Where did the moon come from? The most popular theory is that about 4.5 billion years ago, a protoplanet the size of Mars hit the Earth. Some of the resulting debris, thrown into orbit, coalesced to form the moon.
This idea, known as the “big whack,” can explain a lot in a month. But scientists have lacked evidence of a smoking gun like a crater or fragments of a protoplanet, called Theia.
In a study published Wednesday in the journal Nature, researchers asserted that pieces of Theia survived the impact — but on the ground, sitting at the boundary between Earth’s mantle and core, 1,800 miles below the surface.
“We’re looking deep into the Earth,” said Qian Yuan, a postdoctoral researcher at the California Institute of Technology who led the study. “We found large parts of the impactor Theia.”
The interest of Dr. Yuan began during a planetary geochemistry class while an undergraduate at Arizona State University. He recalled that the professor asked a direct question: “Where is Theia’s influence right now?”
“It really woke me up to have this idea,” said Dr. Yuan.
Some of Theia now make a moon. But if Theia were the size of Mars, about 90 percent of its mass would end up back on Earth. Some of that was melted and mixed with the earth’s minerals. But perhaps some pieces of the protoplanet persisted almost intact.
Dr. Yuan wondered if these fragments might be forming two mysterious structures deep within the Earth, at the boundary between the core and the core. The bridges – one under West Africa, the other under the Pacific Ocean – span an area as large as a continent, and stretch hundreds of miles into the summit.
They were first noticed in the last century, when researchers noticed that seismic waves – tremors caused by earthquakes – slow down when they pass through these regions. It’s hard to tell much about buildings without them being there. Seismic data is like a sonogram of the planet, providing mysterious, impressive views of the structure. They don’t tell the temperature or what the buildings are made of. It is not possible to drill that far into the earth to extract samples.
The blob under West Africa is known as Tuzo, after J. Tuzo Wilson, Canadian geophysicist and pioneer in the theory of plate tectonics. The other, deep in the Pacific Ocean, is called Jason, after W. Jason Morgan, who suggested that hot spots arise from material rising from deep within the mantle.
Some scientists have proposed that perhaps Tuzo and Jason are made of ancient Earth – the shiny parts of a magma ocean that once covered the surface that never mixed with the rest of the mantle. Some thought that these structures might be pieces of oceanic crust that sank into the ocean.
Dr. Yuan noted that Tuzo and Jason’s volume was almost comparable to the moon, leading him and his colleagues to wonder if they might be additional fragments of Theia.
In the Nature paper, they made a series of computer measurements, breaking Theia and Earth into pieces and tracking the movement of the pieces during and after the collision.
When Theia hit Earth, the models found, the collision melted the crust and the outer part of Earth’s mantle, mixing with pieces of Theia. The moon appeared from that cloud of debris.
The simulation also showed that more than 10 percent of Theia’s mantle may have ended up buried deep in the Earth, Dr. Yuan said. Because Theia’s mantle is believed to be richer in iron than Earth’s, those dense fragments could have sunk into Earth’s core. Convection in the cloak then swept pieces of Theia into Tuzo and Jason. (On average, two or three buildings are built.)
“To me, it’s very interesting and novel,” said Paul Tackley, a professor of environmental science at the Swiss Federal Institute of Technology in Zurich who was not involved in the new research.
Dr. Tackley said the simulations provided a compelling idea, but not proof, and Dr. Yuan said it is still possible that the blobs are from the ocean’s crust or the original fossils.
“Our research cannot rule out other reasons,” said Dr. Yuan.
The findings may encourage scientists to look more closely at how a large impact may have influenced the movement of the continental conveyor belt. “It is possible that there were lasting effects on the subsequent evolution of the Earth,” said Dr. Yuan.