The Origin of the Moon

Core Concepts

The clues: what a theory must explain

A successful theory must satisfy four observations at once. (1) Size: the Moon is huge relative to its planet — about 1/81 of Earth's mass. (2) Density and iron: the Moon averages ~3.3 g/cm³ versus Earth's 5.5 — little or no iron core; a body from the same material as Earth should be iron-rich like Earth. (3) Composition: Apollo samples match Earth's oxygen isotope fingerprint almost exactly, yet are strongly depleted in volatiles — easily vaporized elements such as water, potassium, and zinc. (4) Angular momentum: the Earth–Moon system spins fast, as if given an early "twist."

The older suspects — and why each failed

• Fission theory: a fast-spinning young Earth threw off material. Problem: Earth would need impossible spin speeds, and the split material would match Earth's iron-rich composition too exactly.
• Capture theory: the Moon formed elsewhere and was captured. Problem: capture of a body this large is dynamically very difficult, and the Moon's isotopes match Earth's too closely.
• Co-accretion ("double planet"): the Moon formed beside Earth from the same cloud. Problem: a sibling built of the same material should have a similar iron-rich core — the Moon has almost none.

Each suspect explains one clue and contradicts another.

The giant impact hypothesis

About 4.5 billion years ago, a Mars-sized protoplanet nicknamed Theia struck the young Earth. The collision was so violent that much of both bodies vaporized. The dense iron cores merged and sank to Earth's center, while a disk of hot rock vapor and debris — drawn mostly from the mantles of both bodies — spun around Earth and quickly coalesced into the Moon.

This single story explains all the clues: the Moon is big because the collision was big; iron-poor because the cores had already plunged into Earth; Earth-like because much of its material came from Earth's mantle; fast-spinning because the impact's twist survives; and volatile-poor because the searing debris disk boiled them away.

Evidence beyond the story

Isotope fingerprints: oxygen (and titanium) isotope ratios in lunar samples match Earth's mantle far more closely than any other body — a family link. The anorthosite highlands: the bright highlands are made of anorthosite, a rock that floats when molten material crystallizes — direct evidence the young Moon had a magma ocean. Volatile depletion matches a hot, violent birth, and simulations of giant impacts reproduce the Moon's mass, composition, and orbit — while the Moon's tiny (or absent) core fits the impactor's core being swallowed by Earth.

Model limits: the impactor's exact size and the Earth/Theia mix remain debated; high-precision isotope measurements show the Moon matches Earth even more closely than simple models predict, so refinements — multiple impacts, a larger Theia, a vaporized "synestia" — are active research. Treat the giant impact as the strongly favored hypothesis, not a finished story.

After the impact: magma ocean, then tides

The newborn Moon was molten. As the magma ocean cooled, anorthosite floated to form the highland crust; later, about 3–4 billion years ago, radioactive heating drove basaltic floods that filled the low basins — the dark maria. Tides then took over: the Moon's gravity raises tidal bulges on Earth, and friction transfers angular momentum, pushing the Moon away (laser reflectors show ~3.8 cm/year) and lengthening Earth's day. Early on, the Moon was much closer and Earth's day lasted only hours.

Common Confusions