what is the great impact hypothesis

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.

“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

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

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 .

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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How was the moon formed?

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

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?

– Atmosphere of the moon

– How Far is the Moon?

– Every mission to the moon

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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Giant Impact Hypothesis

• Living reference work entry
• First Online: 13 July 2017
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• Hidenori Genda 4  

Part of the book series: Encyclopedia of Earth Sciences Series ((EESS))

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The giant impact hypothesis is one of the theories for the origin of the Moon. In this theory, a Mars-sized object hit Earth obliquely about 4.5 billion years ago, which ejected a lot of materials to form a disk around Earth. From this disk, a single huge moon was formed. Unlike the other hypotheses (the fission, capture, and binary accretion hypotheses), the giant impact hypothesis satisfies almost all physical and chemical constraints of the Moon (Stevenson 1987 ). Thus, the giant impact hypothesis is regarded as the leading theory for the origin of the Moon (e.g., Canup 2004 ).

Rise of Giant Impact Hypothesis

Before the 1970s, the fission, capture, and binary accretion hypotheses had been considered for the origin of the Moon. It is now known that these hypotheses have one or more crucial difficulties in physically making the Moon and explaining the lunar chemical compositions (see review articles Boss 1986 ; Wood 1986 ). However, before the 1970s, the constraints on the...

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Genda, H. (2017). Giant Impact Hypothesis. In: White, W. (eds) Encyclopedia of Geochemistry. Encyclopedia of Earth Sciences Series. Springer, Cham. https://doi.org/10.1007/978-3-319-39193-9_338-1

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Giant Impactor Hypothesis

Where did the moon come from ?

At the time of Project Apollo in the 1960s, there were basically three hypotheses about how the moon formed.

• Double planet (also called the condensation hypothesis ) : The moon and the Earth formed together at about the same time.
• Capture: The Earth's gravity captured the fully formed moon as it wandered by.
• Fission: The young Earth spun so rapidly on its axis that a blob of molten Earth spun off and formed the moon.

But based on the findings of Apollo and some scientific reasoning, none of these hypothesis worked very well.

• If the moon did form alongside the Earth, the composition of the two bodies should be about the same (they aren't).
• The Earth's gravity isn't sufficient to capture something the size of the moon and keep it in orbit.
• The Earth can't spin fast enough for a blob of material the size of the moon to just spin off.

The Giant Impact Theory

Because none of these hypotheses was satisfactory, scientists looked for another explanation.

In the mid-1970s, scientists proposed a new idea called the Giant Impactor (or Ejected Ring) hypothesis. According to this hypothesis, about 4.45 billion years ago, while t­he Earth was still forming, a large object (about the size of Mars ) hit the Earth at an angle. The impact threw debris into space from the Earth's mantle region and overlying crust. The impactor itself melted and merged with the Earth's interior, and the hot debris coalesced to form the moon.

The Giant Impactor hypothesis explains why the moon rocks have a composition similar to the Earth's mantle, why the moon has no iron core (because the iron in the Earth's core and impactor's core remained on Earth), and why moon rocks seem to have been baked and have no volatile compounds. Computer simulations have shown that this hypothesis is feasible.

Distance from Earth to Moon : 240,250 miles (384,400 km)

Diameter: 2,160 miles (3,476 km), or about 27 percent of the Earth's diameter

Mass: 7.35 x 10 22 kilograms, about 1.2 percent of the Earth's mass

Gravity: 1.62 m/s 2 , or 16.6 percent of the Earth's gravity

Mean surface temperature:

sunlight = 266 F (130 C),

­shadow = -292 F (-180 C)

Atmosphere: None

Orbital period: 29.5 day

Lunar day: 29.5 Earth days (the moon is tidally locked to the Earth, so our gravity drags the moon around on its axis and the same side of the moon always faces Earth)

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

October 17, 2012

Giant Impact Theory of Lunar Formation Gains More Credibility

A lingering problem in explaining the genesis of the moon appears to have been solved

By John Matson

The moon may be a chip off the old block after all.

The most commonly invoked explanation for lunar formation holds that a giant protoplanet, sometimes called Theia, struck the newly formed Earth 4.5 billion years ago and created a cloud of debris that quickly coalesced into the moon. But that hypothesis has suffered from a nagging flaw. Simulations of moon-forming collisions have shown Theia would have been the primary donor of lunar material. But analyses of Apollo moon rocks have shown that the moon seems in many ways a chemical clone of Earth, not Theia.

