Although the two are quite 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 the Washington University 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 shown by extensive modeling — the difference in the balance of zinc profiles would most likely be explained by lighter zinc isotopes evaporating away following a collision.