| The Earth's one natural satellite, the Moon, is more than one quarter the size of Earth itself (3,474 km diameter). Because of its smaller size, the Moon's gravity is one-sixth of the Earth's gravity, as we saw demonstrated by the giant leaps of the Apollo astronauts. While there are only two basic types of regions on the Moon's surface, there are many interesting surface features such as craters, mountain ranges, rilles, and lava plains. The structure of the Moon's interior is more difficult to study. The Moon's top layer is a rocky solid, perhaps 800 km thick. Beneath this layer is a partially molten zone. Although it is not known for certain, many lunar geologists believe the Moon may have a small iron core, even though the Moon has no magnetic field. By studying the Moon's surface and interior, geologists can learn about the Moon's geological history and its formation. |