Four and a half billion years ago, a protoplanet called Theia struck the young Earth with enough force to birth the Moon — and, scientists now propose, to leave its own mantle entombed in our planet's deepest interior ever since. Two vast rock formations sitting at the base of Earth's mantle, long mysterious to seismologists, may be the preserved remains of that ancient impactor, held in place by their own density across geological time. The hypothesis, supported by isotopic chemistry and computer modeling, invites us to consider that the violence of planetary formation is not erased but archi
Ancient Impactor's Remains May Lurk in Earth's Deep Mantle
Theia might be just one grave in a planetary cemetery.
Mark
How do we even know these rock formations exist if they're so far down?
Mimi
Seismic waves from earthquakes pass through them and slow down in a distinctive way. It's like listening to sound travel through different materials—we can tell something is there by how the waves behave.
Mark
And the theory is that this is literally a piece of an alien planet?
Mimi
Not alien in the sense of extraterrestrial origin, but yes—material from Theia, a protoplanet that collided with Earth before the Moon existed. The denser parts of Theia's mantle would have sunk and stayed down rather than mixing back into Earth's mantle.
Mark
Why would it stay down instead of mixing?
Mimi
Density. If Theia's mantle was denser than Earth's by just the right amount—between 1.5 and 3.5 percent—it would sink and resist being churned back up. The simulations show that material in that density range would naturally settle near the core.
Mark
How do they know Theia was that dense?
Mimi
They measured hydrogen isotope ratios in Moon rocks. The Moon has an unusual abundance of light hydrogen, which only makes sense if Theia was massive enough to hold onto it and dry enough that water didn't dilute it.
Mark
So the Moon rocks are telling us about the impactor?
Mimi
Exactly. The Moon formed from material ejected during the collision. Its composition is a record of what Theia was like. The isotopes are like a fingerprint.
Mark
What happens if they're wrong?
Mimi
Then these rock blobs are something else—maybe just denser material that crystallized from Earth's early magma ocean. But the evidence is lining up in a way that's hard to ignore. The density estimates match the simulations. The geochemistry of volcanic islands points to these same depths. It's not proven, but it's plausible.
Il Polso
- Two continent-sized rock blobs buried near Earth's core have defied explanation for decades, slowing seismic waves in ways that suggest they are chemically alien to the surrounding mantle.
- A geodynamics student at Arizona State University has proposed the provocative answer: these formations are the surviving mantle of Theia, the Mars-to-Earth-sized protoplanet whose collision with early Earth created the Moon.
- Hydrogen isotope ratios in Apollo lunar samples point to a Theia that was nearly Earth's size and extraordinarily dry, with a mantle dense enough to sink through Earth's interior and resist mixing for billions of years.
- Computer simulations confirm that mantle material in precisely that density range would survive the impact, settle near the core, and account for the mass and position of both rock formations.
- Skeptics caution that the internal structure of these formations is still poorly resolved, and that they may be less solid or massive than current imaging suggests.
- Upcoming NASA and Chinese lunar missions targeting the Moon's south pole crater could retrieve pristine mantle samples that, compared against deep Earth geochemistry, may finally confirm whether Theia's remnants still haunt our planet's core.
Four and a half billion years ago, a protoplanet called Theia struck the young Earth with enough force to birth the Moon — and, scientists now propose, to leave its own mantle entombed in our planet's deepest interior ever since. Two vast rock formations sitting at the base of Earth's mantle, long mysterious to seismologists, may be the preserved remains of that ancient impactor, held in place by their own density across geological time. The hypothesis, supported by isotopic chemistry and computer modeling, invites us to consider that the violence of planetary formation is not erased but archived — written in stone at the very heart of the world we stand on.
Four and a half billion years ago, a body roughly the size of Mars — or perhaps nearly as large as Earth itself — slammed into our young planet and flung enough debris into orbit to form the Moon. Scientists have long wondered what became of the impactor, a protoplanet called Theia. Now a team of researchers believes they have found it, preserved in two enormous rock formations buried at the base of Earth's mantle.
These structures, known as large low-shear velocity provinces, sit beneath West Africa and the Pacific Ocean. Each rises up to 1,000 kilometers tall and stretches several times that distance across. Seismic waves from earthquakes slow abruptly when passing through them — a sign that the rocks are denser and chemically distinct from everything around them. Qian Yuan, a Ph.D. student in geodynamics at Arizona State University, proposes they are not simply relics of Earth's primordial magma ocean, but the surviving mantle of Theia itself.
The case rests on interlocking evidence. Astrophysicist Steven Desch analyzed hydrogen isotope ratios in Apollo Moon rocks and found an unusual abundance of light hydrogen — a signature, he argued, that only a massive, extremely dry protoplanet could have captured and retained. His analysis suggested Theia's mantle was between 1.5 and 3.5 percent denser than Earth's. Yuan then ran simulations showing that material in exactly that density range would survive the collision, resist blending with Earth's mantle, and sink to settle near the core — right where the formations are found. The combined mass of the two blobs is roughly six times that of the Moon, consistent with a Theia nearly Earth's size.
