1 · The Earth before us science and technology

Formation of the Moon

about 4.5 billion years ago The Earth and the Moon

In short

According to the most widely accepted idea, a body almost as big as Mars crashed into the young Earth. A great deal of material was flung into orbit and came together to form the Moon. Moon rocks brought back by astronauts have helped scientists compare what the two worlds are made of.

Why it matters The Moon affects the tides and has kept the Earth company for almost all of its history.

Source: NASA, “Moon Formation”

See it on the line Era quiz Read the full card Printable worksheet

Illustration: Formation of the Moon (The Earth and the Moon) — Illustration created with AI for Storiascopio

Study card about 5 minutes of reading

The story

The Moon: where does it come from?

When you look at it, the Moon always seems the same. Yet for a long time scientists wondered where it came from, and there were several possible answers. Perhaps it had split off from the Earth, still soft, because the Earth was spinning too fast. Perhaps it was a body passing by that was captured by gravity. Or perhaps it had formed next to the Earth, from the same material. Each of these ideas had a flaw, and none explained everything.

The Apollo missions changed everything. Between 1969 and 1972 American astronauts brought back to Earth about 382 kilograms of Moon rocks and dust. Analysing them showed that the Moon is made of materials very like those of the Earth's mantle, with identical amounts of certain kinds of oxygen, but it has very little iron and almost no water or other substances that evaporate easily. It was as if it were made of Earth material that had been baked for a long time and was left without its lighter parts.

The giant impact

The hypothesis that almost all scholars now consider the best was put forward in the mid-1970s, independently, by two groups of American astronomers: William Hartmann and Donald Davis on one side, Alastair Cameron and William Ward on the other. It is called the “giant-impact hypothesis”. It says that when the Earth was still very young, a body about the size of Mars, named Theia (in Greek myth the mother of the Moon), hit it with a glancing blow at high speed.

In the violent collision, a large part of Theia and part of the Earth's mantle were melted and hurled into space. The material, partly vaporised, formed a disc around the Earth, and in a short time, perhaps within a few years or a few centuries, the pieces gathered into a single body, the Moon. Theia's heavier iron core, instead, was mostly absorbed by the Earth. This explains why the Moon has so little iron.

Right after the impact the Moon was only a few tens of thousands of kilometres from the Earth, much closer than today, and the young Earth spun in a few hours: a day may have lasted only five or six hours. Since then the Moon has moved away. We know this precisely because the Apollo astronauts left special mirrors on the lunar surface. By pointing a laser from Earth and measuring the time it takes to come back, we have seen that the Moon is moving away by about 3.8 centimetres a year. That is roughly the speed at which fingernails grow.

The questions still open

The hypothesis is not a certainty. For many years it had a problem: if the Moon is made largely of Theia's material, why is it so like the Earth? The material of a body born in another part of the Solar System should be different. Some scholars think Theia was born close to the Earth and so was similar; others suggest more violent impacts, with more mixing. In 2023 a study suggested that two enormous dense zones in the Earth's mantle, under Africa and under the Pacific Ocean, are remains of Theia, but the matter is debated.

The date is approximate too. The oldest Moon rocks are about 4.4–4.5 billion years old, and estimates for the impact give something between 4.43 and 4.51 billion years ago. We do not know the exact moment, but we know that it happened very early, a few tens of millions of years after the Earth was born.

Why it happened

In the early days of the Solar System there were a great many bodies orbiting the Sun, of all sizes, and close encounters were frequent. The rocky planets grew in just this way: through collisions, some of them very big. That is why scientists consider it normal that a body as big as Mars hit the young Earth.

Computer simulations have shown that a glancing impact, at the right speed and angle, can throw enough material into orbit to form the Moon. This is indirect evidence, but it fits with the rocks brought back by the astronauts, with the Moon's composition and with the fact that it has a very small iron core.

Not all the details fit yet, and so every new discovery, a new Moon rock or a new simulation, may change the story.

What changed

The Moon has left a deep mark on the Earth. Its gravity lifts the waters of the seas twice a day and produces the tides, which have shaped the coasts. According to some scholars the pools left by the tides may have helped life to appear and evolve near the shores, but that is a hypothesis.

The Moon also helps to keep the tilt of the Earth's axis steady, so that it varies only a little, between about 22 and 24.5 degrees. This stability, studied by the French astronomer Jacques Laskar in 1993, keeps the seasons regular. Without a large Moon, the axis might wobble far more and the climate might change violently.

For human beings the Moon has been a calendar, a guide for sailors and a source of myths and poetry. In 1969 it became the first world, besides the Earth, on which human beings landed. Today new missions are preparing to go back, and Moon rocks, which preserve the craters of the impacts of the first billions of years, remain an archive of the history of the Solar System, more intact than the Earth's.

Key people and places

People

William Hartmann and Donald Davis: American astronomers who in 1975 suggested that the Moon formed after a collision between the Earth and a large body.
Alastair Cameron and William Ward: American astronomers who in 1976 developed the same idea with the first calculations.
The Apollo astronauts: Twelve astronauts walked on the Moon between 1969 and 1972 and brought home 382 kilograms of samples.
Jacques Laskar: French astronomer. In 1993 he showed that the Moon stabilises the tilt of the Earth's axis.

Places

Sea of Tranquility, Moon: The large dark plain where Apollo 11 landed in July 1969, the first mission with people on the Moon.
South Pole–Aitken basin, Moon: One of the largest and oldest craters in the Solar System, about 2,500 kilometres across, on the far side of the Moon.
Orbit around the Earth: Right after the impact the Moon was a few tens of thousands of kilometres away; today it is about 384,000.

Good to know

The Moon has a diameter of about 3,475 kilometres, more than a quarter of the Earth's: among the rocky planets none has a moon so large in proportion.
From the Earth we always see the same face of the Moon, because it takes as long to turn once on its axis as to go once around the Earth.
The Apollo astronauts left laser reflectors on the Moon, and some observatories still use them today to measure the distance to the Moon to within a few millimetres.
Moon dust is sharp and sticky: it clung to the spacesuits and caused problems for the astronauts.
One day, a very long time from now, the Moon will be too far away to make total eclipses of the Sun: they are possible today because the Sun is 400 times bigger than the Moon but also about 400 times further away.