Scientists have long agreed that the Moon formed when a protoplanet, called Theia, struck Earth in its infancy some 4.5 billion years ago. Theia’s remains may be located in two continent-sized layers of rock buried deep inside the Earth’s mantle, a group of experts has now proposed.

Seismologists have been puzzled by these two blobs for many years. They sit beneath West Africa and the Pacific Ocean and cross the core like a pair of headphones. Up to 1000 kilometers tall and several times that wide, “they are the largest things in the Earth’s mantle,” says Qian Yuan, a Ph.D. student in geodynamics at Arizona State University (ASU), Tempe. When seismic waves from earthquakes move through the strata, they dramatically slow down, indicating 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 could be substantial puddles of ancient mantle rock that withstood the damage of the impact that created the Moon. Yuan, however, is convinced that the LLSVPs are the core of the alien impactor itself based on new isotopic data and modeling. “This crazy idea is at least possible,” says Yuan, who presented the hypothesis last week at the Lunar and Planetary Science Conference.

For years, the concept has floated in conference rooms and lab corridors. 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.”

Sujoy Mukhopadhyay, a geochemist at the University of California (UC), Davis, finds Yuan’s theory reasonable but is open to other explanations. Evidence from Iceland and Samoa suggests the LLSVPs have existed from the time of the Moon-forming impact. Seismic imaging has shown that magma plumes that feed both islands’ volcanoes extend all the way down to the LLSVPs. According to research by Mukhopadhyay and others over the last ten years, the lava on the islands has an isotopic record of radioactive elements that were only generated during the first 100 million years of Earth’s existence, according to research by Mukhopadhyay and others over the last ten years

Moreover, a new picture of the Moon-forming impactor suggests it could have delivered a cargo of dense rock deep inside the Earth. A cataclysmic impact would have caused volatiles like water to evaporate and escape, while a ring of less dense rocks thrown up in the collision would have eventually coalesced into the Moon. This impact theory was developed in the 1970s to explain why the Moon is dry and doesn’t have much of an iron core. The theory invokes an impactor the size of Mars or—in recent variants—much smaller. ASU Tempe astrophysicist Steven Desch, who is Yuan’s co-author, has just published research that suggests Theia was almost as big as Earth.

Desch and his colleagues measured the ratios of hydrogen to deuterium, a heavier hydrogen isotope, in investigations of Apollo Moon rocks. They discovered that light hydrogen was far more prevalent in several Moon samples than in rocks from Earth. In a 2019 study in Geochemistry, they proposed that Theia must have been enormous to collect and retain so much light hydrogen.

As any water, which is naturally enriched in heavy hydrogen during its production in interstellar space, would have raised the overall deuterium levels, it must also have been relatively dry. 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.

Desch’s density estimates were being modeled by Yuan even before he was made aware of them.His model suggests that after the collision, Theia’s core would have quickly merged with Earth’s.He also looked into what happened to Theia’s mantle. He changed Theia’s size and density to determine under what conditions the material might have survived rather than merging in and falling 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 outcome matched Desch’s deuterium evidence precisely.“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.

The hazy evidence for the LLSVPs themselves is one of several caveats, though. In a study published in Tectonics last year, Barbara Romanowicz, a seismologist at UC Berkeley, and Anne Davaille, a geophysicist at Paris-Saclay University, theorized that their pile-like structure may simply be an illusion produced by interior models that rely on low frequency seismic waves, which obscure small differences.

Rather than reaching up 1000 kilometers, the piles may rise only a few hundred kilometers before breaking off into branched plumes. They might have holes in them, adds Romanowicz. “They may be a bundle of tubes.”

According to Harriet Lau, a geophysicist at UC Berkeley, smaller or less monolithic LLSVPs would also be consistent with an upcoming analysis that shows the LLSVPs are densest at the bottom. Seismometers are used to measure how Earth’s natural vibrations travel into the deep mantle, while GPS stations are used to detect how the Moon’s gravitational pull stretches Earth. “Perhaps the real story behind the density is the distribution depth,” she says.

According to Durham University seismologist Jennifer Jenkins, less enormous LLSVPs could cast doubt on the notion that Theia was roughly the size of a proto-Earth. 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. One reason why scientists desire samples from the Moon’s largest impact crater, on its south pole, where such rocks may be exhumed, is because none of the Apollo samples capture the unaltered mantle. It is a top candidate location for NASA’s return of astronauts to the Moon, and both NASA and China are planning robotic missions to the south pole this decade.

If Theia’s remnants do lie deep in Earth’s mantle, they may not be alone. More frequently near the edges of the LLSVPs, seismologists are observing small, incredibly dense pockets of material in the deep mantle that are only a few hundred kilometers across. 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.