Cite objections to the fission, condensation and capture theories of the origin of the moon.

Home List Science

Índice Show

A few theories
Does Earth have more than one moon?
See a piece of the Moon at the Museum

By John P. Rafferty

The curation of this content is at the discretion of the author, and not necessarily reflective of the views of Encyclopaedia Britannica or its editorial staff. For the most accurate and up-to-date information, consult individual encyclopedia entries about the topics.

The largest and most familiar sight in the night sky is the Moon. Its presence has likely bewitched observers since before the time of modern human beings, millions of years ago. Since then, the Moon has been regarded as a deity by many cultures, and stories have been told of its poetic beauty, its magic and power, its role in transforming people into werewolves and other beasts, and its ability to tip those on the brink over to the side of insanity. We have now traveled to the Moon and studied its rocks and known about its interactions with the tides, but when it comes to how the Moon was created, we still have only a loose collection of theories. What follows are humankind’s best guesses.

Coaccretion is the first of three older sets of ideas that describe how the Moon was formed. This theory posits that the Moon and Earth were formed at the same time from a primordial accretion disk—a disklike flow of gas, plasma, dust, or particles around an astronomical object that slowly collapses inward—which would help explain the geological similarities between the two. Gas from the cloud condensed into material and debris that were pulled in to attach to one or the other of these bodies. Earth happened to pull in more material and increased its mass. Of the two bodies, Earth’s mass allowed it to develop the dominant gravitational pull, and the Moon began to orbit Earth. However, critics noted that this model failed to explain the current angular momentum of the Moon around Earth.
In another set of early Moon-formation theories—one of which sprung from the mind of Sir George Darwin, English astronomer and son of naturalist Charles Darwin—Earth was thought to have once spun so rapidly that chunks of material flew from its surface. This material was thought to have later condensed into the Moon. Although fission theories appeared convincing—since the composition of Earth’s mantle and the Moon were similar—they fizzled over time because no one was able to discover the right combination of properties for a spinning proto-Earth that would generate the right kind of proto-Moon. More specifically, scientists simply did not believe that Earth could ever spin fast enough to throw off pieces of itself. In addition, so far no evidence of such a rapid spinning event on Earth or the Moon has been found.
A third set of older theories posited that the Moon could have formed somewhere else within the solar system but outside Earth’s gravitational influence. Some scientists think that the Moon may have even been in the thrall of another planet for awhile before it broke free. As the theories go, the Moon passed close to Earth sometime later. The path was so close that Earth was able to capture it within its orbit. Although other planets, such as Mars, are thought to have captured small asteroids which have since become de facto moons, scientists have yet to figure out the mechanics behind how Earth could have captured the Moon and forced the Moon’s velocity to brake hard enough for it to remain in orbit. In addition, capture theories fell out of favor after it was discovered that Earth and the Moon were geologically similar to one another.
The first of three theories that rely on the violent collision of a Mars-sized planet called Theia with Earth, this variation presupposes that Theia was made up of different, possibly weaker, material than Earth. When Theia struck, Earth remained relatively intact. Theia, however, broke apart, and the pieces that were left eventually coalesced into the Moon. Although this theory was compelling, it broke down because Earth and the Moon are made up of similar elements (silicon and oxygen, in particular) in similar concentrations.
What if Theia struck the young proto-Earth with such force that both bodies were vaporized? Some scientists propose that a strange rotating bagel-shaped cloud called a synestia could have been created by the impact. They argue that this structure could have acted like a kind of rotating mixing bowl, which blended the chemical elements found within each body evenly. Over time, the material on the very outside of the synestia coalesced into the Moon, while the rest of the material coalesced into Earth.
In this scenario, Theia still strikes Earth, but vaporization did not result, and the debris from the impact still coalesced into the Moon. What’s unique about this theory is that in it the material that makes up Theia happens to be the same stuff that makes up Earth. No harm, no foul, right? So the question becomes: how did Theia form? Perhaps both Theia and Earth formed on opposite sides of the same accretion disk (whose material was spread evenly throughout). Later, something disturbed Theia’s orbit around the Sun and caused it to drift away from its original location, which ultimately resulted in Theia crashing into Earth.
In this theory the early Earth is thought to have been pummeled not by one but by several impacts. Each strike is thought to have created a debris field that eventually coalesced into a tiny moonlet. Later, these smaller moonlets merged with one another to form the Moon. This hypothesis is unique in that it doesn’t rely on one single “smoking gun.” It allows for several events to have grown the Moon incrementally. The model notes that a disk of material would form within hours of each strike and that this material would condense into a single moonlet over the course of a few hundred years. Israeli scientists proposed this idea in early 2017 and argued that the aggregated effect of multiple high-velocity impacts could have produced enough material to form the Moon. They also said, however, that the mechanisms that explain how each of these individual moonlets got together into one larger body have yet to be described.

