Key Takeaways:

  • Two massive, mysterious rock layers (LLSVPS) deep within Earth’s mantle might be leftover debris from Theia, the planet that collided with Earth to form the Moon.

  • The LLSVPs’ density and composition differ from surrounding mantle rock, hinting at an origin outside Earth.

  • New research suggests Theia could have been nearly Earth-sized, explaining the Moon’s lack of water and iron core, and allowing for denser material to sink into Earth’s mantle.

  • Confirmation awaits. The LLSVPs’ structure and the theory of a giant Theia require further investigation.

  • Smaller dense pockets found near LLSVPs might be remnants of additional collisions with smaller planets in Earth’s early history.

It has long been accepted by scientists that the Moon originated from a protoplanet named Theia striking Earth some 4.5 billion years ago when it was still very young. Scientists have now made an astonishing new discovery: Theia’s remains may be discovered in two continent-sized layers of rock that are buried far below the Earth’s mantle.

These two blobs, which straddle the core like a pair of headphones and are located beneath West Africa and the Pacific Ocean, have baffled seismologists for decades. According to Qian Yuan, a geodynamics Ph.D. candidate at Arizona State University (ASU), Tempe, “they are the largest thing in the Earth’s mantle,” standing up to 1000 kilometers tall and spreading out multiple times that wide. When seismic waves from earthquakes travel through the layers, they suddenly slow down, indicating that the layers are denser and have a different chemical composition than the surrounding mantle rock.

Seismologists refer to these regions as large low-shear velocity provinces (LLSVPs), and they may have simply crystallized out of the Earth’s primordial magma ocean. Alternatively, they could be dense pools of early mantle rock that have survived the impact that formed the Moon. But according to Yuan, new isotopic data and modeling suggest that the LLSVPs are actually the interior of the alien impactor. “This crazy idea is at least possible,” says Yuan, who made the presentation at the Lunar and Planetary Science Conference.

For years, the concept has been kicking around meetings and lab corridors. Yet according to Edward Garnero, an ASU Tempe seismologist who was not involved in the research, this is the first time someone has gathered an important amount of evidence and made a convincing argument for it. “I think it’s completely viable until someone tells me it’s not.”

The LLSVPs may have existed since the Moon-forming impact, according to evidence from Iceland and Samoa, according to Sujoy Mukhopadhyay, a geochemist at the University of California, Davis. While he finds Yuan’s theory viable, he is open to alternative theories. Magma plumes that feed the volcanoes on both islands and extend all the way down to the LLSVPs have been traced by seismic imaging. Lavas on the islands contain an isotopic record of radioactive elements that formed only during the first 100 million years of Earth’s history, as Mukhopadhyay and others have discovered over the past 10 years.

Moreover, a new picture of the Moon-forming impactor suggests it could have delivered a cargo of dense rock deep inside Earth. In order to explain why the Moon is dry and lacks a significant iron core, the impact theory was developed in the 1970s. It postulates that during a catastrophic impact, volatiles like water would have evaporated and escaped, while a ring of less dense rocks would have eventually coalesced into the Moon. The theory invoked an impactor the size of Mars or—in recent variants—much smaller. However, astrophysicist Steven Desch of ASU Tempe and co-author of the study Yuan suggests that Theia was almost as large as Earth.

Desch and his colleagues measured the ratios of hydrogen to deuterium, a heavier hydrogen isotope, in their studies of Apollo Moon rocks. They discovered that compared to Earth rocks, some of the Moon samples had far higher concentrations of light hydrogen. They hypothesized in a study published in Geochemistry that Theia must have been massive to be able to absorb and hold onto so much light hydrogen. Additionally, it had to be extremely dry because any water would have increased the overall deuterium levels because water is naturally enriched in heavy hydrogen during its formation in interstellar space. According to Desch, such a large, dry protoplanet would have split into layers with an iron-rich mantle and an iron-depleted core, making it between 2% and 3.5% denser than the Earth today.

Yuan had begun modeling Theia’s fate even before he was made aware of Desch’s density estimates. According to his model, Theia’s core should have quickly merged with Earth’s following the collision. In order to determine what circumstances would have allowed the material to persist rather than blending in and sink to the base of the mantle, he also experimented with Theia’s size and density. According to the simulations, mantle rocks that are 1.5% to 3.5% denser than Earth’s would persist and eventually form piles close to the core. The result lined up perfectly with Desch’s deuterium evidence. “It’s this sweet spot for the density,” Desch says.

The scale of the LLSVPs, which collectively have six times the mass of the Moon, would also be explained by a massive Theia. Yuan claims that only an impactor the size of Theia could have delivered them if they are extraterrestrial.

But there are a lot of limitations, such as the hazy proof for the LLSVPs themselves. According to Barbara Romanowicz, a seismologist at UC Berkeley, and Anne Davaille, a geophysicist at Paris-Saclay University, their pilelike structure may just be an illusion brought about by interior models that depend on low frequency seismic waves, which blur minute differences. This was suggested in a study published in Tectonics. The piles may only rise a few hundred kilometers before splitting off into branched plumes, as opposed to reaching up to 1000 kilometers. “There may be holes in them,” Romanowicz says. “They may be a bundle of tubes.”

According to UC Berkeley geophysicist Harriet Lau, smaller or less monolithic LLSVPs would also be consistent with an upcoming analysis that finds the LLSVPs are densest at the bottom. The analysis is based on two methods of imaging the deep Earth: seismometers to detect how Earth’s natural vibrations travel through the deep mantle, and GPS stations to measure how the Moon’s tidal pull stretches Earth. “Perhaps the real story behind the density is the distribution depth,” she continues.

The hypothesis that Theia was almost as large as a proto-Earth may be complicated by less massive LLSVPs, according to Durham University seismologist Jennifer Jenkins. Yuan’s picture, she adds, “is not inconsistent with what we know, but I’m not entirely convinced.”

According to Desch, the group could verify its hypothesis by comparing the geochemical composition of the rocks from the Moon’s mantle and the lavas on the island. Because the unaltered mantle is not captured in any of the Apollo samples, scientists are interested in samples from the largest impact crater on the Moon’s south pole, where these rocks may be excavated. This decade, NASA and China are planning robotic missions to the south pole, which is a prime candidate location for NASA’s planned astronaut return to the moon.

Perhaps Theia’s remnants are not alone if they are located deep within Earth’s mantle. Seismologists are increasingly seeing small, ultradense pockets of material in the deep mantle, only a few hundred kilometers across, often near the edges of the LLSVPs. Jenkins speculates that they could be the buried remains of iron-rich cores from other dwarf planets that collided with the early Earth. Theia, in fact, might be just one grave in a planetary cemetery.


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