A ‘big bang’ formed the Moon and left footprints deep in the Earth, study suggests | ET REALITY


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

This idea, known as the “big bang,” would explain a lot about the Moon. But scientists are missing irrefutable evidence, such as a crater or pieces of the protoplanet, called Theia.

In a study published Wednesday in the journal NatureResearchers say pieces of Theia survived the impact, but far below, at the boundary between Earth’s mantle and core, 1,800 miles below the surface.

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

Dr. Yuan’s interest began during a planetary geochemistry class while he was a graduate student at Arizona State University. He recalled that the professor asked a simple question: “Where is the Theia impactor right now?”

“It really prompted me to come up with this idea,” Dr. Yuan said.

Part of Theia now forms the moon. But if Theia was the size of Mars, then about 90 percent of its mass ended up on Earth. Some of that certainly melted and mixed with Earth’s minerals. But perhaps some pieces of the protoplanet persisted almost intact.

Dr. Yuan wondered if these pieces could be what form two mysterious structures deep within the Earth, at the boundary between the core and the mantle. The spots (one under West Africa and the other under the Pacific Ocean) cover an area as large as a continent and extend hundreds of kilometers into the mantle.

They were first discovered half a century ago, when researchers realized that seismic waves (the shocks generated by earthquakes) slowed down as they passed through these regions. It is difficult to discern much about the structures other than that they exist. The seismic data is like an ultrasound of the planet, offering blurry, impressionistic views of the structure. They don’t say the temperature or what the structures are made of. It is impossible to drill so deep into the planet to extract samples.

The mass 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 in the Pacific Ocean, is called Jason, after W. Jason Morgan, who suggested that hot spots originated from plumes of material rising from the deep mantle.

Some scientists have proposed that perhaps Tuzo and Jason were made of primordial Earth: crystallized parts 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 oceanic crust that sank into the mantle.

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

In the Nature paper, they carried out a series of computer simulations, breaking Theia and Earth into pieces and tracking the movement of the pieces during and after the collision.

When Theia hit Earth, models show, the collision melted Earth’s crust and outer mantle, mixing them with pieces of Theia. The moon formed from that cloud of debris.

The simulations also showed that more than 10 percent of Theia’s mantle could have ended up embedded in Earth’s deep mantle, Dr. Yuan said. Because Theia’s mantle is thought to have been richer in iron than Earth’s, those denser fragments could have sunk to the boundary between Earth’s core and mantle. Convection in the mantle then dragged the pieces of Theia toward Tuzo and Jason. (Two or three structures were formed in the simulations).

“To me, it’s very interesting and novel,” said Paul Tackley, a professor of geophysics at the Swiss Federal Institute of Technology in Zurich, who was not involved in the new study.

Dr Tackley said the simulations provided a compelling hypothesis, but not proof, and Dr Yuan said it was still possible the spots came from oceanic crust or primordial remains.

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

The findings could spur scientists to take a closer look at how the giant impact may have influenced the movement of the continents’ conveyor belts. “It may have had lasting effects on the subsequent evolution of the Earth,” Dr. Yuan said.

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