The Moon

The Moon is a cold, dry, lifeless ball of rock with an insubstantial atmosphere. Gray and crater-covered, it is the fifth largest moon in the solar system. It looms large in our sky, and was an early target for space exploration. It is the only other world that humans have visited.

structure and atmosphere

This rock ball has an outer layer of calcium-rich, granite-like rock, about 30 miles (48 km) thick on the near side and 46 miles (74 km) thick on the far side. Below is a solid rocky mantle rich in silicate minerals, which becomes partially molten with increased depth. A small iron core may be in the center

THE TERMiNATOR

The left side of the Moon is sunlit. The boundary between this and the unlit part is called the terminator. Here, shadows are longest and surface features, such as craters, are seen to best effect.

The atmosphere is very thin. In total, it is equivalent to the amount of gas released by a landing Apollo craft. The Moon's gravity, one-sixth of Earth's, cannot hold on to the atmosphere, but it is replenished by solar-wind material.

full moon

The Moon is fully lit but the strong, direct sunlight has swamped some of its surface features.

moon data

DIAMETER 2,159 miles (3,475 km)

AvERAGE DiSTANcE fROM EARTH 238,900 miles (384,400 km)

ROTATiON pERIOD 27.32 Earth days

SuRfAcE temperature -240 to 240°F (-150 to 120°C)

SizE coMpARiSQN

interior

The Moon is about a quarter the size of Earth. Its upper crust is cracked. Below is solid crust, and below this is a rocky mantle. Its average density indicates it may have a small iron core.

possible small metallic core thick rocky mantle crust of granitelike rock possible small metallic core

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