How was the moon formed?

Scientists are still unsure as to how the moon formed, but here are three of their best bets.

How was the moon formed?

Giant impact hypothesis

Co-formation theory, capture theory, additional resources, bibliography.

The moon formed a hundred million years after the creation of the solar system . This has left scientists wondering what was the cause of our planet's satellite to birth if it didn't come from the events that formation of the planets. Here are just three of the most plausible explanations. 

The prevailing theory supported by the scientific community, the giant impact hypothesis suggests that the moon formed when an object smashed into early  Earth . Like the other planets, Earth formed from the leftover cloud of dust and gas orbiting the young sun. The early  solar system  was a violent place, and a number of bodies were created that never made it to full planetary status. One of these could have  crashed into Earth  not long after the young planet was created.

Known as Theia, the Mars-sized body collided with Earth, throwing vaporized chunks of the young planet's crust into space. Gravity bound the ejected particles together, creating a moon that is the  largest  in the solar system in relation to its host planet. This sort of formation would explain why the moon is made up predominantly of lighter elements, making it less dense than Earth — the material that formed it came from the crust, while leaving the planet's rocky core untouched. As the material  drew together  around what was left of Theia's core, it would have centered near Earth's ecliptic plane, the path the sun travels through the sky, which is  where the moon orbits today .

According to NASA , "When the young Earth and this rogue body collided, the energy involved was 100 million times larger than the much later event believed to have wiped out the dinosaurs."

Although this is the most popular theory, it is not without its challenges. Most models suggest that more than 60%of the moon should be made up of the material from Theia. But rock samples from the Apollo missions suggest otherwise.

"In terms of composition, the Earth and moon are almost twins, their compositions differing by at most few parts in a million," Alessandra Mastrobuono-Battisti, an astrophysicist at the Israel Institute of Technology in Haifa, told Space.com. "This contradiction has cast a long shadow on the giant-impact model."

In 2020 research published in Nature Geoscience , offered an explanation as to why the moon and Earth have such similar composition. Having studied the isotopes of oxygen in the moon rocks brought to Earth from Apollo astronauts, researchers discovered that there is a small difference when compared with Earth rocks. The samples collected from the deep lunar mantle (the layer below the crust) were much heavier than those found on Earth and "have isotopic compositions that are most representative of the proto-lunar impactor ‘Theia’", the study authors wrote. 

Back in 2017, Israeli researchers proposed an alternative impact theory which suggests that a rain of small debris fell on Earth to create the moon.

"The multiple-impact scenario is a more natural way of explaining the formation of the moon," Raluca Rufu, a researcher at the Weizmann Institute of Science in Israel and lead author of the study, told Space.com. "In the early stages of the solar system, impacts were very abundant; therefore, it is more natural that several common impactors formed the moon, rather than one special one.

Moons can also form at the same time as their parent planet. Under such an explanation, gravity would have caused material in the early solar system to draw together at the same time as gravity bound particles together to form Earth. Such a moon would have a very similar composition to the planet, and would explain the moon's present location. However, although Earth and the moon share much of the same material, the moon is much less dense than our planet, which would likely not be the case if both started with the same heavy elements at their core.

– Does the moon rotate?

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In 2012, researcher Robin Canup, of the Southwest Research Institute in Texas, proposed that Earth and the moon formed at the same time when two massive objects five times the size of Mars crashed into each other.

"After colliding, the two similar-sized bodies then re-collided, forming an early Earth surrounded by a disk of material that combined to form the moon," NASA said . "The re-collision and subsequent merger left the two bodies with the similar chemical compositions seen today.

Perhaps Earth's gravity snagged a passing body, as happened with other moons in the solar system, such as the Martian moons of Phobos and Deimos . Under the capture theory, a rocky body formed elsewhere in the solar system could have been drawn into orbit around Earth. The capture theory would explain the differences in the composition of Earth and its moon. However, such orbiters are often oddly shaped, rather than being spherical bodies like the moon. Their paths don't tend to line up with the ecliptic of their parent planet, also unlike the moon.

Although the co-formation theory and the capture theory both explain some elements of the existence of the moon, they leave many questions unanswered. At present, the giant impact hypothesis seems to cover many of these questions, making it the best model to fit the scientific evidence for how the moon was created.

For more on the giant-impact hypothesis, read "The Big Splat, or How Our Moon Came to be: A Violent Natural History"," by Dana Mackenzie. To learn more about the solar system, check out " Our Solar System: An Exploration of Planets, Moons, Asteroids, and Other Mysteries of Space " by Lisa Reichley. 

