A Mars-sized planet called Theia crashed into Earth roughly 4.5 billion years ago, about 60 million years after the solar system formed. The collision was so violent it blasted enough debris into orbit to form the Moon, and pieces of Theia may still be buried deep inside Earth’s mantle today.
What Was Theia?
Theia was a protoplanet, roughly the size of Mars, that formed in the inner solar system during the chaotic early period when young planets were still sweeping up leftover material and crashing into each other. It’s named after the Greek goddess who was the mother of Selene, the Moon goddess. No one has ever observed Theia directly, of course. Its existence is inferred from what it left behind: the Moon, the chemistry of lunar rocks, and strange structures deep inside our planet.
How the Collision Happened
Computer simulations suggest Theia struck Earth at roughly a 45-degree angle. Hundreds of impact simulations have been run over the years, and that angle consistently produces the best conditions for forming a large satellite like the Moon. The collision would have been unimaginably energetic, far beyond anything in human experience. Both bodies were largely destroyed and reformed in the aftermath.
The impact liquefied Earth’s surface, creating a global ocean of magma thousands of miles deep. It also blasted a massive cloud of vaporized rock into orbit around the newly reformed Earth. Within a relatively short time (perhaps centuries to thousands of years), that orbiting debris coalesced into the Moon.
How Scientists Know It Happened
The strongest evidence comes from oxygen isotopes. Every body in the solar system has a slightly different chemical fingerprint based on the ratios of different oxygen isotopes in its rocks. Mars, meteorites from the asteroid belt, and samples from other sources all carry distinct signatures. But when researchers measure lunar rocks brought back by the Apollo missions, the Moon’s oxygen isotope composition is essentially identical to Earth’s, matching to within 0.2 parts per million. That’s not a coincidence you get from two unrelated objects. It means Earth and the Moon formed from the same pool of material, exactly what you’d expect if a giant impact thoroughly mixed both bodies together.
This near-perfect match actually created a puzzle. In the original version of the giant impact theory, the Moon should have formed mostly from Theia’s material and been chemically different from Earth. The fact that both bodies are so similar means either Theia happened to have the same composition as Earth, or the impact was so energetic that the two planets were completely blended before the Moon separated out.
The Synestia Model
The blending problem led to a more recent refinement of the theory. In the traditional model, the impact ejects a disk of debris that orbits Earth and gradually clumps into the Moon, similar to how Saturn’s rings might form a moonlet. But a more energetic collision could have done something far stranger: turned Earth into a totally new kind of object called a synestia.
A synestia is a rapidly spinning, partially vaporized structure that extends for tens of thousands of kilometers. It’s not quite a planet with an atmosphere, and it’s not a planet with a ring around it. It’s a single enormous cloud of superheated rock vapor, spinning so fast that the outer regions can’t keep up with the inner core. In this model, the Moon condensed inside this vapor cloud, which is why it ended up with the same chemical fingerprint as Earth. Both bodies crystallized from the same well-mixed soup of material.
Pieces of Theia May Still Be Inside Earth
Perhaps the most surprising part of this story is that Theia’s remains may not have fully mixed into Earth after all. Deep in the mantle, straddling Earth’s core like a pair of headphones, sit two continent-sized structures that have puzzled geologists for decades. One lies beneath West Africa, the other beneath the Pacific Ocean. They’re up to 1,000 kilometers tall and several times wider than that, making them the largest structures in Earth’s interior.
These formations, known as large low-shear velocity provinces, behave strangely when seismic waves pass through them. Earthquake waves slow down abruptly inside these zones, which tells geophysicists the material is denser and chemically different from the surrounding mantle rock. One leading explanation is that these blobs are the remains of Theia itself, iron-rich chunks of the impactor that sank to the base of Earth’s mantle after the collision and never fully mixed in.
The scale supports this idea. Together, these two structures contain about six times more mass than the Moon. Only something as large as Theia could have delivered that much foreign material. Isotopic evidence from volcanic hotspots in Iceland and Samoa, where plumes of magma can be traced all the way down to these deep structures, suggests they’ve been sitting there since the time of the Moon-forming impact. The chemical signatures locked in those lavas include radioactive elements that could only have formed during the first 100 million years of Earth’s history.
What the Impact Changed About Earth
The collision didn’t just create the Moon. It fundamentally shaped the planet we live on. The impact is thought to have given Earth its current axial tilt of about 23.5 degrees, which is responsible for our seasons. Before the collision, Earth’s tilt and rotation could have been completely different.
The impact also set Earth spinning much faster than it does today. In the aftermath, a day on Earth may have lasted only about five or six hours. The Moon has been gradually slowing Earth’s rotation through tidal forces ever since, pulling the length of a day from those few hours to the 24 hours we experience now. That tidal braking continues: the Moon’s gravitational pull on Earth’s tides currently lengthens the day by about 2.4 milliseconds per century.
The collision also likely stripped away much of Earth’s original atmosphere and resurfaced the entire planet in molten rock. In a real sense, the Earth that existed before Theia hit it was a different world. The planet we know, with its Moon, its tilt, its seasons, and its particular internal structure, is the product of that ancient collision.