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Theia, perhaps as big as the proto-Earth, would have delivered its densest rocks inside the planet. 

Hagai Perets

Remains of impact that created the Moon may lie deep within Earth

Scientists have long agreed that the Moon formed when a protoplanet, called Theia, struck Earth in its infancy some 4.5 billion years ago. Now, a team of scientists has a provocative new proposal: Theia’s remains can be found in two continent-size layers of rock buried deep in Earth’s mantle.

For decades, seismologists have puzzled over these two blobs, which sit below West Africa and the Pacific Ocean and straddle the core like a pair of headphones. Up to 1000 kilometers tall and several times that wide, “they are the largest thing in the Earth’s mantle,” says Qian Yuan, a Ph.D. student in geodynamics at Arizona State University (ASU), Tempe. Seismic waves from earthquakes abruptly slow down when they pass through the layers, which suggests they are denser and chemically different from the surrounding mantle rock.

The large low-shear velocity provinces (LLSVPs), as seismologists call them, might simply have crystallized out of the depths of Earth’s primordial magma ocean. Or they might be dense puddles of primitive mantle rock that survived the trauma of the Moon-forming impact. But based on new isotopic evidence and modeling, Yuan believes the LLSVPs are the guts of the alien impactor itself. “This crazy idea is at least possible,” says Yuan, who presented the hypothesis last week at the Lunar and Planetary Science Conference.

The idea has rattled around lab corridors and conference halls for years. But Edward Garnero, a seismologist at ASU Tempe who was not involved in the work, says it’s the first time anyone has marshaled multiple lines of evidence and mounted a serious case for it. “I think it’s completely viable until someone tells me it’s not.”

Evidence from Iceland and Samoa suggests the LLSVPs have existed since the time of the Moon-forming impact, says Sujoy Mukhopadhyay, a geochemist at the University of California (UC), Davis, who considers Yuan’s idea plausible but is open to other explanations. Seismic imaging has traced plumes of magma that feed volcanoes on both islands all the way down to the LLSVPs. Over the past decade, Mukhopadhyay and others have discovered that lavas on the islands contain an isotopic record of radioactive elements that formed only during the first 100 million years
of Earth history.

Moreover, a new picture of the Moon-forming impactor suggests it could have delivered a cargo of dense rock deep inside Earth. The impact theory was developed in the 1970s to explain why the Moon is dry and doesn’t have much of an iron core: In a cataclysmic impact, volatiles like water would have vaporized and escaped, while a ring of less dense rocks thrown up in the collision would have eventually coalesced into the Moon. The theory invoked an impactor the size of Mars or—in recent variants—much smaller. But recent work from Yuan’s co-author, ASU Tempe astrophysicist Steven Desch, suggests Theia was nearly as big as Earth.

In studies of Apollo Moon rocks, Desch and his colleagues measured the ratios of hydrogen to deuterium, a heavier hydrogen isotope. Light hydrogen was far more abundant in some of the Moon samples than in Earth rocks, they found. To capture and hold onto so much light hydrogen, Theia must have been massive, they proposed in a 2019 study in Geochemistry. It must also have been quite dry, as any water, which is naturally enriched in heavy hydrogen during its formation in interstellar space, would have raised the overall deuterium levels. Such a dry, large protoplanet would have separated into layers with an iron-depleted core and an iron-rich mantle, Desch says, some 2% to 3.5% denser than present-day Earth.

Even before Yuan learned of Desch’s density estimates, he was modeling Theia’s fate. His model suggests that after the collision, Theia’s core would have quickly merged with Earth’s. He also probed the fate of Theia’s mantle, varying Theia’s size and density to see what conditions would have allowed the material to persist, rather than mixing in, and sink to the mantle’s base. The simulations consistently showed that mantle rocks 1.5% to 3.5% denser than Earth’s would survive and end up as piles near the core. The result lined up perfectly with Desch’s deuterium evidence. “It’s this sweet spot for the density,” Desch says.

A massive Theia would also explain the scale of the LLSVPs, which together contain six times more mass than the Moon. If they are extraterrestrial, Yuan says, only an impactor as large as Theia could have delivered them.

There are many caveats, however, including the fuzzy evidence for the LLSVPs themselves. Their pilelike structure could simply be an illusion created by models of the interior that rely on low frequency seismic waves, which blur small differences, Barbara Romanowicz, a seismologist at UC Berkeley, and Anne Davaille, a geophysicist at Paris-Saclay University, suggested in a study in Tectonics last year. Rather than reaching up 1000 kilometers, the piles may rise only a few hundred kilometers before breaking off into branched plumes. “There may be holes in them,” Romanowicz says. “They may be a bundle of tubes.”

Smaller or less monolithic LLSVPs would also be consistent with a forthcoming analysis that finds the LLSVPs are densest at the bottom, says Harriet Lau, a geophysicist at UC Berkeley. The analysis relies on two ways of imaging the deep Earth: using GPS stations to measure the way the Moon’s tidal pull stretches Earth, and seismometers to sense how Earth’s natural vibrations pass through the deep mantle. “Perhaps the real story behind the density is the distribution depth,” she says.

Less massive LLSVPs could complicate the idea that Theia was nearly the size of a proto-Earth, says Jennifer Jenkins, a seismologist at Durham University. Yuan’s picture, she adds, “is not inconsistent with what we know, but I’m not entirely convinced.”

Desch says the team could test its idea by looking for geochemical similarities between the island lavas and rocks from the Moon’s mantle. None of the Apollo samples capture the unaltered mantle, which is one reason scientists want samples from the Moon’s largest impact crater, on its south pole, where such rocks may be exhumed. NASA and China are both planning robotic missions to the south pole this decade, and it is a leading candidate site for NASA’s return of astronauts to the Moon.

If Theia’s remnants do lie deep in Earth’s mantle, they may not be alone. 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. Maybe they are the sunken remnants of iron-rich cores from other miniature planets that hit early Earth, Jenkins says. Theia, in fact, might be just one grave in a planetary cemetery.  

*Correction, 24 March, 1:30 p.m.: An earlier version of this story misstated Qian Yuan’s Ph.D. focus.