Erick J. Cano et al, "Distinct oxygen isotope compositions of the Earth and Moon", Nature Geoscience, Volume 13, March 2020, https://doi.org/10.1038/s41561-020-0550-0 

Raluca Rufu, "A multiple-impact origin for the Moon", Nature Geoscience, Volume 10, January 2017, https://doi.org/10.1038/ngeo2866

Edward Belbruno et al, " Where Did the Moon Come From? ", The Astronomical Journal, Volume 129, March 2005.

Thomas S. Kruijer and Gregory Archer, "No 182W evidence for early Moon formation", Nature Geoscience, Volume 14, October 2021, https://doi.org/ 10.1038/s41561-021-00820-2

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Scott Dutfield

Scott is a staff writer for  How It Works  magazine and has previously written for other science and knowledge outlets, including BBC Wildlife magazine, World of Animals magazine,  Space.com  and  All About History magazine . Scott has a masters in science and environmental journalism and a bachelor's degree in conservation biology degree from the University of Lincoln in the U.K. During his academic and professional career, Scott has participated in several animal conservation projects, including English bird surveys, wolf monitoring in Germany and leopard tracking in South Africa. 

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An astronaut aboard the International Space Station captures the full Moon as it sets behind Earth's horizon. © NASA

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How did the Moon form?

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.

Moon origin 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. The evidence returned from these missions gave us today's most widely accepted theory. 

  • 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.
  • The accretion hypothesis proposes that the Moon was created along with Earth at its formation.
  • The fission theory  suggests Earth had been spinning so fast that some material broke away and began to orbit the planet.
  • The giant-impact theory is most widely accepted today. This proposes that the Moon formed during a collision between the Earth and another small planet, about the size of the planet Mars . The debris from this impact collected in an orbit around Earth to form the Moon.

A piece of grey rock from the Moon

Lunar meteorite Dar al Gani 400. In 1998, this specimen was found in the Sahara Desert, in Libya. 

Moon rocks from the Apollo missions

The Apollo missions brought back over a third of a tonne of rock and soil from the Moon. This provided some clues on how the Moon may have formed.

'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.'

A fragment of moon rock encased in Perspex on a wooden plaque

This thumbnail-sized piece of Moon rock was gifted to the Museum by President Nixon in 1973. It was collected during the last Apollo space mission. Find out more about our links to the Apollo missions . 

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.'

Proto-Earth and Theia

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.

A full Moon

The Moon may have formed in the wake of a collision between an early proto-planet and an astronomical body called Theia. © Fernando Astasio Avila/ Shutterstock

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.'

The surface of the Moon

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.'

What are the dark spots on the Moon?

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.

Prof Sara Russell explains more about the Moon's formation:

A balancing influence.

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 moon rock in a glass prism

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.'

Find out how the Moon affects life 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.

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Explore gems and minerals, including a piece of Apollo Moon rock, in the Museum's Earth galleries .

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Giant Impact Hypothesis: An evolving legacy of Apollo

LeadLunarOriginsCredit_NASAJPLCaltechT.Pyle

The Moon has always beckoned. Long before our ancestors realized “wandering stars” were actually planets sharing the solar system with Earth, they recognized the Moon was a sort of sibling to our planet. And one of the first big questions to arise was surely: How did the Moon come to be?

Fifty years ago, humans accomplished one of the greatest feats of exploration when we set foot on the Moon. The importance of the Apollo program has been recognized as a political and technological triumph, but less widely appreciated is the scientific windfall brought by the nearly 900 pounds (400 kilograms) of lunar samples Apollo astronauts returned to Earth. These samples have ultimately proven vital to answering the age-old question of how the Moon formed.

Apollo rocks reveal the Moon’s past

Our planet has largely erased the record of its ancient past thanks to a continual re-shaping of its surface through geological activity. But the Moon is essentially dormant, so its heavily cratered surface preserves a record of solar system events going back billions of years. Thus, the Moon is a window into our planet’s primordial history.

A primary goal of the Apollo program was to distinguish among the then-leading theories for how the Moon formed: capture, co-formation, and fission. The capture theory posited the Moon formed independently from Earth, only to be captured by our planet later during a fortuitous close fly-by. The co-formation theory, however, envisioned the Moon grew alongside the Earth, with the pair accumulating mass from the same source of material. A third model, fission, proposed Earth rotated so rapidly that it became unstable, developing a bloated mid-section that shed material from its equator that would eventually become the Moon.

ASYMF1119_02

With the help of Apollo’s cache of lunar samples and data, researchers were introduced to tantalizing new clues and constraints for these three models. For instance, measuring the age of the oldest Apollo samples showed that the Moon must have formed some 4.5 billion years ago, only 60 million years or so after the first grains in our solar system condensed. This means the Moon came to be during the same early epoch that saw the birth of the planets.

