Theia, the Mars-sized planet that slammed into early Earth about 4.5 billion years ago, didn’t survive the collision as a separate world. But it didn’t disappear either. Most of Theia’s mass was absorbed into Earth itself, and a significant portion ended up forming the Moon. In 2023, researchers presented compelling evidence that two massive, continent-sized structures buried deep inside Earth are the remains of Theia, sitting right where they settled billions of years ago.
Two Giant Blobs Near Earth’s Core
In the 1980s, geophysicists noticed something strange. Seismic waves traveling through Earth’s deep interior slowed down dramatically when passing through two enormous regions near the boundary between the mantle and the core. One sits beneath the African continent, the other beneath the Pacific Ocean. These structures, known as large low-velocity provinces, are continent-sized and unusually dense because they contain far more iron than the surrounding mantle rock.
For decades, scientists debated what these blobs actually were. A 2023 study from researchers at Arizona State University and Caltech offered a striking answer: they’re the remains of Theia’s iron-rich mantle. Using high-resolution simulations of the ancient collision, the team showed that the impact’s energy was concentrated in the upper half of Earth’s mantle, leaving the lower mantle relatively cool. Because the deep mantle never fully melted, Theia’s dense, iron-rich material didn’t mix evenly into the rest of the planet. Instead, it sank mostly intact toward the core and settled into two distinct clumps, like blobs of wax in a lava lamp that’s been turned off. Those clumps have remained in place for roughly 4.5 billion years.
A Large Piece Became the Moon
The collision didn’t just reshape Earth. It also flung enough debris into orbit to form the Moon. Models predict that 70 to 90 percent of the Moon’s material originally came from Theia, with the rest coming from Earth’s outer layers. This is why the question of “where is Theia now” has two answers: deep inside Earth, and about 240,000 miles above it.
The exact process of the Moon’s formation is still debated. The traditional view holds that debris from the collision gradually coalesced in orbit over months or years. A newer simulation from NASA suggests the Moon may have formed almost immediately, within hours, as material from both Earth and Theia launched directly into a stable orbit. This faster formation model actually helps explain one of the biggest puzzles in planetary science: why Moon rocks are chemically almost identical to Earth rocks. If the Moon formed quickly from a mix heavily weighted toward Earth’s outer material, the similarity makes more sense.
The Isotope Puzzle
If most of the Moon came from Theia, and Theia formed elsewhere in the solar system, you’d expect Moon rocks to have a noticeably different chemical fingerprint than Earth rocks. They don’t. When scientists measure oxygen isotopes, which act like a planetary ID card, Earth and Moon samples are essentially identical. This is one of the biggest unresolved questions in planetary science, sometimes called the “Isotope Crisis.”
There are a few possible explanations. Theia and proto-Earth may have formed from the same region of the solar disk, making them chemically similar to begin with. Some research suggests they were actually neighbors in the early solar system, orbiting at similar distances from the Sun before their paths crossed. Alternatively, the violence of the impact may have mixed Theia and Earth material so thoroughly that any original differences were erased. Most researchers think the answer involves some combination of both.
The Synestia Theory
Not everyone agrees on exactly how the collision played out. An alternative model proposes that the impact was so violent it didn’t just knock debris into orbit. Instead, it vaporized a significant portion of both bodies, creating a synestia: a rapidly spinning, doughnut-shaped cloud of superheated rock and dust with a molten core at its center. In this scenario, about 10 percent of Earth’s mass would have been vaporized, with the rest turning to liquid rock.
The Moon, in this model, formed inside the synestia rather than from a ring of debris orbiting outside it. A small pocket of liquid rock within the cloud served as a seed, collecting material as the synestia cooled and contracted. Eventually the vapor condensed, the Moon emerged, and the remaining material collapsed back into what became Earth. Theia still exists in this version of events. It’s just that the mixing was even more thorough, which could better explain why Earth and Moon rocks look so alike.
Why Theia Didn’t Just Mix In
The natural question is: why didn’t Theia’s material blend evenly throughout Earth’s interior? The key is temperature. Earlier, lower-resolution models of the giant impact assumed it melted Earth’s mantle all the way through. If that had happened, Theia’s iron-rich rock would have dispersed uniformly, like stirring paint. But the 2023 simulations showed that the lower mantle stayed cooler than previously thought. The impact energy dissipated mostly in the upper mantle, leaving the deep interior solid enough to preserve distinct clumps of foreign material.
Those iron-rich clumps, denser than their surroundings, gradually sank to the lowest point they could reach: the boundary between the mantle and the outer core, roughly 1,800 miles below the surface. And there they’ve sat, largely undisturbed by the slow churning of mantle convection, for nearly the entire history of the planet. They’re among the oldest large-scale features inside Earth.
What This Means for Earth’s History
If the blobs beneath Africa and the Pacific really are pieces of Theia, they aren’t just geological curiosities. These structures are massive enough to influence how heat moves through Earth’s interior, which in turn affects volcanic activity, tectonic plate movement, and even the behavior of Earth’s magnetic field. Some researchers have speculated that the African blob may be connected to the unusual volcanic activity in East Africa. The idea that a foreign planet’s remains are still shaping geological processes 4.5 billion years later is one of the more remarkable findings in recent Earth science.
So Theia is not “somewhere out there.” It’s split between two places you’re already familiar with: beneath your feet and overhead in the night sky.