The Moon's origin is often explained by a large object, called Theia, colliding with early Earth. This impact created a ring of debris that later formed the Moon. However, scientists have proposed several variations of this big-impact theory, as well as other ideas, and research is ongoing to understand how the Moon formed. Other possibilities include the Moon being captured from space, splitting off from Earth, forming at the same time as Earth, or resulting from collisions between smaller objects.
According to the standard big-impact theory, Theia, a Mars-sized object, struck the early Earth, creating a debris ring around Earth. This debris then gathered to form the Moon. The Moon’s oxygen isotope ratios are nearly identical to Earth’s. Oxygen isotope ratios are unique to each object in the solar system, acting like a fingerprint. If Theia had been a separate planet, it likely would have had a different oxygen fingerprint than Earth, as would the material from the collision. Similarly, the Moon’s titanium isotope ratio (Ti/Ti) is extremely close to Earth’s (within 4 parts per million), suggesting that very little, if any, of Theia’s material became part of the Moon.
Formation
One challenge to the long-held theory of the collision is that a Mars-sized object, which likely had a different chemical makeup than Earth, would have caused Earth and the Moon to have different chemical compositions. However, Earth and the Moon do not have different chemical compositions.
Some theories suggest that early in the Solar System's formation, about 4.425 billion years ago, Earth had no large moons and was mostly made of rock and lava. A Mars-sized object called Theia, an early planet, collided with Earth. This impact sent a large amount of material into space. Some of this material escaped, but the rest gathered to form the Moon.
The hypothesis requires a collision between a proto-Earth about 90% the size of today’s Earth and another object the size of Mars (half Earth’s diameter and one-tenth its mass). This object is sometimes called Theia, named after the mother of Selene, the Moon goddess in Greek mythology. This size ratio is necessary for the system to have enough angular momentum to match the Moon’s current orbit. The impact would have sent enough material into orbit around Earth to eventually form the Moon.
Computer models show that the collision needed to be a glancing blow, creating a long arm of material that broke off. The Earth’s uneven shape after the impact caused this material to orbit Earth. The energy from the collision was enormous, possibly vaporizing and melting trillions of tonnes of material. In some parts of Earth, temperatures may have reached 10,000°C (18,000°F).
The Moon’s smaller iron core compared to other rocky planets and moons is explained by Theia’s core merging with Earth’s. The lack of volatile elements in Moon samples is also partly explained by the collision’s energy. This energy would have melted much of the Moon, creating a magma ocean.
The newly formed Moon initially orbited Earth at about one-tenth its current distance. Over time, tidal forces transferred angular momentum from Earth and the Moon to the Moon’s orbit, causing it to move outward. The Moon’s rotation became tidally locked to Earth, so one side always faces Earth. The Moon also absorbed any small moons that previously orbited Earth, which shared Earth’s composition. Since then, the Moon’s geology has developed independently of Earth.
A 2012 study on zinc isotopes in Moon rocks found evidence of volatile depletion consistent with the giant-impact theory. In 2013, a study showed that water in lunar magma is similar in isotopic composition to water in carbonaceous chondrites and Earth.
Derivatives of the hypothesis
The giant-impact hypothesis explains many features of the Earth–Moon system, but some questions remain unanswered. For example, the Moon contains more volatile elements than expected if it formed from a high-energy collision.
Another challenge involves comparing the isotopic compositions of Earth and Moon rocks. In 2001, scientists measured the isotopic signatures of Moon rocks and found they closely match Earth’s rocks, but differ from other objects in the Solar System. This was unexpected because most Moon-forming material was thought to come from a Mars-sized body called Theia. In 2007, researchers at Caltech calculated that Theia having the same isotopic signature as Earth was unlikely (less than 1% chance). A 2012 study of titanium isotopes in Moon samples confirmed the Moon and Earth share the same composition, which conflicts with the idea that the Moon formed far from Earth’s orbit.
To address these issues, a 2012 hypothesis suggested two Mars-sized bodies collided and then collided again, creating a debris disk that eventually formed Earth and the Moon.
