Overview

The Moon is our nearest neighbor in space, a natural satellite orbiting the Earth. It is the frontier of direct human exploration, an outpost that we reached more than a quarter century ago but from which we have since drawn back. But despite our retreat from its surface, the Moon remains of great interest to astronomers. Although originally the Moon was molten, its small mass and radius have made it difficult for it to either generate or retain any appreciable internal heat. It is therefore a dead world, with neither plate tectonic nor volcanic activity. That inactivity when coupled with the Moon's lack of atmosphere means that its surface features are essentially unaltered since its youth. But the Moon has not always been quiescent. Shortly after its formation, it was pelted with a hail of rocky fragments whose size ranged up to 200 kilometers (about 100 miles) in diameter. The small fragments made craters, and the big fragments made huge basins. The basins subsequently flooded with lava (long since congealed) to create several dark, nearly circular plains easily visible to the naked eye. The Earth probably once bore such features, but erosion and plate motions have erased them. On the Moon's windless, rainless, airless surface, however, they remain--a record of events in the early Solar System that gives clues not only to the Moon's birth but to that of the Solar System as well.

In this chapter, we will describe the Moon's surface and why astronomers believe so many of its features were carved by impact. We will see that lunar rocks differ significantly from terrestrial ones and how they point to the Moon's having been born in a cataclysmic event early in the Earth's history. We will also discuss how the Moon affects Earth today through tides and eclipses. But we will begin with a short physical description of the Moon to help us visualize this nearest world.