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Giant Impact Theory of Lunar Formation Gains More Credibility

A lingering problem in explaining the genesis of the moon appears to have been solved

By John Matson

The most commonly invoked explanation for lunar formation holds that a giant protoplanet, sometimes called Theia, struck the newly formed Earth 4.5 billion years ago and created a cloud of debris that quickly coalesced into the moon. Simulations of moon-forming collisions have shown Theia would have been the primary donor of lunar material. But analyses of Apollo moon rocks have shown that the moon seems in many ways a chemical clone of Earth, not Theia.

That thorn may be on its way out. A pair of papers published, one by Canup and one by Matija Ćuk, demonstrate two different ways that a giant impact could produce a moon.

In Canup’s model, the impactor is substantially heftier than the canonical Theia—instead of a Mars-size object colliding with the much larger proto-Earth, her new study proposes a smashup of two comparably sized objects.

A different conception, from Matija Ćuk of the SETI Institute and Sarah Stewart of Harvard, invokes a small, high-velocity projectile smacking into a fast-spinning proto-Earth. Like an interplanetary mortar, that high-energy impact would fling out a cloud of debris composed primarily of material from Earth.