From remote measurements of the Moon’s mass and radius, researchers also know its density is anomalously low, indicating it lacks iron. While about 30 percent of Earth’s mass is trapped in its iron-rich core, the core of the Moon only accounts for a few percent of its total mass. Despite this substantial difference in iron, Apollo samples later revealed that mantle rocks from the Moon and Earth have remarkably similar concentrations of oxygen. And because these lunar and terrestrial rocks are significantly different than meteorites coming from Mars or the asteroid belt, it shows the Moon and Earth’s mantle share a past connection. Additionally, compared with Earth, lunar rocks were also discovered to be more depleted in so-called volatile elements — those that vaporize easily upon heating — a hint that the Moon formed at high-temperatures.

Finally, researchers know that tidal interactions forced the Moon to spiral outward over time, which in turn caused Earth to spin more slowly. This implies the Moon first formed much closer to Earth than it is now. Precise measurements of the Moon’s position using surface reflectors placed during the Apollo program subsequently confirmed this, verifying the Moon’s orbit expands by about an inch each year.

Giant Impact Hypothesis

As is not uncommon in science, the new Apollo data, which was originally intended to test existing theories, instead inspired a new one. In the mid 1970s, researchers proposed the Giant Impact Hypothesis . The new impact scenario envisioned that at the end of its formation, Earth collided with another planet-sized body. This produced a great deal of debris in Earth’s orbit, which in turn coalesced into the Moon. The impacting planet would later be named “Theia,” after the Greek goddess who was the mother of the Moon.

The new theory seemed to reconcile multiple lines of evidence. If the material that formed the Moon originated from the outer layers of Earth and Theia, rather than from their cores, an iron-poor Moon would naturally result. A giant impact that struck Earth obliquely could also account for Earth’s rapid initial spin. Finally, the enormous impact energy associated with such an event would vaporize a substantial portion of the ejecta, accounting for the Moon’s lack of volatile materials.  

Reaction to a violent lunar origin story

The scientific community was initially skeptical of this new model. The impact hypothesis was critiqued as being a contrived, ‘ad hoc’ solution that might represent an extremely unlikely event.

But at the same time, work on other competing models proved increasingly unsatisfying. The energy dissipation needed to capture an intact Moon during a close fly-by seemed implausible, if not impossible. Models of the Moon’s co-formation alongside Earth struggled to explain why the Moon would have obtained a vastly different proportion of iron. Additionally, the current angular momentum of the Earth-Moon system was too low to be explained by a rotationally unstable Earth that flung off enough material to form the Moon. Although, at first, researchers carried out little quantitative work on the giant impact model, it eventually emerged as the most promising idea during a mid-1980s conference on lunar origin, largely due to the weaknesses of competing theories.

But could a giant impact really produce the Moon?  The answer to this question was not obvious. From basic physics, scientists know that ejecta launched from a spherical planet either entirely escape or fall back to the planet’s surface. It does enter into a stable orbit around the planet. However, a large enough impact — one by a body about the size of the planet itself — distorts the shape of the planet, altering its gravitational interactions with the ejecta. 

Additionally, partially vaporized material can be accelerated as gases escape, modifying the material’s trajectory. However, assessing the impact of such effects required a new generation of computer simulations at a scale never before modeled. With then-available technology, such simulations were extremely challenging for computers, but researchers were able to demonstrate that giant impacts could produce orbiting ejecta that might assemble itself into the Moon.

ASYMF1119_04

But thanks to vast computational improvements, by the early 2000s, researchers identified what would later become known as the “canonical” impact theory : a low-velocity collision at about a 45-degree angle by Theia, which had a mass similar to that of Mars. Such an impact produces an iron-depleted disk of material massive enough to form the Moon and leads to a five-hour day on Earth. But over billions of years, tidal interactions then transfer angular momentum to the Moon, which drags the Moon outward while simultaneously slowing down the spin of Earth. This fits well with both Earth’s current 24-hour day, as well as the present orbital distance of the Moon.

Lingering questions

If the Moon were like other astronomical bodies, for which we typically only have remote observations, at this point, we would have likely declared the origin story of the Moon solved. In this case, however, we have physical samples from both the Moon and the Earth that we can compare. Explaining the chemical relationship of those samples has proved to be the biggest challenge to the Giant Impact Hypothesis, inspiring a flurry of work over the past decade on how exactly the Moon came to be.

The conundrum is this: In most giant, disk-forming impacts like those described above, it’s primarily material from the outer portions of Theia that are slingshot into Earth orbit. But we cannot know with certainty what Theia’s composition was when it impacted the Earth. If Theia, like Mars or main-belt asteroids, were made of different material than Earth, then a pre-lunar disk coming from Theia would lead to a Moon with a different composition than our planet.

