Solar System information #7: The Moon

The Moon - our nearest neighbour

The Moon is the only other world in space that Earthlings have ever visited. On July 20th 1969, the Apollo 11 lunar module landed on the Moon and the words "That's one small step for a man, one giant leap for mankind." echoed around the Earth. The Apollo landings went on until December 1972. In all, just 12 American astronauts actually landed on the surface. No one has been to the Moon since then.

On the Moon, there is no air or water: life cannot be sustained. Lunar gravity is only 1/6 of Earth's; not enough to hold on to an atmosphere. Without an atmosphere there is no wind, no rain, virtually no erosion - the Apollo astronauts' boot prints will remain for thousands of years. The Moon is so near to Earth that its gravitational pull has a big effect on our oceans, as we can see from the rise and fall of the tides.

Data from lunar seismometers set up during the Apollo missions suggests that the surface crust of the Moon is about 60km thick on the near side and about 100km thick on the far side. Under the crust is a mantle about 1,000km thick, and under that an iron-rich metallic core about 700km in radius. Around 3,000 weak "moon quakes" occur each year.

The Moon rotates once on its axis in exactly the same time that it takes to orbit once around the Earth. The same side (the near side) is always facing the Earth. The only way of seeing the far side of the Moon is by sending a space probe there.

The surface of the Moon has been observed in detail ever since telescopes were invented in the early 1600s. No other celestial object is near enough for its surface features to be easily visible. It is pockmarked by craters, ranging in size from 270km (for Bailly Crater) all the way down to pea-sized pits in the surface. The far side of the Moon is even more heavily cratered than the near side.

The Apollo astronauts brought back 386kg of rock samples from the Moon. All of these moon rocks were "igneous" - formed from cooling lava. There was no "sedimentary" rock, which is formed by deposits in water.