The giant impact theory proposes that Earth’s Moon formed when a Mars-sized object slammed into the early Earth roughly 4.5 billion years ago, blasting debris into orbit that eventually coalesced into the Moon. It’s the leading scientific explanation for how the Moon came to exist, and it solves several puzzles about the Moon’s composition that older theories couldn’t explain.
The Collision With Theia
The impactor has been named Theia, after a Greek goddess who was the mother of the Moon goddess Selene. Theia was roughly the size of Mars, making it about half the diameter of the early Earth. Simulations suggest it struck Earth at a velocity near or just above escape velocity, at an oblique angle rather than head-on. This glancing blow was energetic enough to tear massive amounts of material from both bodies and fling it into orbit around the battered Earth.
What happened next depends on which version of the theory you follow. In the traditional model, the ejected material formed a ring of debris around Earth, similar to Saturn’s rings but made of molten and vaporized rock. That debris disk then clumped together under its own gravity, likely spending only about 10,000 to 100,000 years close to Earth before the growing Moon migrated outward. Some newer, high-resolution simulations suggest the Moon could have come together in a matter of hours, forming almost immediately from a single massive clump of material rather than slowly accreting from a disk.
Why Older Theories Fell Short
Before the giant impact hypothesis gained traction in the 1970s and 1980s, scientists considered three other explanations. Each had a fatal flaw.
- Co-formation: The Moon formed alongside Earth from the same cloud of dust and gas. This would predict similar densities for both bodies, but the Moon is far less dense than Earth, with a tiny iron core making up only about 1% of its mass. Earth’s iron core, by contrast, accounts for roughly a third of its mass.
- Capture: The Moon formed elsewhere in the solar system and was later captured by Earth’s gravity. If that were true, the Moon should be chemically distinct from Earth, made of different raw materials. It isn’t. The two bodies also share nearly identical oxygen isotope ratios, which would be a remarkable coincidence if they formed in different parts of the solar system. The capture scenario also requires far more angular momentum (rotational and orbital energy) than the Earth-Moon system actually has.
- Fission: A young, molten Earth spun so fast that a chunk of it flew off. This neatly explains the chemical similarity, but it demands an impossibly fast initial spin. The angular momentum needed is much greater than what exists in the system today, with no clear mechanism to shed the excess.
The giant impact theory threads the needle. It explains why the Moon has so little iron (Theia’s iron core, along with much of Earth’s, sank to the center of the merged body rather than ending up in orbit). It accounts for the right amount of angular momentum. And it can, with some refinement, explain the striking chemical match between the two bodies.
The Isotopic Fingerprint
One of the strongest pieces of evidence for the giant impact comes from oxygen isotopes. Every body in the solar system has a slightly different oxygen isotope signature, almost like a chemical fingerprint. Mars has one ratio, meteorites from the asteroid belt have another, and so on. Earth and the Moon, however, are essentially identical. High-precision measurements published in the Proceedings of the National Academy of Sciences found the difference between Earth and Moon oxygen isotopes to be just 0.2 parts per million, which is within the margin of error. For practical purposes, they are the same.
This pattern holds across other elements too. Silicon, sulfur, titanium, and chromium all show distinct isotopic signatures across different planets and asteroids, yet Earth and the Moon match on every count. That level of chemical kinship is hard to produce unless the two bodies share a violent, thoroughly mixed origin.
The Synestia Model
The isotopic match actually posed a problem for early versions of the giant impact theory. If a separate body (Theia) hit Earth and the debris that formed the Moon came mostly from Theia, you’d expect the Moon to carry Theia’s isotopic fingerprint, not Earth’s. For the chemistry to match so precisely, the materials from both bodies needed to be thoroughly blended.
One solution is the synestia model. In this scenario, the collision was even more energetic than traditionally assumed. Rather than producing a neat debris disk, the impact vaporized so much rock that Earth temporarily ballooned into a massive, donut-shaped structure of superheated vapor and molten material extending tens of thousands of kilometers into space. Researchers call this structure a synestia. The key feature is that the proto-Moon orbited inside this cloud of vaporized Earth material long enough for the two to chemically equilibrate. Turbulent mixing, driven by the sheer violence of the collision, blended everything together. The Moon that condensed out of this vapor naturally ended up with the same isotopic composition as Earth’s mantle.
Theia’s Remains Inside Earth
If Theia was absorbed into Earth during the collision, there should be traces of it somewhere. Scientists may have found them. Deep near Earth’s core, seismic waves slow down as they pass through two continent-sized structures, one beneath Africa and one beneath the Pacific Ocean. These formations, known as large low-velocity provinces, are each roughly twice the size of the Moon and contain unusually high levels of iron, making them denser than the surrounding mantle.
A study led by Caltech researchers used simulations to model what would happen to Theia’s iron-rich material after the collision. The results showed that much of the impact energy stayed in the upper half of the mantle, leaving the deeper mantle cooler than earlier models predicted. Because the lower mantle didn’t fully melt, dense blobs of iron-rich material from Theia could sink largely intact to the base of the mantle, settling near the core like wax in a lava lamp that’s been turned off. The simulations confirmed that the physics of the collision could produce both these deep mantle structures and the Moon from a single event. If this is correct, pieces of the world that created our Moon have been sitting beneath our feet for 4.5 billion years.
What the Moon’s Small Core Reveals
The Moon’s interior provides its own line of evidence. Geophysical modeling indicates the Moon has a tiny iron core with a radius of about 340 kilometers, accounting for only around 1% of the Moon’s total mass. Earth’s core, by comparison, makes up about 32% of its mass. This dramatic difference makes sense under the giant impact theory: during the collision, the dense iron from both Theia’s and Earth’s cores would have been pulled inward by gravity, merging at Earth’s center. The lighter, rocky material blasted into orbit carried very little iron with it, producing a Moon that is almost entirely rock.
This lopsided iron distribution is difficult to explain with the co-formation or capture theories but falls naturally out of the physics of a giant impact. The collision essentially sorted the materials by density, keeping the heavy iron on Earth and sending the lighter silicate rock into orbit.