How Was the Moon Formed? The Giant Impact and the Evidence for It
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The Moon is too big. No other rocky planet has a satellite anything like a quarter its own diameter, and for a century the three obvious ways of getting one, that it was captured passing by, that it spun off a fast-turning Earth, or that it formed alongside the Earth from the same cloud, each failed some test. The answer that has held since the 1980s is more violent: the Moon is the wreckage of a collision, four and a half billion years ago, between the newly formed Earth and another planet the size of Mars.
Three ideas that did not work
Capture, in which a wandering body was caught by Earth's gravity, needs a way of slowing it down, which gravity alone does not provide, and a captured moon would be expected to have a different composition from Earth. Fission, proposed by Charles Darwin's son George in 1879, has a molten early Earth spinning so fast that a lump of it flew off, leaving the Pacific basin as the scar; it requires the Earth to have spun once every two and a half hours, and there is no way to get rid of that spin afterwards. Co-formation, the two bodies condensing together from the same material, predicts a Moon with an iron core like the Earth's, and the Moon has almost none.
The three ideas were still competing in July 1969 when Apollo 11 brought back the first rocks, and the rocks were the first real evidence. Over six landings the astronauts collected 382 kilograms of them, and what the samples showed ruled all three out.
What the rocks said
The lunar samples told four things at once:
- •The Moon is dry: it had almost no water and almost none of the volatile elements, such as sodium and potassium, that boil off at moderate temperatures, as if the material had been strongly heated
- •The Moon is iron-poor: its density and its tiny core show that it is made mostly of the kind of rock found in the Earth's mantle, not of a whole planet's worth of material
- •The Moon was once molten to great depth: the pale highland crust is a rock that forms by floating on a global ocean of magma
- •The Moon is made of Earth: the ratios of oxygen isotopes in lunar rock match Earth's exactly, while every meteorite from elsewhere in the solar system differs
The giant impact
In 1975 two papers proposed the same idea independently: that late in the Earth's formation, when it was nearly full size, a body about the mass of Mars struck it a glancing blow. The impactor's iron core sank into the Earth and merged with its own, which explains why the Moon has so little iron. The rocky mantles of both bodies were partly vaporised and thrown into orbit as a disc of hot debris, which explains the missing volatiles and the magma ocean, and the disc coalesced into the Moon within a few years or decades. Computer simulations from the 1980s onward showed that the collision could produce a Moon of the right mass in the right orbit, and the hypothetical impactor was named Theia, after the Greek goddess who was mother of the Moon.
The impact also explains why the Earth spins as fast as it does and why the Moon's orbit is tilted to the Earth's equator, and it gives a date. Lunar rocks and the decay of radioactive elements in meteorites put the event at about 4.5 billion years ago, some 30 to 100 million years after the solar system began to form, and the Moon has been drifting away from the Earth ever since at, currently, 3.8 centimetres a year.
What does not yet fit
The awkward part is the isotopes. In the simplest simulations the Moon is made mostly of Theia, not Earth, and Theia, having formed elsewhere in the solar system, should have had different isotope ratios, as every other body does. Yet the Moon's oxygen, titanium and tungsten match Earth's to within measurement error. Several solutions are being tested: that Theia happened to form near enough to Earth to share its composition; that the impact was so energetic that both bodies vaporised and mixed thoroughly before the Moon condensed; that the young Earth was spinning very fast, so that a smaller impactor could throw off mostly Earth material; or that there were two impacts rather than one. Seismic images of two continent-sized blobs of dense rock deep in the Earth's mantle have been proposed, since 2023, as the buried remains of Theia itself.
Why it matters
The Moon is why the Earth is habitable in the way it is. Its pull raises the tides, which may have helped life move from sea to land, and its mass steadies the tilt of the Earth's axis, which would otherwise wander chaotically over millions of years and swing the climate with it, as the axis of Mars does. The giant impact also stripped the early Earth of its first atmosphere and reset the planet's clock, and the Moon's cratered surface, unweathered since, is the best record of the bombardment the young Earth went through. The samples that decided the argument are still being studied; a vial sealed on the Moon by Apollo 17 in 1972 was opened for the first time in 2022, and the rocks China returned from the far side in 2024 are the first from there.
The takeaway
The Moon formed about 4.5 billion years ago when a Mars-sized body struck the young Earth, merging its iron core with Earth's and throwing a disc of vaporised mantle rock into orbit that gathered into the Moon. That explains why the Moon is dry, iron-poor, once molten and chemically a twin of the Earth's mantle; why it is so close a twin is the part still being worked out.