The Moon Was Created By A Massive Collision Of Earth And A Mars-Sized Planet

The idea that the Moon was once part of Earth was suggested as far back as 1898, but it wasn’t until the mid-70s that the giant impact hypothesis — which suggests that the Moon was formed via the massive collision between Earth and another Mars-sized body — first gained favor. In 2001, Canup and Asphaug published solid calculations in support of the hypothesis, but physical proof of the impact has yet to be found. But according to new research published today in Nature, that may have changed.

A paper published today in Nature by Frédéric Moynier and Randal Paniello of Washington University in St. Louis, and James Day of Scripps, shares findings of a chemical analysis of Moon rocks that shows a fractional difference in their makeup as compared to samples from the Earth. Although the two are markedly similar, it’s been previously shown that Moon rocks lack volatile elements, which suggests they may have evaporated during the incredibly intense heat and pressure created during an impact event. But if the hypothesis that light elements actually evaporated from Moon rocks during their formation is correct, you’d expect to find evidence of elements being layered by mass — heavier elements would condense first, and so on.

That process is known as isotopic fractionation — a concept central to carbon dating — and Moynier and team’s results suggest they found exactly that. They compared the blend of zinc isotopes in Moon rocks and Earth samples, and found that the Moon rocks held slightly higher proportions of heavier zinc isotopes. If the Moon was indeed once part of Earth — which has been modeled extensively — the difference in the balance of zinc profiles would most likely be explained by lighter zinc isotopes evaporating away following a collision.

“When a rock is melted and then evaporated, the light isotopes enter the vapor phase faster than the heavy isotopes, so you end up with a vapor enriched in the light isotopes and a solid residue enriched in the heavier isotopes,” Moynier said. “If you lose the vapor, the residue will be enriched in the heavy isotopes compared to the starting material.”

This is the first time anyone’s published results showing such fractionation in Moon rocks, and in addition to showing that that the Moon rocks differ in chemical composition to Earth, they also show similar differences — low concentrations of zinc, but comparatively high proportions of heavier zinc isotopes — to Martian rock material. The Earth and Mars have chemical makeups similar to chondrites, which are meteors that have not been melted or altered, and thus are simliar to the makeup of rocky material born in the early universe. The way Moon rocks differ from all three suggests that it must have been born under conditions of extreme heat and volatility, like if it condensed out of vaporized rock.

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