The Giant Impact Hypothesis (1975): How the Moon Formed

William Hartmann and Donald Davis proposed the giant impact hypothesis in 1975: the Moon formed from debris blasted off the young Earth when a body at least the size of a large moon struck it. The idea explained the Moon’s small iron core and became the leading explanation.

How did the Moon form? The question in 1975

By the mid-1970s the Moon had been visited and sampled, yet nobody could say where it came from. Three explanations competed. In capture, Earth seized a Moon that had formed elsewhere. In fission, a fast-spinning Earth flung off a piece of itself. In co-formation, the two grew side by side from the same debris. Each model had serious problems, and none comfortably explained a basic fact: compared with Earth, the Moon has only a small iron core. The puzzle sat near the center of Solar System science.

What the paper found

• Big bodies were common late in planet building. Late planet formation included bodies of satellite size or larger, not just rubble.
• A giant collision was plausible. With such bodies around, a large impact on the early Earth was something to expect, not a freak accident.
• The Moon could be made from the debris. Such an impact would eject material, largely from Earth’s iron-poor outer layers, which could then gather into the Moon.
• The iron puzzle gets an answer. Because the debris came mostly from rock rather than metal, a Moon built from it ends up with a small iron core.

How they did it

This is a theory paper, not an observation. Hartmann and Davis started from how planets grow: through collisions among planetesimals, the solid bodies, from boulders up to small worlds, that merge to build planets. Their key step was to ask how large the biggest of Earth’s neighbors would have been near the end of that process. If several bodies of satellite size or larger were still around, one of them hitting Earth was not a coincidence.

They then followed the consequences. Earth’s outer layers are poor in iron, because most of the planet’s metal sits in its core. An impact that excavated those layers would throw iron-poor rock into orbit, and a Moon assembled from that rock would be iron-poor too. One violent event did the work that three gentler models could not.

Why it mattered

The paper offered a way out of a stalemate. A single collision could explain the Moon’s low iron content without the problems that hobbled capture, fission and co-formation. It also changed the picture of the young Solar System: not a calm nursery, but a place where violent collisions shape planets.

That violent picture spread. It reached the giant planets in the Nice model of migrating giant planets, and it reached Earth’s later history in the Alvarez team’s evidence that an asteroid ended the age of dinosaurs.

What we know now

Half a century on, the giant impact hypothesis is still the favored account of the Moon’s origin. Computer simulations made it concrete: in 2001, Canup and Asphaug modeled a collision with a Mars-sized impactor, a body now often called Theia.

One problem is still open. Earth and Moon share a near-identical isotopic makeup, a kind of chemical fingerprint written in the ratios of different forms of the same elements. Matching that fingerprint in impact simulations is still an active research problem.

Who wrote it

William K. Hartmann and Donald R. Davis were planetary scientists working on how bodies collide and grow. The idea has two independent origins: Alastair Cameron and William Ward proposed a similar giant impact in 1976. Semantic Scholar lists about 715 citations for the 1975 paper (October 2026).

Read the original

Hartmann, W. K. and Davis, D. R. (1975). “Satellite-sized planetesimals and lunar origin.” Icarus 24, 504-515. doi:10.1016/0019-1035(75)90070-6

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