Throughout their childhood, Earth and Theia lived in harmony but everything changed when gravitational disturbances attacked
TL;DR
Scientists have proposed that two massive rock formations deep within Earth’s mantle, known as large low-shear velocity provinces (LLSVPs), might be the remnants of the protoplanet Theia, which collided with Earth 4.5 billion years ago to form the Moon. These formations, located beneath West Africa and the Pacific Ocean, are denser and chemically distinct from the surrounding mantle. Researchers are using new seismic and isotopic data to investigate whether Theia’s dense mantle survived and sank into Earth’s core. If true, this discovery could change our understanding of Earth’s structure and early history.
For a long time, scientists have agreed that the Moon was formed after a protoplanet called Theia collided with the early Earth about 4.5 billion years ago. Now, a team of researchers has a new bold idea: The remains of Theia may be hidden in two massive layers of rock located deep within Earth’s mantle.
For decades, seismologists have been intrigued by these two large formations, which sit beneath West Africa and the Pacific Ocean and resemble a pair of headphones around the Earth’s core. These formations are up to 1,000 kilometers tall and much wider. “They are the largest features in the Earth’s mantle,” says Qian Yuan, a Ph.D. student in geodynamics at Arizona State University (ASU) in Tempe. Seismic waves from earthquakes slow down significantly as they pass through these layers, which suggests the formations are denser and chemically distinct from the surrounding mantle.
Known as large low-shear velocity provinces (LLSVPs), these structures might have crystallized from Earth’s ancient magma ocean or could be dense remnants of the primitive mantle that survived the violent Moon-forming impact. However, based on new isotopic data and models, Yuan thinks these LLSVPs could actually be the remnants of Theia itself. “This wild idea is at least plausible,” says Yuan, who presented this hypothesis at the recent Lunar and Planetary Science Conference.
This theory has been floating around for years in labs and conferences, but Edward Garnero, a seismologist at ASU, Tempe, who was not part of the study, believes this is the first time a solid case has been built with multiple lines of evidence. “I think it’s entirely possible until proven otherwise.”
Evidence from places like Iceland and Samoa supports the notion that these LLSVPs have existed since the Moon-forming event, according to Sujoy Mukhopadhyay, a geochemist from the University of California, Davis, who finds Yuan’s theory convincing but is also open to other explanations. Seismic studies have shown that magma plumes feeding volcanoes on these islands originate from the LLSVPs. In the past decade, researchers like Mukhopadhyay have discovered that lava from these volcanoes contains a radioactive isotopic record that dates back to Earth’s first 100 million years.
Additionally, recent findings about the Moon-forming impactor suggest it could have deposited dense material deep within Earth. The impact theory, developed in the 1970s, explained why the Moon lacks significant water and has a small iron core: a massive collision would have vaporized volatile substances like water, while less dense debris from the impact would have formed the Moon. The original theory suggested an impactor the size of Mars or smaller, but recent work by Steven Desch, Yuan’s co-author and an astrophysicist at ASU, suggests Theia was nearly as large as Earth.
In their studies of Apollo Moon rocks, Desch and his colleagues found that some Moon samples had a much higher ratio of light hydrogen (hydrogen to deuterium) compared to Earth rocks. To capture and retain this much light hydrogen, Theia would have needed to be a large, dry protoplanet, they proposed in a 2019 Geochemistry study. Such a large protoplanet would have developed an iron-depleted core and an iron-rich mantle, Desch adds, making Theia’s mantle 2% to 3.5% denser than Earth’s mantle today.
Even before learning of Desch’s density calculations, Yuan was modeling Theia’s destiny. His models showed that Theia’s core would have quickly merged with Earth’s after the impact. Yuan also tested various sizes and densities of Theia’s mantle to determine how this material could survive and sink to the base of Earth’s mantle rather than mix in. His simulations consistently showed that material 1.5% to 3.5% denser than Earth’s mantle could remain intact and accumulate near the core, aligning well with Desch’s deuterium data. “It’s the perfect density range,” says Desch.
A massive Theia would also explain the size of the LLSVPs, which together are six times heavier than the Moon. If these formations are indeed extraterrestrial, Yuan argues, only an impactor the size of Theia could account for them.
There are, however, several uncertainties, including the incomplete data on LLSVPs. Their structure could simply be an illusion created by seismic models that use low-frequency waves, which blur finer details, according to a study in Tectonics by Barbara Romanowicz, a seismologist at UC Berkeley, and Anne Davaille, a geophysicist at Paris-Saclay University. The “blobs” may actually extend only a few hundred kilometers upward before branching off into plumes. “There could be gaps in them,” Romanowicz says. “They could be a collection of tubes.”
Smaller or less uniform LLSVPs would align with new research suggesting that the LLSVPs are densest at their base, says Harriet Lau, a geophysicist at UC Berkeley. This research uses GPS to measure how the Moon’s gravitational pull affects Earth and seismometers to track how vibrations pass through the deep mantle. “The real story might be about how the density changes with depth,” she suggests.
Less massive LLSVPs would complicate the idea that Theia was nearly the size of Earth, says Jennifer Jenkins, a seismologist at Durham University. Yuan’s theory, she notes, “fits with what we know, but I’m still not completely convinced.”
Desch proposes that further testing could involve comparing the chemical makeup of lava from islands like Iceland and Samoa to rocks from the Moon’s mantle. Unfortunately, none of the Apollo samples contain unaltered Moon mantle material, which is why scientists are eager to collect samples from the Moon’s largest impact crater, located at the south pole. Both NASA and China are planning robotic missions to the Moon’s south pole in the coming decade, and NASA has also prioritized the location for future astronaut missions.
If Theia’s remains do lie deep within Earth’s mantle, they may not be alone. Seismologists have been detecting small, ultradense pockets of material in the deep mantle, often near the edges of the LLSVPs. Perhaps these pockets are the remnants of other small protoplanets that collided with Earth early in its history, Jenkins suggests. Theia, in fact, might be just one of many planetary remains buried within Earth.