OisotopesEMVtall

Instead, data derived from Apollo lunar samples increasingly show that the Earth and Moon are almost chemically indistinguishable, not just for oxygen, but for many other elements too. Solving this “isotopic crisis” requires explaining how the collision of two independently formed planets, each with their own distinct history and composition, could have produced two such indistinguishable offspring.   

One potential and feasible explanation is that Theia did have an Earth-like composition, perhaps due to both bodies forming at a similar distance from the Sun from shared material. In fact, there is evidence that the impactors that delivered the final 40 percent of Earth’s mass were quite Earth-like . However, new analyses of lunar samples highlight one elemental similarity between Earth and the Moon that doesn’t exactly add up , and it involves the element tungsten.

Tungsten is a particularly useful for understanding planet origin for two reasons: it tends to be incorporated into a planet’s metallic core as it forms, and one flavor (or isotope) of tungsten is produced by the radioactive decay of the element hafnium, which was prevalent only during the first roughly 60 million years of solar system history.

Unlike tungsten, hafnium does not tend to be incorporated into a planet’s core, and instead remains within its mantle. Thus, if a planet’s core formed during the first 60 million years — as was likely true for both Theia and early Earth — the abundance of a particular flavor of tungsten in its mantle would have been extremely sensitive to the timing of its core’s formation. In other words, even if Theia had been Earth-like in elements like oxygen by virtue of forming near Earth, an additional coincidence would be needed to produce the needed Earth-Moon tungsten match. Current estimates suggest such a coincidence would have been highly improbable .

An alternative concept envisions that the giant impact produced a disk that was at first chemically distinct from the Earth, but eventually vaporized portions of the Earth mixed together with vapor in the disk, equalizing their compositions . In this “equilibration” model, the mixing of material essentially erased the chemical signature of Theia in the Moon-forming disk.

Equilibration is an appealing process because it could account for why Earth and the Moon show similarities across many elements, including tungsten. However, such mixing must occur rapidly, because it likely only took the Moon a few hundred years to form in the disk. Whether such efficient mixing occurred over such a short time period remains uncertain.

Variations of the Giant Impact Hypothesis

In 2012, researchers made an important discovery by showing that certain special gravitational interactions with the Sun could have allowed Earth to slow its rotation by a factor of two or more by siphoning angular momentum from Earth’s spin to its orbit around the Sun . And if this is possible, it means the Earth’s rotation rate just after the Moon formed could then have been even faster than previously assumed — spinning about once every 2 hours instead of 5 hours — indicating an even more forceful impact with Theia.

Researchers have proposed a variety of “high-angular momentum” impacts that could produce such rapidly rotating Earths , including some that lead to a disk and planet with nearly equal mixtures of material from both Theia and early Earth. The exact slowdown needed to explain a larger, higher-energy impact, however, would require a narrow range of parameters that are, as yet, still quite uncertain, making the scenario’s overall likelihood unclear. 

But what if the Moon were the product of multiple impacts, rather than just one? Recent alternative models consider the Moon formed via tens of smaller impacts with the Earth , rather than a single, giant impact. In this scenario, a relatively small impact creates a moonlet whose orbit spirals outward. A later impact produces another moonlet, whose orbital expansion could cause it to merge with the prior outer moonlet. A full-sized Moon built up by many smaller impactors with a range of compositions is more likely to end up with an Earth-like composition than a Moon produced by a single impact. However, the problem with this theory is that moonlets formed by different impacts don’t necessarily merge. Instead, it’s more likely that such moonlets would get ejected from orbit or eventually collide with Earth.

A final question is whether lunar impact simulations have considered all important aspects of a Moon-forming collision. Prior studies have generally found similar outcomes even when different computational approaches are adopted. However, a new paper proposes that if the Earth’s mantle was molten at the time of the giant impact — due to heating from a recent prior impact — it would have been more heated up more than previously predicted, leading to a more Earth-like disk, even for a giant impact scenario.    

Where do we go from here?

Thus, we find lunar origin models at a crossroads of sorts. On one hand, many once-uncertain aspects of the Giant Impact Hypothesis have been validated. Current planet-formation models predict that giant impacts were commonplace in the inner solar system as Earth grew. Thousands of increasingly sophisticated simulations have established that many (if not most) of such giant impacts would produce disks and moons. The Moon’s bulk lack of iron, which is difficult to explain in competing models like intact capture, results naturally from a large impact. This is because the material that coalesced into the Moon comes from the outer mantles of the colliding bodies rather than from their iron-rich cores.   

However, explaining other characteristics still poses a difficult challenge. Specifically, it’s hard to account for the ever-growing list of elemental similarities between the Earth and Moon, as revealed by lunar samples. One would expect the collision of two planets to have left some trace of their compositional differences, and yet — at least based on current data — such differences are not evident.

