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A ‘Big Whack’ Formed the Moon and Left Traces Deep in Earth, a Study SuggestsTwo enormous blobs deep inside Earth could be remnants of the birth of the moon.
An artist’s concept showing protoplanet Theia, left, colliding with Earth to eventually form the moon, right, possibly leaving two giant structures embedded in the Earth’s mantle 4.5 billion years ago.Credit...Hernán CañellasNov. 1, 2023
Where did the moon come from? The most popular theory says that about 4.5 billion years ago, a Mars-size protoplanet slammed into Earth. Some of the resulting debris, tossed into orbit, coalesced to form the moon.
This idea, known as the “big whack,” would explain much about the moon. But scientists have lacked smoking-gun evidence like a crater or pieces of the protoplanet, named Theia.
In a study published on Wednesday in the journal Nature, researchers assert that pieces of Theia did survive the impact — but way down, sitting on the boundary between Earth’s mantle and core, 1,800 miles below the surface.
“We looked into the deeper Earth,” said Qian Yuan, a postdoctoral researcher at the California Institute of Technology who led the research. “We found big chunks of the impactor Theia.”
Dr. Yuan’s interest started during a planetary geochemistry class while he was a graduate student at Arizona State University. He recalled that the professor asked a straightforward question: “Where is the impactor Theia right now?”
“It really sparked me to have this idea,” Dr. Yuan said.
Some of Theia now makes up the moon. But if Theia was the size of Mars, then about 90 percent of its mass ended up back on Earth. Some of that certainly melted and mixed into Earth’s minerals. But perhaps some pieces of the protoplanet persisted almost intact.
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