A catastrophic collision between Earth and another body probably created the Moon. Computer models predict that the Moon was derived from the impacting body, and yet Earth and the Moon are chemically almost identical. Two new models show how this scenario could arise if Earth was spinning faster at the point of impact than it does today.
One model, by Robin Canup of the Southwest Research Institute in Boulder, Colorado, suggests that if the impactor was larger than previous models have allowed — perhaps even Earth-sized — a collision could create a planet and a disk of Moon-forming material, both with the same composition and made from the impactor and its target.
Meanwhile, Matija Ćuk and Sarah Stewart at Harvard University in Cambridge, Massachusetts, show that if Earth used to spin faster than it does today, even a small, fast impactor could cause Earth to shake off a disk of material that could subsequently form the Moon.