Researchers are pretty sure we got our favorite satellite, the Moon, after a planet, Theia, collided with proto-Earth 4.5 billion years ago. What is not certain are the details of Theia's fate. Was it a hit-and-run, or did the two planets' mantles fuse together?
Qian Yuan, Earth scientist at Arizona State University, and his colleagues suggested recently a new line of evidence to support the latter hypothesis, which suggests that Theia not only fused with Earth, but that we may know exactly where the remains of its mantle are on Earth.
Giant impact hypothesis
“Compared to the moon, much less is (known) about Theia,” says Yuan. & # 39; The moon is there. You have examples. People have been there … few people care too much about the impactor. "
Much of the work around the giant impact hypothesis involves comparing isotopes found on the Moon with those on Earth. Their similarities in composition suggest that the Moon was made from a patch of ancient Earth, meaning something like a giant impact knocked it off our pale blue dot.
Original models estimated that the impactor, Theia, was about the size of Mars (half the size of Earth today). However, some recent studies suggest it was perhaps four times the size of Mars, or about the size of proto-Earth. Regardless, most researchers agree that the core – the densest part – of Theia fused with Earth's core incredibly quickly after impact, in a matter of hours.
Meanwhile on Earth
Today, the Earth's mantle is not completely uniform. About 8 percent of it differs slightly from the rest, forming two large stacks near the core-mantle boundary. These two poles are called Large Low-Shear-Velocity Provinces (LLSVPs), so named because seismic waves, called shear waves, move about 1 or 2 percent slower when they pass through them. And they are huge: one is under the African continent and the other under the Pacific Ocean.
Some researchers believe that the LLSVPs slow the shear waves because they have a higher temperature than the rest of the mantle. Others, like Yuan and his colleagues, think they are not only hotter, but also closer and different in composition.
Yuan says he was in a planetary geochemistry class when the idea arose that the LLSVPs may have been related to Theia. As he puts it, he was inside ASU professor Micha Zolotov& # 39; s class, learn about the giant impact hypothesis for the formation of the moon. Zolotov said the weakest part of the theory was the hypothetical planet Theia – no one had ever found any direct evidence to support its existence. It's completely gone. There's no evidence of this in meteorites, the asteroid belt, anywhere. When Zolotov said this, Yuan recalls, “It hit me so hard. I thought (Theia) would have entered Earth after the impact. Is it possible it entered Earth and formed the LLSVPs? "
Looking for Theia
Yuan & # 39; s first step was to perform some simple calculations, first comparing the size of the two LLSVPs with the size of Mars' mantle – a rough estimate for that of Theia. He found that the two LLSVPs were 80 or 90 percent the size of the Martian mantle. When he added the moon? “Almost a perfect match,” he says. "So then I thought: it's not that crazy."
He cited a 2012 Nature paper geochemist Sujoy Mukhopadhyay at the University of California, Davis, who investigated noble gasotopes of volcanic basalt in IcelandMukhopadhyay had shown that the mantle is heterogeneous, with at least two separate sources, and that those sources are at least 4.5 billion years old. That is, older than the moon. “That was in line with our hypothesis,” says Yuan. One of the sources could be the mantle of Theia, which was preserved in the Earth's mantle after impact.
Then Yuan turned to ASU astrophysicist Steven Desch, which had been published new in 2019 estimates for the composition of Theia herselfDesch, along with Katharine Robinson of the Lunar and Planetary Institute in Houston, used the composition of lunar samples from the Apollo missions to model a likely Theia, and concluded that it was much larger than expected – about the size of 1 proto-Earth, or 4 Mars planets. More importantly for Yuan, Desch, and Robinson, Theia's mantle had a higher amount of iron oxide than Earth's. This means it was denser, so if the two planets collided, Theia's mantle would sink.
Yuan and Desch teamed up to figure out what Theia's mantle composition should look like to make it look like today's LLSVP's after 4.5 billion years of mantle convection. They found that if Theia had a greater density than Desch's earlier estimate, the mantle would have sunk too much and form a global layer instead of two stacks. Instead, their calculations showed that the estimates for Theia's size and density were spot on.
“What really stands out about (the study) is how creative it is,” Susannah Dorfman, one geoscientist at Michigan State University who was not involved in the study, says. "How it connects two fields that looked at problems in two different ways."
Dorfman explains that the simulations they ran were solid. The only place the idea could fall apart would be if the assumptions they made were found to be incorrect, such as Theia's specific iron oxide composition and how its density relates to that of Earth. Still, she says, "when you can get a seemingly inevitable result from a certain set of starting conditions, that feels wonderful."
Yuan is the first to emphasize that there is still a lot of uncertainty surrounding the new idea. "We have to emphasize that it is a hypothesis, and we are only proposing it for the first time," he says. "It's still very new."
"I hope more researchers will test our hypothesis to gather more evidence to prove or disprove it," added Yuan. He says that an obvious next step is to compare the compositions of noble gasotopes in lunar samples with those of LLSVPs. "There is no reason why they should have a chemical relationship unless they inherited it from the ancestor, Theia."
Dorfman says Yuan's hypothesis has already attracted a lot of attention. “I've seen (Yuan's) talk on YouTube,” she says. "He's watched 40,000 times. For a student lecture at a conference, it's incredible … This is one of the things the pandemic is bringing us, we can reach a much wider audience."
You can watch Yuan & # 39; s speech Below.