The Moon should be made mostly of the impactor. But it's made mostly of Earth.
Four and a half billion years ago, a violent collision may have forged the Moon from the wreckage of a young Earth — yet despite half a century of study and the rare gift of actual lunar rocks, science cannot fully account for what happened. The Moon's chemical kinship with Earth defies the very models built to explain its birth, leaving planetary scientists caught between elegant theory and stubborn evidence. This is not merely a question about a distant satellite; it is an unresolved chapter in the story of how our own world came to be.
- The leading theory of a catastrophic impact 4.51 billion years ago is widely accepted, yet scientists cannot even agree on the size of the impactor — estimates range from Mercury-sized to nearly half the mass of Earth, a discrepancy that changes everything downstream.
- Apollo rock samples, including the ancient Genesis rock, confirm the Moon was once a global ocean of magma thousands of degrees deep, but this very evidence sharpens the mystery rather than resolving it.
- The Moon's rocks are chemically far too similar to Earth's — every simulation that explains the physics of the collision predicts a Moon built mostly from the impactor's alien material, yet the geology says otherwise.
- Laboratory experiments recreating pressures of 250,000 atmospheres and temperatures exceeding 1700°C are probing how minerals crystallized as the Moon cooled, but the results keep exposing gaps in classical models rather than closing them.
- Researchers now suspect an impactor nearly half Earth's size may better explain the chemical similarity, yet even this revision leaves the geometry of the collision in contradiction with what the rocks reveal.
Fifty years after Apollo 17 departed the lunar surface, the Moon's origin remains one of planetary science's most stubborn puzzles. The rocks brought home have been studied intensely, yet scientists still disagree on the most fundamental details of how Earth's companion world was born.
The prevailing theory holds that roughly 4.51 billion years ago, a body called Theia struck the early Earth with catastrophic force, and the debris coalesced into the Moon. But consensus ends there. Estimates of Theia's size range from roughly Mercury-sized to nearly half the mass of present-day Earth — a difference that reshapes every subsequent calculation about what the collision produced.
At Vrije Universiteit Amsterdam, planetary scientist Wim van Westrenen recreates the Moon's ancient interior in miniature, heating tiny samples to over 1700 degrees Celsius under pressures of 250,000 Earth atmospheres. His goal is to understand how a Moon that began as a glowing globe of magma slowly crystallized into solid rock. The Apollo samples point clearly to that molten origin: the Genesis rock, nearly 4.46 billion years old, is composed almost entirely of plagioclase — a light mineral that floated to the surface of a magma ocean extending 1,700 kilometers deep. The pale highlands we see today are, in essence, the ancient roof of that vast liquid world.
Yet the deeper mystery resists every model. Classical impact theory predicts the Moon should be made predominantly of Theia's material, bearing a chemical signature distinct from Earth's. Instead, lunar rocks are strikingly Earth-like — a fact that no simulation has satisfactorily explained. Larger-impactor models offer a partial answer, but they introduce new geometric contradictions that the evidence refuses to resolve.
Van Westrenen's laboratory results continue to expose patterns of mineral crystallization that current simulations cannot predict. Every model that successfully reproduces the physical architecture of the Earth-Moon system stumbles when confronted with the actual chemistry of the rocks. After half a century, and with the extraordinary advantage of genuine lunar material in hand, the question of what truly happened in those first moments after impact remains, as yet, unanswered.
Fifty years after astronauts left the Moon, we still don't know how it got here. Apollo 17 lifted off from the Moon's northeastern quadrant in 1972, and the rocks it brought home have been studied intensely ever since. Yet planetary scientists remain divided on the most basic question: when did the Moon form, and what exactly created it?
The consensus rests on a dramatic collision. Roughly 4.51 billion years ago, an object called Theia—named by lunar scientists—struck the early Earth with catastrophic force. The impact was so violent it fundamentally reshaped our planet's history. But here's where agreement breaks down: nobody can agree on Theia's size. Some models suggest it was roughly the size of Mercury. Others propose something far larger—an object nearly half the mass of present-day Earth. The difference matters enormously, because it changes everything about what happened next.