“The giant impact theory explains many traits of the system—that’s why it’s favored—but this [discrepancy] is a little tricky,” says planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colo., who played a key role in developing the Theia idea. “This has been a thorn in the side of the impact theory for some time.”

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That thorn may be on its way out. A pair of papers published online October 17 in Science, one by Canup and one by planetary scientists at the SETI Institute in Mountain View, Calif., and Harvard University, demonstrate two different ways that a giant impact could produce a moon with the observed chemical similarities to Earth.

In Canup’s model, the impactor is substantially heftier than the canonical Theia—instead of a Mars-size object colliding with the much larger proto-Earth, her new study proposes a smashup of two comparably sized objects. “The set of impacts that I identify that can do this involve a much larger impactor than had been considered before,” Canup says. “The type of impact that I’m advocating here is the collision of two half-Earth-mass objects. They merge to form the Earth.” The moon would then form from the leftover debris, naturally explaining its similarities to Earth.

A different conception, from Matija Ćuk of the SETI Institute and Sarah Stewart of Harvard, invokes a small, high-velocity projectile smacking into a fast-spinning proto-Earth. Like an interplanetary mortar, that high-energy impact would fling out a cloud of debris composed primarily of material from Earth. “The crucial difference is that Earth is spinning faster,” Ćuk says. “If you hit it hard it’s easier for the pieces to fly into space.”

Both studies build on the recent finding by Ćuk and Stewart that gravitational interactions with the sun can quickly sap angular momentum from the newborn Earth-moon system. As a result, Earth may have been spinning much faster after lunar formation than had previously been thought plausible— a day on Earth may have lasted only two or three hours immediately after the impact. And the possibility of a fast-spinning Earth opens the door to types of collisions that had not been considered viable before.

Indeed, the true impact of the new studies is not in the specifics of the revised lunar-forming models but in the fact that such revisions now appear plausible, says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz. “It’s not so much that they’ve come up with a model that works; it’s that they have taken away a constraint that we thought was sacrosanct for the last 20 years,” he says.

Ćuk, too, foresees the opening of a new chapter in unraveling the story of the moon’s birth. “This is going to be, I hope, the first of a new batch of papers, rather than the final word,” he says. “The thing that really surprised me and Sarah is, we didn’t try very hard—this kind of came out pretty much by itself. So that’s promising. We didn’t have to look far and wide for something that worked.”

The only catch is that Theia’s size and the magnitude of its impact, which once seemed to be fairly well understood, are now open to debate. And many more plausible scenarios that can explain the Earth-moon system may now come to the fore. “That is my worry—I wonder if moon formation may have become an unsolvable problem,” Asphaug says. “If you can have an Earth that is spinning with pretty much any spin rate, suddenly all bets are off.”

Giant Impact Hypothesis: Theory on how the Moon was formed

Various theories have been proposed on the formation of the moon but none explains all the points precisely. The Giant Impact Hypothesis is the currently favored theory on how the moon was formed. It says that the moon was formed about 4.5 billion years ago, a few million years after the formation of the solar system, due to the collision of earth with a planet about the size of Mars.

According to this theory a Mars sized planet once orbited the sun not far away from earth. This early planet has been named Theia, after the Greek titan who gave birth to the Moon goddess, Selene. About 30-50 million years after the solar system began to form, Theia collided with Earth. The collision resulted in Theia being partially absorbed into earth, but a significant amount of debris from both Theia and Earth were sprayed around our planet. Gravity pulled the debris into orbit around earth and as the fragments collided, they began to quickly coalesce together to form today’s moon.

The Giant Impact theory on how the moon was formed is supported by some evidence including: the identical direction of the Earth’s spin and the Moon’s orbit, Moon samples that indicate the surface of the Moon was once molten just like it should have been after the collision, the Moon’s relatively small iron core and evidence of similar collisions in other star systems (that result in debris disks). Also giant collisions are consistent with the leading theories of the formation of the solar system. Finally, the moon has exactly the same oxygen isotope composition as the Earth, unlike other planets in the Solar System, indicating that the moon should have been formed from material in Earth’s neighborhood.

One of the points against the theory is that the energy from such a collision would have produced a global ocean of magma on earth but there is no evidence that earth had such a magma ocean. Other remaining questions include when the Moon lost its share of volatile elements and why Venus, which also experienced giant impacts during its formation, does not host a similar moon.

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Learn about this topic in these articles:, origin of moon.