Further support comes from volcanic plumes feeding islands like Iceland and Samoa, which have been traced back to these deep formations. The lavas carry isotopic fingerprints of radioactive elements that existed only in the first 100 million years of Earth's history, suggesting an unbroken connection to that ancient material.
Uncertainties persist. Some seismologists argue the formations may be less solid or coherent than they appear, possibly riddled with gaps or composed of branching plumes rather than dense, unified masses. If so, reconciling them with a near-Earth-sized Theia becomes more difficult.
The next test may come from the Moon. NASA and China are both planning missions to the lunar south pole this decade, targeting a vast impact crater where mantle rocks may have been excavated and exposed. Comparing those samples with deep Earth geochemistry could confirm whether Theia's remnants truly endure in our planet's interior. And if they do, researchers suspect Theia may not be alone — scattered ultradense pockets detected throughout the deep mantle hint that Earth's interior may be a museum of ancient planetary collisions, not merely a graveyard.
Four and a half billion years ago, something the size of Mars—or perhaps nearly as large as Earth itself—collided with our planet in a cataclysm so violent it created the Moon. For decades, scientists have wondered what became of the impactor, a protoplanet called Theia. Now a team of researchers believes they have found it: entombed in two massive blobs of rock buried so deep in Earth's mantle that they sit like a pair of headphones straddling the planet's core.
These formations, known to seismologists as large low-shear velocity provinces, or LLSVPs, have long been a puzzle. One sits beneath West Africa, the other beneath the Pacific Ocean. Each reaches up to 1,000 kilometers tall and stretches several times that distance across. When seismic waves from earthquakes pass through them, the waves slow down abruptly—a sign that these rocks are denser and chemically distinct from the surrounding mantle. Qian Yuan, a geodynamics Ph.D. student at Arizona State University, has proposed that these continent-sized formations are not simply crystallized remnants of Earth's primordial magma ocean, but rather the surviving mantle of Theia itself, preserved in the depths for billions of years.
The hypothesis rests on converging lines of evidence. Steven Desch, an astrophysicist at ASU, studied hydrogen isotope ratios in Apollo Moon rocks and found that some samples contained far more light hydrogen than Earth rocks do. This abundance, he reasoned, could only have been captured and retained by a massive, extremely dry protoplanet. A smaller impactor would have lacked the gravitational pull to hold onto so much light hydrogen. Desch's analysis suggested Theia was nearly as large as Earth itself, with a mantle somewhere between 1.5 and 3.5 percent denser than Earth's mantle today. Yuan then ran computer simulations to see what would happen to such a body after collision. His models showed that mantle rocks with that exact density range would survive the impact, resist mixing with Earth's mantle, and sink to settle near the core—precisely where the LLSVPs are found.
The scale fits too. The two rock formations together contain roughly six times more mass than the Moon itself. Only an impactor as large as Theia could have delivered such a quantity of material to Earth's depths. Additional support comes from geochemical studies of volcanic islands. Researchers have traced plumes of magma feeding volcanoes in Iceland and Samoa down to the LLSVPs themselves. The lavas erupted at these islands contain isotopic signatures of radioactive elements that formed only in the first 100 million years of Earth's history, suggesting a direct connection to material from that ancient epoch.
Yet significant uncertainties remain. The very structure of the LLSVPs is debated. Some seismologists argue that what appears to be two massive, coherent piles may actually be an optical illusion created by the low-frequency seismic waves used to image Earth's interior. The formations might be riddled with holes, or composed of branching plumes rather than solid blocks. If the LLSVPs are smaller or less monolithic than currently believed, they could be harder to reconcile with a Theia nearly the size of Earth. Some researchers remain unconvinced by the hypothesis, though none have ruled it out entirely.
The next chapter of this investigation may unfold on the Moon itself. NASA and China are both planning robotic missions to the Moon's south pole this decade, targeting the Moon's largest impact crater where pristine mantle rocks may have been excavated and exposed. If scientists can retrieve samples of the Moon's unaltered mantle and compare their geochemistry to the deep Earth rocks, they may finally confirm whether Theia's ghost still dwells in Earth's depths. And if it does, Yuan and others suspect Theia may not be alone—seismologists are increasingly detecting small, ultradense pockets scattered through the deep mantle, possibly the sunken cores of other planetary bodies that struck early Earth. The deep interior, it seems, may be less a graveyard than a museum of planetary collisions.
Citazioni salienti
They are the largest thing in the Earth's mantle— Qian Yuan, Arizona State University
I think it's completely viable until someone tells me it's not— Edward Garnero, seismologist at Arizona State University
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