Researchers have proposed many new, creative explanations for how an impact (or impacts) could have produced a Moon so chemically similar to Earth. However, the new ideas impose additional constraints — for example that Theia must have had similar concentrations and flavors of both oxygen and tungsten, or that the angular momentum of the Earth-Moon system has drastically changed from its initial value. Thus, the impact theory still grapples with the question it faced nearly half a century ago:  Would such an event have been likely, or does it require the Moon to be the product of a very unusual event? 

Making headway depends on developments across several fronts. It’s not clear that existing models can account for all known traits of the Moon, including its volatile content and the tilt of its orbit relative to the plane of the solar system. Researchers will need to employ next-generation models to link the varied origin scenarios to predict the Moon’s properties, which will then be tested by comparing them to observations.

Fortunately, NASA and other countries are planning upcoming robotic and human Moon missions that hope to provide crucial new constraints. For example, new lunar samples may more fully reveal the Moon’s composition at depth, or improved measurements of lunar seismic activity and heat flow may better constrain the Moon’s internal composition and initial thermal state.

Ultimately, we will continue to pursue the answer for how our Moon came to be, not only so we can understand the history of our home world, but more generally, so we can unravel what our nearest cosmic neighbor can tell us about the formation and evolution of inner planets — both in our solar system and beyond.

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JWST detects methane in atmosphere of scorching hot WASP-80 b

How The Moon Was Formed: The Giant Impact Hypothesis

We don’t know all the details yet, but we have a good idea of the true origins of our only natural satellite.

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  • Lunar research began in earnest when Apollo astronauts brought moon rocks back to Earth in 1969.
  • We are learning more about the moon than ever, as techniques for analyzing the chemical composition of old and new lunar samples continue to advance.

As one of Earth’s most familiar sights in the sky, the moon has inspired billions of people to gaze upward in wonder. Early in humanity’s history, we constructed myths about this silvery orb, and later, we pursued a space race to explore it on foot. Always, there was a standout mystery: how did the moon form and find a home orbiting our blue planet?

🌒 You love the cosmos. So do we. Let’s nerd out over it together.

Apollo astronauts kick-started scientific research to answer this question when they returned from the moon in 1969 with about 48 pounds of lunar rock and dust . By measuring the age of the rocks, scientists learned that the moon formed about 4.5 billion years ago, amidst the chaotic early years of our Solar System’s own formation. Today’s tools and techniques can analyze the chemistry of lunar material in ways that were impossible just 50 years ago, revealing more detail than ever before about the story of our moon.

☄️ The Giant Impact Hypothesis Remains the Best Explanation

The generally accepted model of the moon’s creation assumes that a massive object, dubbed Theia, crashed directly into Earth 4.51 billion years ago, when our planet was still busy growing to its current size and forming its core. The resulting impact vaporized part of young Earth’s mantle , tossing rocks and gasses outward. After some time, the ejected matter (a combination of Earth material and Theia material) began orbiting our planet. The clumps of gas, dust, and rock collided and stuck together.

After just a few thousand years—recent models reveal this surprisingly short period—they coalesced into a spherical shape that continued orbiting Earth. The early moon rock was so hot that it was an entirely molten world, and it took 150 to 200 million years to cool and crystallize into its familiar, gray, rocky exterior. Theia was the catalyst for our planet’s formation, too, as it helped push heavier elements like nickel and iron toward the core.

three lab technicians at the lyndon b johnson space center in houston texas examine a lump of rock brought back from the fra mauro area of the moon by the apollo 14 mission

“Over the last 50 years, the Giant Impact Hypothesis has become the favored explanation, which I believe is the best approximation of what likely happened given the geochemical data we’ve been able to collect,” geochemist Erick Cano of the University of New Mexico in Albuquerque tells Popular Mechanics in an email.

While the Giant Impact Hypothesis is generally accepted, we still have many mysteries about the moon’s history.

The biggest challenge to planetary scientists trying to reconstruct the story of the moon is that their clues come from “very processed” rocks, Anthony Gargano, another geochemist at the University of New Mexico, tells Popular Mechanics . The moon has undergone billions of years of changes since its inception. Our satellite experienced vaporization, magma, and crystallization, all of which transformed the rocks.

🌝 Studying the Moon’s Chemical Composition for Clues

close up view  of apollo 16 lunar sample as scientists try to learn more about how the moon formed

Luckily, measurement technologies used to study planet formation are rapidly improving. Scientists are able to measure chemical compositions in ways they were not able to in the Apollo days. For example, we can now examine a slice of moon rock under an electron microscope or even study a grain of moon dust using atom probe tomography (APT). This technique distinguishes atomic-level differences in materials.

Measurement of stable isotopes is also particularly informative. Oxygen, for example, comes in light and heavy varieties, with the “heavy” version having two more neutrons in its atomic nucleus than the “light” version. The amounts of each isotope present on the lunar samples reveals more about processes that shaped the environment on the moon.

