Animated model of the structure of the Moon. The familiar near side is partially covered in dark basins called maria or seas. The far side lacks seas, and is heavily cratered. It is thought that its crust is around 50 kilometres thick, below which there is a thick mantle dark orange. This surrounds the small molten iron_nickel core bright orange. The Moon磗 core is relatively much smaller than the cores of the terrestrial planets. This is thought to be due to the method of its formation: the Moon formed when a Mars_sized body collided with the early Earth. The collision ejected a huge amount of material from Earth into space, but mostly from its outer layers, which contain far less iron than the core.