…1980s that a model emerged—the giant-impact hypothesis—that eventually gained the support of most lunar scientists.

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Ask Astro: What happened after the giant impact that created the Moon?

Q: If the Giant Impact Hypothesis is correct and the Moon is made from Earth and Theia (the Mars-sized object that collided with Earth), what happened to the rest of Theia after the collision?

A: In the 1970s, Donald R. Davis and I suggested that the Moon was formed when a Mars-sized planetesimal, later called Theia, struck a newly formed Earth about 4.5 billion years ago. At the time, the Giant Impact Hypothesis had very little to say about what happened to the impactor itself.

In the years since, many researchers have modeled what the impact may have looked like. After slamming into Earth, the outer rocky shells of both Earth and Theia were blasted into a disk of debris around our planet. From this disk, the Moon coalesced; thus, models indicate most of Theia’s material ended up as part of the Moon. Any iron core that Theia may have had was consumed by Earth’s own core.

The Giant Impact Hypothesis is, as your question alludes to, not yet settled. One issue with the hypothesis is that samples of lunar rocks reveal that the Moon and Earth have very similar ratios of isotopes — the equivalent of an elemental fingerprint for celestial objects. For example, the ratio of oxygen-16 to oxygen-18 is about the same in the Moon as in Earth. These results are surprising, since no other major bodies in the solar system are that alike, especially in oxygen concentrations. A few years ago, simulations seemed to show that Theia had actually originated in the distant solar system, complicating the hypothesis further.

It is true that meteorites from other parts of the solar system bare no isotopic resemblance to either Earth or the Moon. But a potential explanation comes in the form of a rare group of meteorites called enstatite chondrites, which are also nearly identical to Earth. These meteorites may have been the building blocks of Earth and, as a recent paper indicates, Theia as well. Additionally, the simulations are somewhat questionable and Theia may have originated more locally.

This suggests that the former planetesimal is a long-lost twin of our planet. For now, scientists must rely on models to solve the puzzle of Theia, but if a stray meteorite from the collision is found, it may provide the final clues needed to put the mystery to rest.

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• Published: 22 March 2021

Isotopic evidence for the formation of the Moon in a canonical giant impact

• Sune G. Nielsen   ORCID: orcid.org/0000-0002-0458-3739 1 , 2 ,
• David V. Bekaert 1 &
• Maureen Auro 1  

Nature Communications volume  12 , Article number:  1817 ( 2021 ) Cite this article

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• Early solar system
• Geochemistry

Isotopic measurements of lunar and terrestrial rocks have revealed that, unlike any other body in the solar system, the Moon is indistinguishable from the Earth for nearly every isotopic system. This observation, however, contradicts predictions by the standard model for the origin of the Moon, the canonical giant impact. Here we show that the vanadium isotopic composition of the Moon is offset from that of the bulk silicate Earth by 0.18 ± 0.04 parts per thousand towards the chondritic value. This offset most likely results from isotope fractionation on proto-Earth during the main stage of terrestrial core formation (pre-giant impact), followed by a canonical giant impact where ~80% of the Moon originates from the impactor of chondritic composition. Our data refute the possibility of post-giant impact equilibration between the Earth and Moon, and implies that the impactor and proto-Earth mainly accreted from a common isotopic reservoir in the inner solar system.

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Introduction.

There is a general consensus that the last major event in Earth’s accretion corresponded to the Moon-forming giant impact 1 , but considerable uncertainty remains regarding the exact conditions of this episode 2 . Canonical giant impact models that reproduce the Earth-Moon system require a roughly Mars-sized impactor (called Theia) to have collided with proto-Earth after ~90% accretion was complete 3 , 4 . These models all result in the Moon predominantly deriving from the impactor, which implies that the Earth and Moon should exhibit distinct isotopic compositions if Theia and proto-Earth formed from different isotopic reservoirs in the solar system. However, extensive isotopic analyses have revealed undetectable or limited difference between the Earth and Moon for elements such as O, Ti and Cr 5 , 6 , 7 , which all exhibit large variations among chondrites and differentiated meteorites 6 , 8 , 9 . These observations could be explained if (i) the initial dynamic simulations were not capturing the actual geometry of the giant impact itself 10 , 11 , (ii) post-giant impact equilibration between the Earth and Moon materials led to their isotopic homogenization 2 , 12 , or (iii) the impactor and proto-Earth mainly accreted from a common isotopic reservoir (best represented by enstatite chondrites) 13 .

