PART I: Introduction

Impact craters are those craters which form when a meteorite strikes the surface of a planet. Such craters are found on all solar system objects which have surfaces-the terrestrial planets, the Moon, and many of the satellites of the outer planets. The occurrence and appearance of impact craters then tell us about the history of the cratering event. On the Earth impact craters are not easily recognized because of the intense weathering and erosion processes that wear away its surface. On the Moon over 80 percent of the surface looks much the same as it did 3-1/2 billion years ago, and hence the rugged, heavily cratered terrain tells us about bombardment events before then. About half of the Martian surface is also very old, cratered terrain. Two-thirds of Mercury’s surface is also heavily cratered.

Various geological clues and studies of the rocks returned from the Moon by the Apollo astronauts indicate that asteroid-sized chunks of matter were more abundant in the solar system some 3.9 billion years ago. During that era, considered to be the last stage of planetary accretion, the young planets were subject to intense bombardment by these objects. The Earth itself would have been struck many times, so that its crust was broken up and modified by the repeated impacts. Although plate tectonics, mountain building, weathering and erosion have all but hidden the craters formed on the Earth, the relatively unweathered lunar surface still bears testimony of this intense period of bombardment.