Life › Origin of life › Formation of Earth
Giant impact forms the Moon
c. 4.5 billion years ago· Event· Earth, climate & extinctions
Strong evidenceGood evidence; dates or details still being refined.
A Mars-sized body, nicknamed Theia, probably struck the young Earth; debris from the crash formed the Moon.
See it on the Life map →What happened
About 4.5 billion years ago, a planet-sized body hit the growing Earth at a glancing angle. The blow melted much of both bodies and flung a ring of hot debris into orbit, which gathered into the Moon. The young Moon was far closer than today and Earth spun much faster, with a day of only a few hours. Tides have since slowed Earth's spin and pushed the Moon outward, now about 3.8 centimeters a year.
Why it matters
The Moon steadies Earth's tilt and therefore its seasons, drives the tides, and slowed the day to 24 hours. Some scientists think these helped life begin or later thrive.
The evidence (fossils, genes, rocks)
Apollo moon rocks (1969–1972) have oxygen isotopes almost identical to Earth's; the Moon has a tiny iron core and was once a magma ocean; laser reflectors left by astronauts measure its retreat.
Still debated
If Theia was a separate planet, why are Earth and Moon so chemically alike? One 2023 idea says dense blobs deep in Earth's mantle are Theia's remains.
Connections (2)
▸Formation of Earth was soon followed by Giant impact forms the MoonStrong evidence
The connection
While Earth was still growing, within perhaps tens of millions of years of its start, a Mars-sized body struck it at an angle. The blast melted Earth's surface and threw rock into orbit, which gathered into the Moon. The impact also likely set Earth's fast early spin and may have tilted its axis.
When and how
About 4.5 billion years ago in the inner solar system; proposed by William Hartmann and Donald Davis in 1975 and by Alastair Cameron and William Ward in 1976.
How sure are we?
Apollo moon rocks match Earth's oxygen isotopes almost exactly; the Moon has little iron; computer simulations reproduce the Earth–Moon spin and orbit.
Debates and open questions
The exact timing (estimates run from about 4.51 to 4.35 billion years ago) and Theia's size are debated; some models use several smaller impacts instead.
▸Giant impact forms the Moon steadied Earth's tilt, possibly aiding Origin of lifeDebated
The connection
The Moon's pull keeps Earth's tilt near 23 degrees, damping wild climate swings over millions of years. The early Moon, much closer, also raised strong tides that may have washed shorelines wet and dry, concentrating chemicals. Both are possible aids to life, not proven requirements.
When and how
The Moon formed about 4.5 billion years ago; Jacques Laskar and colleagues modeled its steadying effect on Earth's tilt in 1993; tidal-pool origin ideas followed, for example Richard Lathe's in 2004.
How sure are we?
Computer models show Earth's tilt would wander more without the Moon, and rocks show tides were stronger in the past. The link to life itself is inference.
Debates and open questions
Later studies suggest a moonless Earth's tilt might wander less than first thought; tides matter only if life began on shores rather than at deep vents.