Early studies calculated the average value of oxygen isotopes in lunar rock found at several different regions of the moon, Cano says. Because those studies took an average of the measurements, scientists today know that the results were misleading; the measurements indicated that the moon’s chemical composition was virtually identical to Earth’s, but that evidence goes against the idea of a moon containing material from a secondary body colliding with Earth. One explanation to justify the identical chemical composition is that meteor impacts delivered the oxygen.

Thanks to a different approach that examined the same samples, a study in March 2020 cleared up the confusion. The evidence , which Cano and other researchers presented in Nature Geoscience , examined each sample separately with high-precision measurement tools, finding distinct characteristics in each one. Scientists concluded that the moon appears to have different oxygen isotope compositions from our planet.

This data, found in samples from deep inside the lunar mantle, 30 miles beneath the surface, supports a giant impact origin story. Furthermore, this reveals more about the mysterious Theia. “Our findings imply that the distinct oxygen isotope compositions of Theia and Earth were not completely homogenized by the moon-forming impact, thus providing quantitative evidence that Theia could have formed farther from the sun than did Earth,” the researchers note in their paper.

Another NASA-led study also reveals more about the geochemistry of the giant impact. Planetary scientists know that the element chlorine vaporizes at low temperatures, so they used chlorine to track planet formation. Earth has an abundance of light chlorine. In contrast, the moon rocks scientists examined contained more of the heavy chlorine isotope. A sound explanation is that as Earth and the moon reformed after the impact, the larger-bodied Earth drew away most of the light chlorine. “The chlorine loss from the moon likely happened during a high-energy and heat event, which points to the Giant Impact theory,” Gargano, one of the lead researchers, says in a NASA press release. The team’s work was published in September 2020 in the Proceedings of the National Academy of Sciences .

🧪 Where Did the Moon’s Carbon Come From?

Recently, scientists at several Japanese universities and the Japan Aerospace Exploration Agency found a surprise on the moon in the form of carbon ion emissions from the moon’s surface. They used data collected during the KAGUYA mission, Japan’s second mission to explore the moon from orbit. Launched in 2007, it created the most detailed topographical model we have of our rocky neighbor with the aid of 15 different instruments. Investigations of the data it collected over almost two years about the moon’s geology are challenging previous research on lunar samples.

Scientists previously believed there was not much carbon at all on the moon, even though this volatile element normally influences the formation and evolution of planetary bodies. Yet, the estimated carbon emissions KAGUYA found on the moon’s surface were far greater in quantity than expected, researchers reported in Science Advances in May 2020. Instruments showed that carbon ions were distributed across almost the entire lunar surface. Therefore, it must be indigenous to the moon, researchers concluded.

This evidence means the carbon must have been embedded in the moon during its formation or soon afterward. The study also notes that the moon’s basaltic plains emit far more carbon ion emissions than the highlands. It’s evidence for carbon existing on the moon for billions of years, rather than entering later from outside sources such as solar wind or meteorites. Instruments were detecting carbon emissions at a rate of about 5.0 × 10⁴ per square centimeter per second, which is far greater than solar wind and micrometeoroids could supply, according to the study.

The Story of the Moon Is Still Taking Shape

crescent moon against stars background

In the same year, researchers in Germany uncovered another compelling piece of the story, evidence that the moon took shape just a few thousand years after the impact. The study , published in July 2020 in the journal Science Advances , found that ejected matter from Thiea and Earth condensed into a magma ocean 600 miles deep. It took 150 to 200 million years for that liquid rock to fully crystallize, according to the computer simulation models researchers used in this study. Previous estimates said the moon took just 35 million years to cool into a solid crust.

Russia’s Luna missions have collected lunar material as well. China’s recent Chang’e-5 probe collected samples from the dark side of the moon. The area the Apollo rocks came from is only a small region of the moon, so it’s like trying to put together a giant puzzle when you have only a few pieces, Cano says.

Putting together data from all of these experiments and missions will be the key to painting a clearer picture of the moon’s experiences since its birth 4.5 billion years ago. So far, we don’t have access to data from some of those countries, such as China.

“Even with just the current samples and data we have available, scientists are still coming up with new ideas regarding the details of lunar formation,” Cano says. Still, an overwhelming amount of chemical evidence exists to support the Giant Impact Hypothesis, Gargano says. At this point, the work is all about filling in the details.

Cano agrees. “In my opinion, the current data we have is enough to make a reasonable hypothesis about the moon’s origin. However, in order to determine the specific details of its formation, we would likely need to return to the lunar surface and collect more samples and do a more in-depth geological study,” Cano says.

