Abstract
The impact crater record in the inner Solar System is owed to two impactor populations with dierent size distributions. The old population, responsible for an intense period of bombardment that ended ~3.8 Ga ago, is virtually identical in size distribution to the present main belt asteroids. Many asteroids were probably ejected from the main belt during an epoch of orbital migration of the giant planets. These results confirm that an inner Solar System impact cataclysm occurred ~3.9 Ga. The second population, responsible for craters younger than ~3.8 Gy matches the size distribution of near earth asteroids. These impactors are also derived from the asteroid belt, but by a combination of chaotic gravitational resonances and non-gravitational processes that are size-dependent.
