Formation of the Moon: The Giant Impact With Theia

The formation of the Moon is best explained by the giant impact hypothesis: about 4.51 to 4.45 billion years ago, a planet-sized body often called Theia crashed into the young Earth, and debris from the collision gathered in orbit to form the Moon. The idea was proposed in the 1970s and became the leading theory after scientists studied the rocks brought back by the Apollo missions. It explains much about the Moon, but some puzzles, above all the close chemical match between Earth and Moon, remain unsolved.

Background: an unusual moon

Earth’s Moon is large compared with its planet. Its diameter is about 27 percent of Earth’s, a much bigger ratio than for the moons of any other planet; only the dwarf planet Pluto and its moon Charon have a larger ratio. Its average density, about 3.3 grams per cubic centimetre, is much lower than Earth’s 5.5, because it has only a small iron core.

Before the Space Age, three main theories competed. The fission theory, developed in the late 19th century by the astronomer George Darwin, son of Charles Darwin, proposed that a fast-spinning Earth flung off the Moon. The capture theory suggested that the Moon formed elsewhere and was caught by Earth’s gravity. The co-accretion theory held that Earth and Moon grew side by side from the same cloud of debris. Each theory struggled to explain all the facts.

The Apollo evidence

Between 1969 and 1972, six Apollo crews landed on the Moon, beginning with the Apollo 11 Moon landing, and returned about 382 kilograms of samples. Analysis of these rocks transformed lunar science.

The samples showed that the Moon is very dry and depleted in volatile elements, such as potassium, sodium and zinc, which evaporate at relatively low temperatures. That pointed to a hot, violent origin.

The bright lunar highlands turned out to be made largely of anorthosite, a rock rich in the light mineral plagioclase feldspar. Scientists concluded that the young Moon had been covered by a global ocean of magma hundreds of kilometres deep, and that plagioclase crystals floated to its surface to form the first crust. A famous example is the “Genesis Rock”, sample 15415, collected by the Apollo 15 astronauts David Scott and James Irwin in 1971. Melting an entire Moon so soon after its birth required an enormous energy source.

The giant impact hypothesis

In 1975 the American planetary scientists William K. Hartmann and Donald R. Davis published a paper in the journal Icarus proposing that a large body struck the early Earth and that the Moon formed from the debris. Independently, Alastair Cameron and William Ward at Harvard argued in 1976 that a Mars-sized impact could explain the high angular momentum, the total amount of spin and orbital motion, of the Earth-Moon system.

The idea attracted little attention at first. That changed at a conference on the origin of the Moon held in Kona, Hawaii, in October 1984, where the giant impact emerged as the favoured explanation. The proceedings, published in 1986, became a standard reference.

What happened in the impact

In the classic version, modelled in detail by Robin Canup and Erik Asphaug in 2001, Theia was roughly the size of Mars, about one-tenth of Earth’s mass. It struck the proto-Earth a glancing, oblique blow rather than head-on, near the end of Earth’s growth.

The collision melted much of the Earth and blasted a disc of molten and vaporised rock into orbit. Theia’s iron core largely merged with Earth’s core, which explains why the Moon has so little iron. The material in the disc then clumped together, perhaps within as little as a few years to a few centuries, to form the Moon.

The newborn Moon orbited much closer to Earth than today, perhaps at only a few tens of thousands of kilometres compared with about 384,400 kilometres now. Tidal forces have pushed it steadily outwards ever since; laser measurements show it is still receding by about 3.8 centimetres a year. Earth’s day was also far shorter then, probably only a few hours long.

The name Theia comes from Greek mythology, where Theia was the Titaness who was mother of Selene, goddess of the Moon. It came into common use among scientists around 2000.

When did it happen?

Dating the impact is difficult. Estimates fall into two broad groups:

• Early dates, about 4.51 billion years ago, come from some studies of lunar zircon crystals and from models of planet formation.
• Later dates, about 4.45 to 4.35 billion years ago, come from the ages of some lunar crustal rocks and from certain isotope systems.

