A ‘big blow’ shaped the moon and left marks deep in the Earth, a study suggests

Where did the moon come from? The most popular theory says that a protoplanet the size of Mars collided with Earth about 4.5 billion years ago. Some of the resulting debris, flung into orbit, coalesced to form the moon.

This idea, known as the “big blow,” would explain a lot about the moon. But scientists lacked evidence such as a crater or pieces of the protoplanet, called Theia.

This is evident from a study published on Wednesday in the journal Natureresearchers claim that parts of Theia survived the impact, but far below, at the boundary between the Earth’s mantle and core, 3,000 kilometers below the surface.

“We looked at the deeper Earth,” said Qian Yuan, a postdoctoral researcher at the California Institute of Technology who led the study. “We found large pieces of the Theia impactor.”

The interest of Dr. Yuan started in a planetary geochemistry class while still a student at Arizona State University. He remembered the professor asking a simple question: “Where is the impactor Theia right now?”

“It really gave me this idea,” said Dr. Yuan.

Part of Theia now forms the moon. But if Theia was the size of Mars, about 90 percent of its mass ended up back on Earth. Some of it is certainly melted and mixed with the minerals of the earth. But perhaps some parts of the protoplanet remained virtually intact.

Dr. Yuan wondered whether these pieces might represent two mysterious structures deep within the Earth, at the boundary between the core and the mantle. The blobs – one under West Africa, the other under the Pacific Ocean – cover an area the size of a continent and extend hundreds of kilometers into the mantle.

They were first noticed half a century ago, when researchers realized that seismic waves – the shaking caused by earthquakes – slowed down as they passed through these areas. It’s hard to discern much about the structures other than that they exist. The seismic data is like a sonogram of the planet, providing faint, impressionistic images of its structure. They don’t tell you what the temperature is or what the structures are made of. It’s impossible to drill that far into the planet to retrieve samples.

The blob beneath West Africa is known as Tuzo, after J. Tuzo Wilson, a Canadian geophysicist and pioneer in the theory of plate tectonics. The other, deep beneath the Pacific Ocean, is called Jason, after W. Jason Morgan, who suggested that the hot spots came from plumes of material rising from the deep mantle.

Some scientists have proposed that Tuzo and Jason may have been created from primordial Earth: crystallized portions of the magma ocean that once covered the surface and never mixed with the rest of the mantle. Others thought the structures could be pieces of ocean crust that sank into the mantle.

Dr. Yuan noticed that the volume of Tuzo and Jason was roughly comparable to that of the moon, leading him and his colleagues to wonder if they might be extra pieces of Theia.

In the Nature paper, they ran a series of computer simulations, splitting Theia and Earth into pieces and tracking the pieces’ motion during and after the collision.

When Theia hit Earth, the models found, the collision melted the crust and outer mantle, mixing with bits of Theia. The moon was created from that cloud of debris.

The simulations also showed that more than 10 percent of Theia’s mantle could have ended up in Earth’s deep mantle, said Dr. Yuan. Because Theia’s mantle is believed to have been richer in iron than Earth’s, those denser parts could have sunk toward Earth’s core-mantle boundary. Convection in the mantle then brought the Theia pieces to Tuzo and Jason. (In the simulations, two or three structures formed.)

“For me it is very interesting and new,” says Paul Tackley, professor of geophysics 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 hypothesis, but not proof, and Dr. Yuan said it was still possible that the blobs came from ocean crust or primordial remains.

“Our study cannot rule out other reasons,” said Dr. Yuan.

The findings could prompt scientists to take a closer look at how the giant impact affected the conveyor belt’s movement across the continents. “It could have had long-lasting consequences for Earth’s subsequent evolution,” said Dr. Yuan.