The giant impact that made the Moon
What happened
Within the first hundred million years after Earth formed, a world about the size of Mars struck the young planet a glancing blow. NASA calls this the giant-impact hypothesis and names the impactor Theia. The collision melted much of the Earth and flung a huge quantity of vaporized rock into orbit, where it gathered into the Moon. The theory accounts for features that are otherwise hard to explain: the Moon's small iron core, signs that it was once molten to great depth, and the near-identical oxygen-isotope makeup of lunar and terrestrial rock, as if both were cut from the same material.
Why it matters
The impact set Earth spinning fast and left it with an unusually large Moon. Over billions of years that Moon has steadied Earth's tilt and raised the tides, both of which shaped the environments life later evolved in.
How we know
The strongest evidence is the Apollo Moon rocks, whose oxygen isotopes match Earth's and whose chemistry shows a body once molten and short on iron. Simulations of a Mars-sized impact reproduce the angular momentum of the Earth-Moon system, though matching those near-identical isotopes exactly is still an open problem, known as the lunar isotopic crisis.
Sources
- NASA (Astrobiology Program). What was the Earth like right after it formed? (2024) · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- Canup & Asphaug. Origin of the Moon in a giant impact near the end of the Earth's formation (2001) · Peer-reviewed (author-declared)doi.org · Cited as a "journal" source (no stronger domain match).
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