We won’t have to wait long for another batch of lunar samples to inform our lingering questions about how the moon came to be. NASA will launch a human return to the moon by 2024 with the Artemis mission .

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Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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Billions of years ago, a version of our Earth that looks very different than the one we live on today was hit by an object about the size of Mars, called Theia – and out of that collision the Moon was formed. How exactly that formation occurred is a scientific puzzle researchers have studied for decades, without a conclusive answer.

Most theories claim the Moon formed out of the debris of this collision, coalescing in orbit over months or years. A new simulation puts forth a different theory – the Moon may have formed immediately, in a matter of hours, when material from the Earth and Theia was launched directly into orbit after the impact.

“This opens up a whole new range of possible starting places for the Moon’s evolution,” said Jacob Kegerreis, a postdoctoral researcher at NASA’s Ames Research Center in California’s Silicon Valley, and lead author of the paper on these results published in The Astrophysical Journal Letters. “We went into this project not knowing exactly what the outcomes of these high-resolution simulations would be. So, on top of the big eye-opener that standard resolutions can give you misleading answers, it was extra exciting that the new results could include a tantalisingly Moon-like satellite in orbit.”

The simulations used in this research are some of the most detailed of their kind, operating at the highest resolution of any simulation run to study the Moon’s origins or other giant impacts.  This extra computational power showed that lower-resolution simulations can miss out on important aspects of these kinds of collisions, allowing researchers to see new behaviors emerge in a way previous studies just couldn’t see.

A Puzzle of Planetary History

Understanding the Moon’s origins requires using what we know about the Moon – our knowledge of its mass, orbit, and the precise analysis of lunar rock samples – and coming up with scenarios that could lead to what we see today.

Previously prevailing theories could explain some aspects of the Moon’s properties quite well, such as its mass and orbit, but with some major caveats. One outstanding mystery has been why the composition of the Moon is so similar to Earth’s. Scientists can study the composition of a material based on its isotopic signature, a chemical clue to how and where an object was created. The lunar samples scientists have been able to study in labs show very similar isotopic signatures to rocks from Earth, unlike rocks from Mars or elsewhere in the solar system. This makes it likely that much of the material that makes up the Moon originally came from Earth.

In previous scenarios where Theia sprayed out into orbit and mixed with only a little material from Earth, it’s less likely we’d see such strong similarities – unless Theia was also isotopically similar to Earth, an unlikely coincidence. In this theory, more Earth material is used to create the Moon, particularly its outer layers, which could help to explain this similarity in composition.

There have been other theories proposed to explain these similarities in composition, such as the synestia model – where the Moon is formed inside a swirl of vaporized rock from the collision – but these arguably struggle to explain the Moon’s current orbit.

This faster, single-stage formation theory offers a cleaner and more elegant explanation for both these outstanding issues. It could also give new ways to find answers for other unsolved mysteries. This scenario can put the Moon into a wide orbit with an interior that isn’t fully molten, potentially explaining properties like the Moon’s tilted orbit and thin crust – making it one of the most enticing explanations for the Moon’s origins yet.

Getting closer to confirming which of these theories is correct will require analysis of future lunar samples brought back to Earth for study from NASA’s future Artemis missions. As scientists gain access to samples from other parts of the Moon and from deeper beneath the Moon’s surface, they will be able to compare how real-world data matches up to these simulated scenarios, and what they indicate about how the Moon has evolved over its billions of years of history.

A Shared Origin

Beyond simply learning more about the Moon, these studies can bring us closer to understanding how our own Earth became the life-harboring world it is today.

“The more we learn about how the Moon came to be, the more we discover about the evolution of our own Earth,” said Vincent Eke, a researcher at Durham University and a co-author on the paper. “Their histories are intertwined – and could be echoed in the stories of other planets changed by similar or very different collisions.”

The cosmos is filled with collisions – impacts are an essential part of how planetary bodies form and evolve. On Earth, we know that the impact with Theia and other changes throughout its history are part of how it was able to gather the materials necessary for life. The better scientists can simulate and analyze what’s at play in these collisions, the more prepared we are to understand how a planet could evolve to be habitable like our own Earth.

This research is a collaborative effort between Ames and Durham University, supported by the Institute for Computational Cosmology’s Planetary Giant Impact Research group . The simulations used were run using the open-source SWIFT, ( SPH with Inter-Dependent Fine-grained Tasking ) code, carried out on the DiRAC (Distributed Research Utilizing Advanced Computing) Memory Intensive service (“COSMA”), hosted by Durham University on behalf of the DiRAC High-Performance Computing facility.

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Members of the news media interested in covering this topic should reach out to the  NASA Ames newsroom .

