The origin of the
Earth's single satellite, so large in relation to its parent planet, has intrigued
mankind from the beginning of history.
Three basic explanations have been offered:
- Capture: the Moon initially formed in another orbit in the early solar system, and was somehow captured into its present orbit around Earth.
- Binary planet: Moon and Earth accreted from a common cluster of nebular material, which segregated into a gravitationally coupled doublet.
- Fission: the Moon was torn out of the Earth at some early stage of its evolution by unknown but tremendous forces.
The first two theories were dismissed during the early phases of
lunar exploration, when it became known that the
oxygen isotopic ratios of lunar rocks were virtually identical to that of the Earth's
mantle. The final theory, that of
fission, came into focus as being the most likely. However, even this had flaws.
The Earth has a large iron core, whilst the Moon has none. In addition, the mean density of the Earth is higher than that of the Moon.
A refinement of the fission theory was developed. Currently it states that the Moon formed from a collision with the Earth in a fairly late stage of the Earth's development.
A roughly Mars-sized protoplanet collides with the protoEarth at a near-zero relative velocity. The mantle of the Earth is shattered at the point of contact, and debris is ejected into orbit around the damaged planet: since the iron core of the Earth has already formed at this stage, none of this material is contained in the ejecta.
The ejected mantle material coalesces into a single body, the Moon, which achieves a stable orbit around the Earth. This results in an Earth with a still-preserved iron core, and mantle containing original and impact material as well as a Moon which has a) a similar mantle composition to Earth and b) no iron core, which accounts for the lower overall density of the satellite.