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Birth of the Moon
c. 4.5 billion years ago, probably in a giant impact· Event· Universe & Earth
Mostly agreedGood evidence; some details, dates or numbers uncertain.
The Moon probably formed when a Mars-sized body slammed into the young Earth about 4.5 billion years ago.
See it on the History map →What happened
In the leading theory, a planet-sized object nicknamed Theia struck Earth a glancing blow. The impact melted much of Earth and hurled rock into orbit, which quickly gathered into the Moon. The early Moon was far closer, raising much stronger tides. It has been drifting away ever since, now by about 3.8 cm (1.5 inches) a year. The Curious Science map covers the details.
Why it matters
The Moon steadies the tilt of Earth's axis, which keeps climates more stable over long periods, and it drives the tides.
How we know
Apollo Moon rocks (1969–1972) closely match Earth's chemistry and show the Moon was once molten; computer simulations of the impact reproduce the Earth–Moon system.
Still debated
Why Earth and Moon are so chemically alike still puzzles scientists, and some favor other impact scenarios; Theia's size and the exact timing are uncertain.
Connections (3)
▸Birth of the Sun and Earth was soon followed by Birth of the MoonMostly agreed
The connection
In the crowded early solar system, Earth was still growing through collisions. Within roughly 100 million years of the solar system's start, a body about the size of Mars, called Theia, struck it. Debris thrown into orbit around Earth gathered into the Moon.
When and how
In the inner solar system, about 4.5 billion years ago; the giant-impact idea was proposed by William Hartmann and Donald Davis in 1975.
How sure are we?
Apollo rocks show the Moon shares Earth's oxygen 'fingerprint' and has little iron, as an impact that blasted off Earth's outer layers would produce.
Debates and open questions
The exact timing (estimates run from about 4.51 to 4.35 billion years ago) and why Theia left so little chemical trace are debated.
▸Birth of the Moon created tides that may have aided First lifeDebated
The connection
The young Moon was much closer, so tides were far stronger. Some scientists propose that tides repeatedly flooded and dried coastal pools, concentrating chemicals in cycles that could have helped build life's first molecules. The Moon also steadied Earth's tilt, stabilizing climate later on.
When and how
On early Earth's shorelines, roughly 4–3.5 billion years ago; the fast-tide idea was argued by biologist Richard Lathe in 2004.
How sure are we?
This is a hypothesis. Lab experiments show wet-dry cycles help link molecules into chains, but no direct record of tides' role exists.
Debates and open questions
Many researchers favor deep-sea hot vents, where tides matter little, so the Moon may not have been needed for life to begin.
▸Apollo 11 Moon landing returned rocks that hold clues to the Birth of the MoonMostly agreed
The connection
Apollo astronauts brought back 382 kg of Moon rock. It is dry, low in iron, and its oxygen isotopes nearly match Earth's. These clues led scientists in 1975 to propose the giant-impact theory: a Mars-sized body hit the young Earth, and the debris formed the Moon. The Curious Science map covers it in depth.
When and how
Six Apollo landings, 1969–1972; samples studied in Houston and labs worldwide; William Hartmann and Donald Davis published the impact idea in 1975.
How sure are we?
Lab analysis of the samples and computer simulations of the impact support the theory, now the leading explanation.
Debates and open questions
The rocks match Earth too closely: simple models predict more material from the impactor. Variants, such as a hotter or faster collision, try to explain it.