Part of the difference may reflect how long the lunar magma ocean took to solidify rather than the date of the impact itself. Most scientists place the event roughly 50 to 150 million years after the birth of the Solar System, during the final stage of the formation of the Earth.

Open questions

The isotope puzzle

The greatest challenge to the classic model is chemistry. Every body in the Solar System has a characteristic “fingerprint” of isotopes, slightly different versions of the same element. Mars and many meteorites differ measurably from Earth in their oxygen isotopes. Yet lunar rocks are almost identical to Earth rocks in oxygen, titanium, tungsten and other isotopes.

Simulations of the classic impact suggest that the Moon should be made mostly of Theia’s material, so it ought to look different from Earth. Some studies have reported a very small difference in oxygen isotopes, but others have found none.

Alternative versions

Several modified scenarios try to solve the puzzle:

• A faster-spinning Earth. In 2012 Matija Ćuk and Sarah Stewart proposed that a smaller impactor struck a rapidly rotating Earth, throwing out mostly Earth material.
• Equal-sized bodies. Robin Canup suggested in 2012 that two bodies of similar size collided and thoroughly mixed.
• A synestia. Simon Lock and Sarah Stewart have proposed that a very energetic impact turned the Earth into a spinning, doughnut-shaped cloud of vaporised rock, from which the Moon condensed.
• Many impacts. Some researchers argue that a series of smaller collisions built up the Moon gradually.
• A similar Theia. Theia may simply have formed in the same part of the Solar System as Earth from similar material.

Where Theia ended up is also debated. A 2023 study suggested that two huge regions of unusual rock deep in Earth’s mantle, beneath Africa and the Pacific, could be remnants of Theia, but this remains a hypothesis.

Effects on the early Earth

For Earth, the impact was a planet-wide catastrophe. Models suggest it melted much or all of the mantle, creating a deep magma ocean, and may have stripped away part of any early atmosphere. For a time, the surface was probably wrapped in a hot atmosphere of rock vapour that slowly cooled and condensed.

The collision also delivered Theia’s metal and rock to the growing planet, adding to Earth’s final mass. Some scientists think the impact helps explain why Earth’s mantle contains certain elements in the proportions it does, although this is model-dependent.

Once the debris had settled, the Moon began to influence Earth. Its strong early tides, raised when it was much closer, would have been far larger than today’s. Over billions of years, the tidal exchange of energy slowed Earth’s rotation to the 24-hour day we know.

Myths and facts

• Myth: the Moon was torn out of the Pacific Ocean basin. This was a popular version of the old fission theory. The Pacific basin is far younger than the Moon, formed by plate tectonics within the last 200 million years or so.
• Myth: the giant impact is proven fact. It is the leading scientific explanation and fits most evidence, but its details, including the size of Theia and the timing, are not settled.
• Fact: Moon rocks are older than almost all Earth rocks. Because the Moon has no plate tectonics, its highland crust preserves rocks more than 4 billion years old, which are rare on Earth.

By the numbers

Timeline

• c. 4,510–4,450 million years ago: Theia collides with the proto-Earth; the Moon forms from the debris disc.
• c. 4,450–4,350 million years ago: The lunar magma ocean crystallises, forming the anorthosite highlands.
• AD 1879 onwards: George Darwin develops the fission theory of the Moon’s origin.
• AD 1969–1972: Six Apollo missions return about 382 kg of lunar samples.
• AD 1975: Hartmann and Davis publish the giant impact hypothesis.
• AD 1976: Cameron and Ward independently propose a Mars-sized impact.
• AD 1984, October: The Kona conference on the origin of the Moon favours the giant impact.
• AD 2001: Canup and Asphaug publish an influential computer model of the impact.
• AD 2012: New models are proposed to explain the isotopic similarity of Earth and Moon.

Why it matters

The giant impact shaped the Earth as well as creating the Moon. It may have set Earth’s spin and tilt, and the Moon’s gravity now helps stabilise that tilt and raises the ocean tides. The Moon’s ancient, unaltered surface also preserves a record of the early Solar System, including the impacts of the Late Heavy Bombardment, that has been erased on Earth.