Museum planetary science researcher Prof Sara Russell explains the origins of Earth's closest companion.

Analysis of samples brought back from the NASA Apollo missions suggest that the Earth and Moon are a result of a giant impact between an early proto-planet and an astronomical body called Theia.

A few theories

'There used to be a number of theories about how the Moon was made and it was one of the aims of the Apollo program to figure out how we got to have our Moon,' says Sara.

Prior to the Apollo mission research there were three theories about how the Moon formed.

Capture theory suggests that the Moon was a wandering body (like an asteroid) that formed elsewhere in the solar system and was captured by Earth's gravity as it passed nearby. In contrast, accretion theory suggested that the Moon was created along with Earth at its formation. Finally, according to the fission scenario, Earth had been spinning so fast that some material broke away and began to orbit the planet.

What is most widely accepted today is the giant-impact theory. It proposes that the Moon formed during a collision between the Earth and another small planet, about the size of Mars. The debris from this impact collected in an orbit around Earth to form the Moon.

Lunar meteorite Dar al Gani 400

The Apollo missions brought back over a third of a tonne of rock and soil from the Moon.

'When the Apollo rocks came back, they showed that the Earth and the Moon have some remarkable chemical and isotopic similarities, suggesting that they have a linked history,' says Sara.

'If the Moon had been created elsewhere and was captured by the Earth's gravity we would expect its composition to be very different from the Earth's.

'If the Moon was created at the same time, or broke off the Earth, then we would expect the type and proportion of minerals on the Moon to be the same as on Earth. But they are slightly different.'

The minerals on the Moon contain less water than similar terrestrial rocks. The Moon is rich in material that forms quickly at a high temperature.

'In the seventies and eighties there was a lot of debate which led to an almost universal acceptance of the giant impact model.'

Lunar meteorites are also an important source of data for studying the origins of the Moon.

'In some ways meteorites can tell us more about the Moon than Apollo samples because meteorites come from all over the surface of the Moon,' adds Sara, 'while Apollo samples come from just one place near the equator on the near side of the Moon.'

Before Earth and the Moon, there were proto-Earth and Theia (a roughly Mars-sized planet).

The giant-impact model suggests that at some point in Earth's very early history, these two bodies collided.

During this massive collision, nearly all of Earth and Theia melted and reformed as one body, with a small part of the new mass spinning off to become the Moon as we know it.

Scientists have experimented with modelling the impact, changing the size of Theia to test what happens at different sizes and impact angles, trying to get the nearest possible match.

'People are now tending to gravitate towards the idea that early Earth and Theia were made of almost exactly the same materials to begin with, as they were within the same neighbourhood as the solar system was forming,' explains Sara.

'If the two bodies had come from the same place and were made of similar stuff to begin with, this would also explain how similar their composition is.'

A lunar landscape showing the Antoniadi Crater near the Moon's south pole

The mineralogy of Earth and the Moon are so close that it's possible to observe Moon-like landscapes without jetting off into space.

'If you look at the lunar surface, it looks pale grey with dark splodges,' Sara says. 'The pale grey is a rock called anorthosite. It forms as molten rock cools down and lighter materials float to the top, and the dark areas are another rock type called basalt.'

Similar anorthosite can be seen on the Isle of Rum in Scotland. What's more, most of the ocean floor is basalt - it's the most common surface on all the inner planets in our solar system.

'However, what is really special on the Moon, that we can't ever replicate on Earth, is that the Moon is geologically rather dead,' Sara says.

The Moon hasn't had volcanoes for billions of years, so its surface is remarkably unchanged. This is also why impact craters are so clear.

By looking at the Moon we can tell a lot about what the Earth was like four billion years ago.

Having a moon as large as ours is something that's unique in our solar system.

'While other planets have tiny moons, the Earth's Moon is almost the size of Mars,' Sara says.

'If you look at other similar planets to ours, they wobble quite a lot in their orbit (the North Pole moves) and as a result the climate is much more unpredictable.'

A piece of Anorthosite breccia moon rock displayed in a glass prism

The Moon has helped stabilise Earth's orbit and reduced polar motion. This has aided in producing our planet's relatively stable climate.

'It's a subject of quite a lot of scientific debate as to how important the Moon has been in making it possible for life to exist on Earth.'

Does Earth have more than one moon?

There may indeed be several objects in orbit around Earth. But to the best of our knowledge they are objects that the planet has drawn into its orbit - most likely captured asteroids. These natural satellites don't share the same important history as the Moon and they likely exist only temporarily in Earth's orbit.