When one looks at the conditions which make advanced life on the Earth possible, one of the surprises is the importance of the Moon . The present standard model is that the Moon was formed when Earth experienced a collision with a Mars-sized body early in its history. It is appropriate to examine alternative hypotheses to evaluate the strength of the standard model. The alternative hypotheses advanced have been:

  • The Earth and Moon formed together as a planetary pair.

There are major problems with all three alternative scenarios for the formation of the Moon, but they offer looks at some interesting physical concepts.

This scenario envisions the early Earth in its molten state spinning rapidly and forming a tidal bulge at the equator which led to the separation of the mass of the Moon and its coalescence as a separate body. Sometimes called the "fission" model, this hypothesis was put forward by George Darwin in 1879. Part of the model was the proposal that a resonance with the Sun and solar tides which gave the necessary extra boost to separate off a large molten mass which then solidified to form the Moon. In support of this hypothesis, Osmond Fischer proposed that the Pacific Ocean represents the unhealed scar where the Moon used to be. Further support came from the fact that the Moon's density is very nearly equal to the density of the Earth's mantle .

The arguments against this hypothesis involve a number of interesting studies.

While advocates of this model differ in their proposed origin of the interloper, the fact that the age of the Moon is similar to the solar system makes it likely that the source of the material is from within the solar system. Dalrymple points out that the oxygen isotope concentration is identical in the Earth and Moon and the undifferentiated meteorites, but different from that in more primitive meteorites. This would seem to restrict the source of the Moon to the same general neighborhood of the solar system as the Earth, but the bulk composition of the Earth with its large iron core is greatly different from that of the Moon.

Often called the "double planet hypothesis", this proposal is that the Earth and Moon formed at about the same time in the same region of the solar system and were close enough together to form a bound system with each other. It certainly seems like a reasonable hypothesis.

IMAGES

  1. New Moon-Formation Theory Also Raises Questions About Early Earth

    hypothesis for moon formation

  2. Hence, That's How Was The Moon Formed...!!!

    hypothesis for moon formation

  3. Where did the Moon come from?

    hypothesis for moon formation

  4. PPT

    hypothesis for moon formation

  5. Artwork showing a theory for the Moon's formation

    hypothesis for moon formation

  6. How did the moon form// GIANT IMPACT HYPOTHESIS//FORMATION OF MOON

    hypothesis for moon formation

COMMENTS

  1. Why Do We Have a Moon?

    No concrete evidence explains why there is a moon. The best hypothesis presented is the Giant Impactor hypothesis: It suggests that around 4.45 billion years ago, while the Earth was still forming, a large object hit the Earth at an angle.

  2. What Are Some Examples of a Good Hypothesis?

    Strong hypotheses are most often written in the, “If A occurs, then B will occur” format and are presented as statements, not questions. Good hypotheses also are clear and keep variables in mind, defining them in easy-to-measure terms.

  3. What Is Nebular Theory?

    The solar nebular theory explains the formation and evolution of the solar system. It is the most widely accepted model, also known as the “solar nebular hypothesis.” Formation of the sun

  4. Giant-impact hypothesis

    The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon was formed from the ejecta of a collision between

  5. How Was the Moon Formed?

    The prevailing theory supported by the scientific community, the giant impact hypothesis suggests that the moon formed when an object smashed

  6. How did the Moon form?

    The accretion hypothesis proposes that the Moon was created along with Earth at its formation. The fission theory suggests Earth had been spinning so fast that

  7. How the Giant Impact Hypothesis Can Help Explain the ...

    How did the moon form? The Giant Impact Hypothesis suggests that the moon was formed as a result of the collision of Earth and another

  8. Giant Impact Hypothesis: An evolving legacy of Apollo

    The capture theory posited the Moon formed independently from Earth, only to be captured by our planet later during a fortuitous close fly-by.

  9. How The Moon Was Formed: The Giant Impact Hypothesis

    ☄️ The Giant Impact Hypothesis Remains the Best Explanation. The generally accepted model of the moon's creation assumes that a massive object

  10. How Did the Moon Form?

    Invite the students to share and record their thoughts, then provide additional input as needed. 1. The co-formation hypothesis states that the Moon formed in

  11. Origin of the Moon

    The standard giant-impact hypothesis suggests that a Mars-sized body, called Theia, impacted the proto-Earth, creating a large debris ring around Earth, which

  12. Collision May Have Formed the Moon in Mere Hours, Simulations

    In this theory, more Earth material is used to create the Moon, particularly its outer layers, which could help to explain this similarity in

  13. The Origin of the Moon: The Giant-Impact Theory

    Most planetary scientists expected that lunar samples brought to Earth at the end of each of the six Apollo missions would confirm one of three leading

  14. Hypotheses for the Formation of Earth's Moon

    Hypothesis for Moon formation. Often called the "double planet hypothesis", this proposal is that the Earth and Moon formed at about the same time in the same