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Researchers put forward the idea after computer simulations found that a collision with a second, sibling moon in Earth's early history might solve the longstanding puzzle of why the two faces of the moon differ so dramatically.
While the near side, which always faces the Earth, is low-lying and relatively flat, the far side is high and mountainous, with a crust tens of kilometres thicker.
The idea builds on what planetary scientists call the "big impact" model of the moon, in which a planet the size of Mars slammed into the Earth in the early days of the solar system and knocked out a vast shower of rocky debris, which later coalesced as the moon.