The solar system in which the infant Earth found itself was an unsettled environment, filled with lumps of rock whizzing around on irregular orbits. Some 4.5 billion years ago one of these, a Mars-sized body, clobbered our planet. The result was a comprehensive rearrangement. Some of the impacting material stuck, while the rest was blasted into orbit along with bits of Earth excavated by the collision, where it formed the moon.
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