Recent studies of V isotope ( 50 V and 51 V) variations in chondrites and terrestrial rocks have revealed that the bulk silicate Earth (BSE) is uniformly enriched in 51 V (δ 51 V BSE  = −0.856 ± 0.020‰; n  = 76, 2SE; where δ 51 V sample  = (( 51 V/ 50 V) sample /( 51 V/ 50 V) AA − 1) × 1000, with AA corresponding to the Alfa Aesar standard; see Methods section) relative to average chondrites (δ 51 V = −1.089 ± 0.029‰, n  = 14, 2SE), by Δ 51 V BSE-chondrites  = 0.233 ± 0.037‰ (2SE) 14 . It has been proposed that V isotope variations in bulk carbonaceous could have a nucleosynthetic origin 15 , but subsequently it was found that all V isotope variations in bulk chondrites can be accounted for by recent production of 50 V by GCR spallation processes 16 . The invariant V isotope composition of all chondrites implies that nucleosynthetic V isotope anomalies must be very small and cannot induce planetary scale V isotope heterogeneity. Early solar system irradiation may also have induced production of 50 V by spallation reactions, which has been inferred for some CAIs 17 , but given the uniform bulk V isotope compositions in chondrites with highly variable CAI abundances such a process is also unlikely to account for planetary scale V isotope variations. Collectively, these arguments imply that V isotope variations among terrestrial planets do not reflect differences in the composition of their accretionary materials, but are the result of planetary differentiation processes 14 . Although it is currently not certain why the silicate Earth and chondrites are distinct in terms of V isotopes 15 , the most likely scenario is that core formation processes prior to the giant impact caused V isotope fractionation that resulted in the BSE having a heavy V isotope composition relative to chondrites 14 . This scenario is supported by the recent observation that the V isotope composition of the bulk silicate mars (BSM) is also enriched in 51 V relative to chondrites (Δ 51 V BSM-chondrites  = 0.067 ± 0.042‰ 14 ). Metal-silicate V isotope fractionation factors required to explain the observed isotope offsets between planetary (BSE, BSM) and chondritic compositions are in good agreement with each other 14 , therefore providing empirical support for systematic V isotope fractionation during high pressure-high temperature planetary differentiation, before the Moon-forming event.

Considering the V isotope difference between BSE and chondrites, it follows that if bulk Theia was composed primarily of chondritic material then it should have been significantly lighter than the silicate proto-Earth. As a consequence, the canonical dynamic simulations of the giant impact 3 , 4 would imply that the Moon is characterized by a V isotope composition intermediate between Earth and chondrites. On the other hand, alternative giant impact geometries 10 , 11 and post-impact Earth-Moon equilibration scenarios 2 , 12 would result in largely indistinguishable V isotope compositions for Earth and the Moon, even if Theia and proto-Earth were initially different in terms of δ 51 V.

Here we show that the Earth and Moon indeed exhibit different V isotope compositions, which strongly implies that the canonical giant impact offers the best explanation for the formation of the Moon.

Results and discussion

Lunar v isotope data and correction for cosmic ray exposure.

Recent analyses of lunar rocks have found that many samples exhibit δ 51 V significantly lighter than both Earth and chondrites 16 . Based on a strong correlation between V isotopes and cosmic ray exposure (CRE) ages, this observation was interpreted to reflect production of 50 V due to galactic cosmic ray (GCR) interaction with primarily Fe atoms (see Supplementary Note  1 ) at the surface of the Moon 16 . These authors, however, concluded that the Earth and Moon have indistinguishable V isotope compositions due to the large uncertainties on their V isotopic measurements and the large GCR effects on most of their lunar samples. Here, we have analyzed one lunar soil and three lunar basalts with confirmed substantial GCR effects (samples 10084, 15495, 15556, and 70215; GCR exposure ages >100 Myr, Supplementary Table  1 ) and these all reveal variably light V isotope compositions that follow the previously found relationship between V isotopes and CRE ages (Fig.  1 ; Supplementary Table  1 ). In addition, we present new V isotope data for five Apollo mission lunar rocks that have been shown to record very young GCR exposure ages (samples 12004, 74255, 68815, 68115, and 14321; GCR exposure ages between 2 and 49 Myr, Supplementary Table  1 ). We also analyzed one recently excavated lunar meteorite (LAP02205) that has a very young GCR exposure age (~4 Myr; Supplementary Table  1 ). These six samples reveal very limited V isotope variation and an uncorrected average δ 51 V = −1.077 ± 0.039‰ (2SE), suggesting that this value closely resembles lunar rocks prior to irradiation. When we combine our new V isotope data for lunar samples with previously reported results, we obtain a strong correlation with CRE ages (Fig.  1 ). The y-intercept of the best-fit line to these data corresponds to the irradiation-free composition of the lunar samples, which has a value of δ 51 V Moon  = −1.037 ± 0.031‰ ( n  = 26, 2SE), intermediate between chondrites and BSE.

