Giant-impact hypothesis

The idea that a body about the size of Mars struck the newly formed Earth, and that the debris thrown out gathered to form the Moon.

The rocks brought back by Apollo showed that lunar rock closely resembles Earth's mantle, yet has little iron and is poor in substances that evaporate easily. In 1975 William Hartmann and Donald Davis proposed that this is explained if a large impact blasted rock from Earth's outer layers into space, where it collected into the Moon^[1].

The shortage of iron follows because Earth's iron had already sunk to the core by the time of the impact; the shortage of volatile substances because they were lost from the hot debris. It remains the standard account of the Moon's origin, but questions are open. Earth and Moon are too alike in composition, with no clear trace of the body that struck. Simulations of many different kinds of collision continue.