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A. Morbidelli, J.M. Petit, B. Gladman (Observatory of Nice, France), J. Chambers (NASA Ames (USA); Armagh Observatory (UK))
At the end of the main accretional period of the terrestrial planets, a few percent of the initial planetesimal population is left on highly-inclined orbits in the inner solar system (Petit et al., 2000). We show that this final depletion of this leftover population would cause an extended bombardment of all of the terrestrial planets, slowly decaying with a time scale of order 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 would 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, consistent with the Petit et al.~(2000) estimate for this population. If a Terminal Lunar Cataclysm (a spike in the cratering record ~3.9~Ga ago) really occurred on the Moon, it was not caused by the highly--inclined leftover population, because of the monotonic diminishment of the latter.