner solar system. The final depletion of this leftover population would cause an extended bombardment of all of the terrestrial planets, slowly decaying with a timescale on the order of 60 Ma. Because of the large impact velocities dictated by the high inclinations, these projectiles would produce craters much larger than those formed by asteroids of equal size on typical current near‐Earth asteroid orbits: on the Moon, basins could have been formed by bodies as small as 20 km in diameter, and 10 km craters could be produced by 400 m impactors. To account for the observed lunar crater record, the initial population of highly‐inclined leftovers would need to be a few times that presently in the main asteroid belt, at all sizes, in agreement with the simulations of the primordial sculpting of both these populations. If a terminal lunar cataclysm (a spike in the crater record ∼3.9 Ga ago) really occurred on the Moon, it was not caused by the highly‐inclined leftover population, because of the monotonic decay of the latter.