Formation of the Moon Several mechanisms have been proposed for the formation of the Moon, such as co-accretion with the Earth from the protoplanetary disk, capture of a rogue asteroid or fission from the Earth. The collision hypothesis The hypothesis that is now the most widely accepted is that in which a Mars-sized body collided with the Earth. Refinements of this hypothesis indicate that the impact occurred after the Earth's core had formed, and that the crust-mantle regions of both the impactor and the Earth were vaporised and then mixed during the impact. The collision hypothesis accounts satisfactorily for the geochemical, mineralogical and isotopic compositions of the lunar samples as represented by material returned by Apollo and Luna missions, and by lunar meteorites. The timing of the giant impact event is set at around 4.51 billion years ago. The impact period The final turbulent stages of Solar System formation were traced out by intense cratering of the planets by asteroids and comets. Following the giant impact that formed the Moon, the Earth suffered a prolonged period of bombardment by smaller projectiles. Although no trace of this bombardment remains on the Earth's surface, having been erased by subsequent geological processing, the scars are visible as craters of many different sizes on the Moon's surface. This epoch of bombardment lasted until ~ 3.9 billion years ago, and gradually decreased over 400 million years to 3.5 billion years ago, as determined by relative crater counting on the Moon. During this period, the surface of the Earth was unfriendly to life, as it was heated and melted by bombardment.

The waning Moon obtained with the Wide Field Imager on the ESO 2.2m (La Silla) telescope.