The study will appear tomorrow in the journal Nature.
The study also found that the moon is younger than previously believed.
Prior studies had found that it formed when Earth was about 30 million years old. But now it appears that the impact occurred 30 million years after that.
"And it could have [been] as late as 140 million years after the solar system began to condense," said Alan Brandon of NASA Johnson Space Center in Houston, Texas.
Brandon's commentary on the new study also appears in tomorrow's Nature.
The key to the new study was a new method for looking at trace isotopes in moon rocks, particularly of the element tungsten-182.
Tungsten-182 is formed by the radioactive decay of halfnium-182, which has a half-life of nine million years.
By comparing the abundance of tungsten-182 to that of another isotope, tungsten-184—which is not formed from halfnium—it's possible to determine how old rocks were when the magma that formed them solidified.
"This nice thing is that, [geologically speaking], this radio-isotopic clock is very short-lived," Brandon said. That makes it extremely useful for dating events early in the life of the solar system.
This "clock," though well-known, had been difficult to use for the moon. That's because the rocks collected by Apollo astronauts had been sitting on the lunar surface for billions of years.
"During that whole time, they've been bombarded with cosmic rays," Brandon said. "Cosmic rays will hit an atom and change it.
"If you don't correct for that you get the wrong radiometric [date] on your clock," Brandon said.
The new study gets around the problem by using tiny fragments of metal contained in the rocks.
Yet another surprise was that the moon's early ocean doesn't appear to have solidified until about 120 million years after the beginning of the solar system. (Explore an interactive solar system.)
That's 60 million years after the most likely date for the creation of the moon.
"That means that the moon stayed molten for quite a while," Brandon said.
He noted that computer models had suggested the magma should solidify much more quickly.
Brandon was involved in another study published in Nature earlier this year on Martian meteorites that suggested the red planet had magma oceans that were also slow to solidify.
Scientists aren't sure if the same thing happened on Earth, Brandon said.
However, "We're seeing magma oceans that last a lot longer [than previously believed] on all planets, and it bodes interesting things for Earth."
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