Wim van Westrenen, a lunar and planetary scientist at Vrije Universiteit Amsterdam, has spent his career trying to solve this puzzle. In his laboratory, he recreates the Moon's interior conditions by heating tiny samples to more than 1700 degrees Celsius—five times hotter than a conventional oven—while subjecting them to pressures equivalent to 250,000 Earth atmospheres. This allows him to simulate what happened deep inside the Moon as it cooled from its initial state: a glowing ball of magma, thousands of degrees hot, with no solid rock yet formed. The real question, van Westrenen explains, is how much time elapsed between impact and the moment minerals began to crystallize. That timing is extraordinarily difficult to pin down.
The Apollo samples offer crucial clues. The Genesis rock, collected in 1971 by Apollo 15 astronauts, is nearly 4.46 billion years old and composed almost entirely of plagioclase, a white mineral that floats to the top of molten material because it is so light. To produce the vast quantities of plagioclase visible across the lunar surface requires an enormous magma ocean—van Westrenen estimates the entire Moon was molten, with magma extending 1,700 kilometers deep to the center. As the magma cooled, the lighter plagioclase crystals rose to the surface, creating the white-colored regions we see today. In essence, we are looking at the roof of an ancient, colossal body of liquid rock.
Yet here lies the central mystery that has resisted solution: the Moon's chemical composition should be radically different from Earth's, but it isn't. Classical impact models, refined over twenty-five years, predict that most of the Moon should be made from material originating with Theia, the impactor. The rocks should bear Theia's chemical signature, not Earth's. But geologists consistently find the opposite. The Moon is far more Earth-like than any model predicts it should be.
The latest hydrodynamic simulations suggest a larger impactor offers the best explanation for this chemical similarity. If Theia was roughly half Earth's current size, and Earth was still forming at the time of impact, then the collision would have completed Earth's formation while creating the Moon from a mixture of debris. But even this scenario leaves problems unsolved. In the classical giant-impact model, Theia would need to strike Earth at a glancing angle, with half the impactor missing the planet entirely and entering orbit to form the Moon. Yet this geometry should produce a Moon composed primarily of Theia material. The observations contradict the theory.
Van Westrenen's laboratory work has revealed something the old models cannot explain: when a deep magma ocean solidifies, the sequence of mineral formation follows patterns that current simulations fail to predict. The chemical composition problem persists. Every simulation that successfully recreates the physical properties of the Earth-Moon system—the masses, the orbital mechanics, the overall structure—falls short when it comes to matching what we actually find in the rocks.
The formation of the Moon remains unresolved, van Westrenen acknowledges, despite half a century of study and the unprecedented advantage of having actual lunar material in hand. The Moon's origins are linked directly to Earth's own history; understanding one requires understanding the other. As researchers continue to heat and compress tiny samples in laboratories, trying to reverse-engineer the conditions of 4.51 billion years ago, the fundamental question persists: what actually happened in those first moments after impact, and why does the evidence refuse to fit the theories we have built?
Citações Notáveis
The Moon rocks are far more Earth-like than they should be— Wim van Westrenen, Vrije Universiteit Amsterdam
The whole Moon was actually molten; 1700 kilometers of magma all the way down to the center— Wim van Westrenen
A Conversa do Hearth Outra perspectiva sobre a história
If we've had Moon rocks for fifty years, why haven't we solved this?
Because the rocks tell us something the physics models say shouldn't be possible. The Moon should be made mostly of the impactor, but it's made mostly of Earth. That's the puzzle.
So the current theories are wrong?
Not entirely wrong. They explain the mechanics—how a collision could create an orbiting body. But they can't explain the chemistry. There's a gap between what the equations predict and what the rocks show.
What does van Westrenen's lab actually do that's different?
He recreates the Moon's interior in miniature—extreme heat and pressure—to watch how minerals form as magma cools. It's like running a slow-motion film of the Moon's first million years, compressed into hours.
And that reveals what?
That the sequence of crystallization doesn't match what the old models predict. The minerals form in ways that suggest the impact and cooling process were more complex than we thought.
Does he have a new theory about Theia's size?
The evidence points toward a larger impactor—maybe half Earth's mass—but even that doesn't fully resolve the chemical similarity problem. It's a better fit, but not a complete answer.
What happens next? Do we need a completely new model?
Possibly. The lab work keeps challenging the classical assumptions. Eventually, either the models will evolve or we'll discover something about the impact process we haven't considered yet.