Traditionally, the Moon is believed to have formed from debris ejected by a giant impact on early Earth. However, models struggle to explain why Earth and Moon rocks have similar isotopic compositions while also accounting for the system’s angular momentum. A 2022 study found that giant impacts could immediately place a satellite with a mass and iron content similar to the Moon into orbit far beyond Earth’s Roche limit. Even if satellites initially enter the Roche limit, they may survive by being partially stripped and moved to stable orbits. These satellites have molten outer layers made of about 60% Earth material, which might explain the Moon’s Earth-like isotopic composition. Immediate formation could allow the Moon to have a tilted orbit and offer a simpler explanation for its origin.
In 2004, Russian astrophysicist Nikolai Gorkavyi proposed the multiple large asteroid impacts model. This idea, supported by Russian astronomers in 2013 and planetary researchers at the Weizmann Institute in 2017, suggests the Moon formed from repeated impacts by large asteroids (1–100 km) over millions of years. These impacts could have blasted enough Earth debris into orbit to form a disk that eventually merged into a single moon.
In 2018, researchers at Harvard and UC Davis developed models showing that planetary collisions can create a synestia—a vaporized rock and metal disk extending beyond the Moon’s orbit. Over time, the synestia would cool and form a satellite while reforming the impacted planet.
Other hypotheses
This hypothesis suggests that the Moon was pulled into orbit around the Earth by gravity. This idea was widely accepted until the 1980s. Some evidence supporting it includes the Moon’s size, its orbit around Earth, and its tidal locking, which means the same side always faces Earth.
A challenge with this hypothesis is explaining how the Moon could be captured without colliding with Earth or being thrown away. Scientists think Earth may have had a thick atmosphere in the past, which could have slowed the Moon’s movement as it passed through, allowing it to be captured. This idea might also explain the unusual orbits of some of Jupiter and Saturn’s moons. However, it does not explain why Earth and the Moon have nearly identical oxygen isotope ratios, which are chemical markers that suggest a shared origin.
This hypothesis, now proven incorrect, suggests that a fast-spinning early Earth ejected a piece of itself to form the Moon. It was first proposed by George Darwin, the son of the famous scientist Charles Darwin, in 1879. This idea remained popular until the Apollo missions. In 1925, Otto Ampferer, an Austrian geologist, suggested that the Moon’s formation might have caused the movement of Earth’s continents.
Scientists once thought the Pacific Ocean was a mark left by this event. However, today it is known that the ocean floor in this region is only about 200 million years old, while the Moon is much older. The Moon is not made of oceanic crust but of material from Earth’s mantle, which formed deep inside Earth before the time of the first complex life on Earth, known as the Precambrian era.
The accretion hypothesis states that Earth and the Moon formed together from the early material in the Solar System’s disk. This idea struggles to explain the Earth-Moon system’s angular momentum, or the amount of spinning motion, and why the Moon has a much smaller iron core compared to Earth (25% of the Moon’s radius versus 50% for Earth).
A more extreme idea, proposed in 1997 by Russian scientist Vladimir Anisichkin, suggests the Moon formed from an explosion of Earth’s early core. Another theory, proposed in 2010 by Dutch scientists Rob de Meijer and Wim van Westrenen, claims the Moon may have formed from a nuclear explosion caused by the rapid spinning of Earth’s early core. This spinning could have gathered heavy elements like thorium and uranium near Earth’s equator and between its core and mantle. If these elements were concentrated enough, they might have caused a powerful nuclear reaction that ejected material into space, forming the Moon. A similar natural nuclear reaction has been observed on Earth, but on a much smaller scale. This theory explains both the similarities and differences in the chemical makeup of Earth and the Moon.
Additional theories and studies
In 2011, scientists suggested that a second moon may have existed 4.5 billion years ago. This moon might have hit the current Moon during its forming process.
One idea, which is just a possibility, is that Earth might have pulled the Moon from Venus.
Using a method called uranium-lead dating on tiny pieces of a mineral found in samples from the Apollo 14 mission, scientists found the Moon to be about 4.53 billion years old.
Scientists using a tool called Mini-RF on NASA's Lunar Reconnaissance Orbiter (LRO) found that the Moon's under-the-surface layers might contain more metals, such as iron and titanium, than previously thought.
In July 2020, scientists reported that the Moon formed about 4.425 billion years ago, which is 85 million years later than earlier estimates. They also found that the Moon had a large ocean of molten rock for about 200 million years, longer than previously believed.
On November 1, 2023, scientists used computer models to suggest that pieces of a planet-like object called Theia, which may have hit Earth long ago, could still be inside Earth. This collision might have led to the formation of the Moon.