The Giant Impact Hypothesis: The Best Explanation We Have for Where the Moon Came From

The Moon is unusually large for a planet our size. Mars has two moons, both captured asteroids, each small enough to look like a lumpy potato from close range. Earth's Moon is a full 27 percent of Earth's diameter, massive enough to stabilise our axial tilt and drive the tides that shaped coastal ecosystems for billions of years. Something extraordinary must have made it.

The leading explanation is this: roughly 4.5 billion years ago, in the early solar system when planetary collisions were routine, a body about the size of Mars struck the proto-Earth at an oblique angle. Planetary scientists call this impactor Theia, after the Titan in Greek mythology who was the mother of Selene, goddess of the Moon. The collision was not a head-on smash. It was a glancing blow, which matters enormously for what came next.

The impact released energy on a scale that staggers any intuition. Both Theia and the outer layers of proto-Earth were vaporised and flung into orbit as a disc of molten rock and gas. Over a timescale estimated at decades to centuries, that disc coalesced under its own gravity into the Moon. The inner Earth survived, mostly intact, which is why we still have a planet at all.

Computer simulations of this process, run repeatedly since the hypothesis was formalised in the 1970s and refined extensively since, consistently produce a Moon-sized object in Earth orbit. No other proposed origin mechanism does this reliably. A Moon that formed elsewhere and was captured by Earth's gravity would require an implausibly precise set of conditions. A Moon that condensed alongside Earth from the same cloud of material cannot explain why the Moon has almost no iron core while Earth's is enormous.

The Apollo missions returned 382 kilograms of lunar rock between 1969 and 1972. Those samples have been analysed continuously ever since, and they carry a fingerprint. The ratio of oxygen isotopes in lunar rock is nearly identical to Earth's, and distinctly different from Mars rocks or most meteorites. This suggests the Moon formed from material that was, at some point, part of the same body as Earth, or mixed thoroughly with it during the impact.

The near-identical isotope ratios were once a problem for the hypothesis, because early models predicted that most of the disc material came from Theia, not Earth, and Theia should have had a different isotopic signature. More recent simulations, including work published in the journal Nature in 2012 by researchers at the Harvard-Smithsonian Center for Astrophysics, proposed a higher-energy impact that mixed the two bodies far more completely. The isotope match, once a puzzle, now supports the hypothesis rather than undermining it.

No hypothesis in planetary science is finished, and this one has open edges. The Moon is drier than almost any rock on Earth, which fits a violent high-temperature origin. But water ice has been confirmed in permanently shadowed craters near the lunar poles, detected by India's Chandrayaan-1 mission in 2008 and confirmed by subsequent observations. Where that water came from, whether it arrived later via comets and asteroids, or survived the impact in some form, remains an active research question.

The precise nature of Theia is also unknown. Whether it was a rocky body similar to Mars, or something with a different composition, affects the simulations in ways that researchers are still working through. Lunar samples returned by China's Chang'e-5 mission in 2020 have added new data points, and the samples that Artemis missions aim to return from the lunar south pole may resolve some of the remaining questions about water and composition.

Science does not require a hypothesis to be perfect. It requires it to be better than the alternatives, and testable. The Giant Impact Hypothesis has survived five decades of scrutiny, lunar sample analysis, isotope geochemistry, and increasingly sophisticated computational modelling. Every competing explanation, co-formation, capture, fission from a rapidly spinning Earth, fails on at least one of the constraints the rock record sets.

The Moon's size, its iron-poor composition, its isotopic kinship with Earth, and its current orbital dynamics all point back to the same event. A Mars-sized body hit a young Earth, and the sky caught fire for a geological instant. What cooled and gathered in the weeks and centuries after that is what you see rising over any horizon on a clear night.