The origin of planetary impactors in the inner solar system

Insights into the history of the inner solar system can be derived from the impact cratering record of the Moon, Mars, Venus, and Mercury and from the size distributions of asteroid populations. Old craters from a unique period of heavy bombardment that ended È3.8 billion years ago were made by asteroids that were dynamically ejected from the main asteroid belt, possibly due to the orbital migration of the giant planets. The impactors of the past È3.8 billion years have a size distribution quite different from that of the main belt asteroids but very similar to that of near-Earth asteroids. The Moon and all the terrestrial planets were resurfaced during a period of intense impact cratering that occurred between the time of their accretion, È4.5 billion years ago (Ga), and È3.85 Ga. The lunar cratering record and the radiometrically dated Apollo samples have shown that the intense bombardment of th