The Moon lit by Earthshine, with the solar corona behind the limb.
THE EVOLUTION OF THE MOON
 
Some scientists consider the Earth and Moon a double planet. Before the discovery of Pluto's moon Charon, our Moon was the largest in relation to its primary (the main planet). Elsewhere in the Solar System, the story is different. Though the Galilean moons are roughly the same size as our Moon, they are dwarfed by Jupiter, whereas the Earth and Moon are closer in size. The relatively large size of the Moon and the notion that Earth and Moon are a double planet have influenced ideas of how the Moon came into being.
 
So how did it get there ? Why doesn't it go round the Sun like the other planets ? There are four main theories how the Moon formed and became our satellite: separation, co-accretion, capture, and collision.
 
Separation
Separation, once known as the fission hypothesis, was once the most popular idea of how the Moon formed. It suggests that during the early history of the Solar System the spinning of the molten globe caused it to deformed into an egg-shape. As the egg become more and more stretched, it assumed a dumbbell shape. One of the globular dumbbell ends broke away to form the Moon.
 
Co-accretion
In the co-accretion hypothesis, the Moon simply formed at the same time as the Earth as a double planet. Just as the Earth was growing in size, it had a companion in a stable orbit, also growing in size; swallowing up stray Solar System bodies.
 
Capture
Capture involves having the Moon form somewhere else in the Solar System. Its orbit may have been perturbed or modified by the another planet, perhaps by a collision, putting it in an orbit of the Sun which brings it close to Earth. As it passed close by, it was captured by the Earth's gravitational pull.
 
Collision
Collision is the currently most popular idea of how the Moon formed. A large meteorite or planetesimal collided with Earth. As it struck, the surface was smashed, and a blob of molten rock from the Earth's mantle was jetted out into space. This and the other bits left over from the collision went into orbit around Earth and formed the Moon. This idea fits with all we have learnt about the Moon so far, including the type of rocks it's made of.
 
Evolution
The surface of the primitive Moon is best imagined as seething ocean of molten rock, bombarded and stirred by giant impacts. These impacts would have turned the crust over many times, mixing it thoroughly. When it started to cool just slightly, calcium and aluminium rich minerals in the molten mantle called feldspars began to crystallise and float to the surface of the magma ocean as "rockbergs". These on the surface formed a mineral known as anorthosite, found today in the lunar highlands.
 
The evolution of the Moon since has been dominated by two processes. Cratering and flood volcanism. All the rocks on the Moon have been formed by the cooling or freezing of molten lava. Although not exactly the same, they comprise the materials that make up different kinds of lavas on Earth.
 

Lunar rocks contain minerals typical of volcanism - pyrexene (purple), ilmenite (black), feldspar (white/brown) and olivine.
 
 
As the surface cooled and solidified, bombardment by meteorites continued and the surface became heavily cratered. The larger impacts formed basins. The bigger impacts also cracked the surface deep down and made it easier for lavas to fill the low lying basins.
 

The dark maria of the Moon were formed by lava.
 
The lavas erupting first had a lot of iron and titanium in them. The oldest maria are rich in these elements. But because these minerals became concentrated deeper within the Moon, the lavas erupted later contain less of these heavy metals. Such differences in the minerals on the surface and laboratory analysis of Apollo samples allow the age of different processes and events to be established.
 

Iron crystals (3 thousandths of a centimetre across) formed in the lunar highlands from hot vapour.
 
The Moon today is generally regarded as a dead planet, frozen in time, its surface occasionally disrupted by a larger meteorite. There has been speculation though, that the Moon isn't completely dead and that it has a small iron core which remains molten. There are reported sightings of gas too, possibly volcanic, escaping from the surface.
 
The Moon has had a very complex and violent early history, preserved by the lack of air and water which would otherwise wear away the clues to its past. From the rocks returned to Earth and from careful geological mapping of the different terrains and landforms a fair understanding has emerged of how the Moon evolved.