In this study, Fe/V ratios of all samples were measured on minor splits of the dissolved samples that were processed for V isotope measurements. The best-fit line and gray 2SE envelope through all the data, calculated by taking into account 2SE errors in both x and y, has a y-intercept of δ 51 V Moon  = −1.037 ± 0.031 (2SE). This value represents our best estimate for the irradiation free V isotope composition of the Moon. Orange squares are from this study, green circles from ref. 16 .

Vanadium isotope homogeneity of the Moon

The samples investigated here and elsewhere 16 represent a very diverse set of lunar lithologies covering low and high Ti basalts from different lunar mantle source regions 18 , as well as several KREEP-rich (K, Rare Earth Element and Phosphorus) samples corresponding to highly evolved magmas 18 . In agreement with a previous study 16 , we conclude that, although V isotope fractionation can be significant at high temperature 19 , fractional crystallization did not induce any detectable V isotope fractionation on the Moon as KREEP-rich samples are indistinguishable from mare basalts (Supplementary Table  1 ). The lack of magmatic V isotope fractionation on the Moon could be related to a lower oxygen fugacity that may have resulted in V occupying a single valence state in lunar magmas, and thus attenuated any redox-driven V isotope fractionation 16 . Given that different types of mare basalts exhibit invariant V isotope compositions that are identical to KREEP-rich rocks (Supplementary Table  1 ), different regions of the lunar mantle are unlikely to record any detectable V isotope variation. As a result, we infer that the bulk Moon (apart from the very surface that is affected by GCR effects) is homogenous with respect to V isotopes (δ 51 V Moon  = −1.037 ± 0.031‰; Supplementary Note  2 ), with an isotopic difference of Δ 51 V BSE-Moon  = 0.181 ± 0.035‰ (2SE) between the silicate Earth and Moon (Fig.  2 ; Supplementary Note  2 ).

Error bars are 2SE for terrestrial samples and the two studies of lunar samples (square markers; Supplementary Note  2 ). Chondrite data are individual samples with 2 SD error bars (circle markers) 14 . Individual data for terrestrial samples previously compiled 14 . Lunar sample data can be found in Supplementary Table  1 . Error-weighted averages and 2SE for each reservoir are shown as vertical gray bars behind each sample group (Supplementary Note  2 and ref. 14 ).

No V isotope effects by evaporation, core formation or late accretion

The V isotopic difference between Earth and the Moon cannot realistically be explained via kinetic isotope fractionation due to partial volatilization of V during the Moon-forming event. First because V is relatively refractory under both nebular and planetary magma ocean conditions 20 , 21 , and therefore unlikely to have been significantly volatilized. However, even if volatilization induced V isotope effects, then isotopic fractionation during either (i) partial condensation of an originally BSE-like vapor phase or (ii) evaporation of a partially molten proto-Moon 22 would both produce heavy isotope enrichments relative to the BSE, which is opposite to what we observe here for lunar V. If the Moon represents a partial condensate of a protolunar disk 23 that resulted in a light V isotope composition of the Moon relative to Earth, then we would expect similarly refractory elements like Ti and Sr to show similar stable isotope offsets as V, which is not observed 24 , 25 . Furthermore, equilibrium isotope exchange reactions in the protolunar disk may be expected to produce limited isotope fractionation because V, like Si or Ti, is associated with at least one atom of oxygen (e.g., VO, VO 2 , V 4 O 10 ) in both the solid and gas phases 26 , which limits the potential for significant equilibrium isotope fractionation 27 . The partial vaporization behavior and thermodynamics of V under protolunar disk conditions are unknown, making quantitative assessments of such equilibrium effects very difficult. For Si, the isotopic offset expected between the Earth and the Moon from liquid–vapor separation within the silicate vapor atmosphere 28 is ~3 times smaller than the one reported here for V, and is not observed in natural samples 29 . By analogy to Si, equilibrium isotope fractionation is, therefore, also unlikely to account for the V isotopic offset between Earth and the Moon.

