TUCSON, ARIZONA — Scientists have revised an earlier theory about how the moon was formed via a single, slow collision between Earth and Mars-sized planet Theia, with broken-off parts of Theia forming the moon.


According to a new study in The Planetary Science Journal, the problem with the previous theory was that the moon shares much of its chemistry with Earth, not Theia, and that it requires improbably low initial velocity.


To explain both phenomena, the new theory suggests an alternative version of events. Rather than hitting the Earth once, at low speed, merging then and there and forming the moon, Theia initially hit Earth at higher speeds in a ‘hit and run’ collision, and then, between 100,000 and 1 million years later, the two struck each other again, resulting in a collision that more fully merged the two.


Using computer simulations of the massive impacts, scientists concluded that this version of the moon’s history is a better fit than what is known as the “giant-impact hypothesis.”