Earth’s natural satellite, the Moon, is thought to have formed about 4,5 billion years ago as a result of a collision between the young Earth (which was smaller than the Earth today) and an object about the size of Mars.
The impact spewed out a huge amount of rocky debris, which eventually coalesced to form the Moon. Even though it has only 1,2 percent of the mass fo Earth, the Moon is the fifth-largest planetary satellite in the Solar System.
Its gravity influences Earth’s oceans, and when full, the Moon is the brightest object in our sky after the Sun. However it is too small to retain a substantial atmosphere, and geologic activity is no longer there. However, scientists discovered some recent activity.
Twelve men have walked on its surface during the Apollo missions and have brought back to Earth over 380 kg (838 lb) of lunar rock.
The Moon has an elliptical orbit around Earth, so the distance between the two varies. However, the average distance is 380 000 km (235 000 miles).
A day on the Moon is 27.32 Earth days, the same as a year (duration to orbit Earth). This phenomenon is called synchronous rotation and keeps one side of the Moon permanently facing Earth. That’s why there is one “far/dark side of the Moon”, the part of the Moon we never see, because of its rotation.
The lunar crust is about 30 miles (48 km) thick on the near side and about 46 miles (74 km) thick on the far side. The upper part of the rocky mantle is solid but it’s deeper parts are semi-molten. The core consists of an outer part of molten iron around a solid, iron-rich inner part.
The Moon’s atmosphere is extremely thin, with a total mass of only about 24.6 tons and partly because of this, the atmosphere can’t hold a stable surface temperature, so it varies widely, from as low as -274°F (-170°C) at night to as high as 257°F (125°C) by day. However, the measured average temperature is -20°C (-4°F)
The Moon’s gravity is only one-sixth that of Earth, and the lunar atmosphere is escaping all the time, but it is also replenished by the solar wind and outgassing from lunar rocks. Moon’s atmosphere is mostly made out of neon, helium, hydrogen, and argon. The neon and hydrogen have mainly been deposited by the solar wind. The argon and helium are mainly derived form radioactive decay processes in lunar rocks.
There is more to say about the surface structures, events, mapping and exploration of the Moon. However, this will be a topic for another Space Learning Web article.
Source: Smithsonian Nature Guide – Stars and Planets