The origin of the moon's craters
(PhysOrg.com) -- Moon's craters, together with samples of the surface returned during the Apollo program, tell the story of impacts from two different populations of small bodies. The first rocky collection was gradually depleted over time: About 3.85 billion years ago (the Moon formed about 4.5 billion years ago) there was a cataclysmic heavy bombardment of material onto the surface that lasted only a few hundred million years. The second collection of bodies appears not to have been depleted, however, and their impacts have continued at a steady pace. We observe this latter group today as Earth-orbit-crossing objects, and their numbers are apparently renewed at about the same rate that they are lost. All these details are important to our understanding of the Moon, the history of the bombardment of the Earth by the same populations of asteroids, and - not least - to a better understanding of how the...