Formation of the lunar core could have sequestered some V, although it would not have left the silicate Moon with a lighter V isotope composition than BSE. The lunar core is indeed expected to be more reduced than the silicate Moon 30 , and theoretical considerations of stable V isotope fractionation predict that more reduced forms of V are isotopically lighter 31 . Therefore, lunar core formation would have rendered the silicate Moon heavier than BSE, assuming they both started with a BSE-like isotope composition, which is opposite to the observed difference. In addition, metal-silicate equilibration experiments at 1.5 GPa detected no significant V isotope fractionation 32 , implying that low pressure core formation like that of the Moon would not induce any planetary scale V isotope variation. Regarding terrestrial core formation, it is commonly considered that the main phase of metal segregation (pre-Moon formation) readily accounts for the depletion of V in the silicate Earth 33 , 34 , 35 , with 40–50% of terrestrial V now residing in the core 35 . This depletion is due to the mildly siderophile nature of V in metal-silicate equilibration experiments over a large range of pressures and temperatures 34 , 35 , 36 , which invariably requires large amounts of V to have entered the core throughout Earth accretion. For this reason, it is not likely that post-giant impact terrestrial core formation processes produced the heavy V isotopic signature of the Earth relative to that of the Moon 14 .

Late accretion of <2% chondritic material to Earth after the giant impact 37 appears incapable of inducing any change in the V isotope composition of BSE because chondritic material has V concentrations that are similar to BSE 15 . Post giant impact equilibration between Earth and the lunar debris disk or synestia 2 , 12 is likewise unlikely to account for the observed V isotopic difference. First because the temperatures at which this process would have taken place 2 are so high that stable isotope fractionation should be highly attenuated. Secondly, if V isotopes had been fractionated during such a process, then we would expect to see similar or larger stable isotope effects for many other elements including Mg and Ti, which is not the case 24 , 38 . Instead, we show that the Earth-Moon V isotopic difference can be readily accounted for by mixing between proto-Earth and a chondritic impactor in the framework of canonical giant impact simulations, in which the Moon is dominated by material from Theia 3 , 4 .

Canonical giant impact model

We carried out two-component isotope mixing calculations considering a system with pre-impact (proto-Earth, Theia) and post-impact (Earth, Moon, escaping mass) components 39 (Supplementary Note  3 ). We found that all sizes of Theia previously investigated by dynamic simulations (0.8*M Mars  ≤ M Theia  ≤ 0.45*M Earth , where M Mars , M Theia and M Earth are the masses of Theia, Mars and Earth, respectively) can be reconciled with the observed Δ 51 V BSE-Moon  = 0.181 ± 0.035‰ as long as the Moon contains at least ~60% material from Theia (Fig.  3 ). We note that the calculations assume the silicate portion of Theia to be broadly chondritic, akin to what has been observed for Mars 14 . If Theia had been affected by minor V isotope fractionation during core formation then the results of the mixing calculations would invariably render the fraction of Theia in the Moon higher than what we present here. The minimum amount of Theia material in the Moon is only found when considering the smallest size of Theia combined with the smallest potential V isotopic difference between Earth and Moon, Δ 51 V BSE-Moon  = 0.146‰. Increasing M Theia results in higher fractions of Theia being incorporated into the Moon (e.g., ≥75% for M Theia  = 0.45*M Earth ; Fig.  3 and Table  1 ). The mass balance required to generate the observed Δ 51 V BSE-Moon has been reproduced by the canonical Giant Impact simulations 3 , 4 , large impactor sizes 11 , as well as hit-and-run simulations 40 . The latter two model types, however, tend to produce relative fractions of Theia in the Earth and Moon ( δf T  ≡ [ φ E / φ M  − 1] × 100, where φ E and φ M are the mass fractions of the silicate portions of the Earth and Moon derived from Theia, respectively) that are relatively similar, thus corresponding to δf T ~ 0 ± 30% 11 , 40 . Conversely, the canonical giant impact simulations generally produce significantly more negative values 3 . Our models reveal that it is only possible to reproduce Δ 51 V Earth-Moon  = 0.181 ± 0.035‰ when −100% < δf T  < −40% (Table  1 ). This range of δf T is very rarely obtained for fast spinning proto-Earth 10 , hit-and-run scenarios 40 , and for simulations where Theia is larger than 0.15 M Earth 11 . We therefore conclude that, although it is possible to account for the V isotopic difference between Earth and Moon via multiple different giant impact scenarios, the canonical simulations 3 , 4 provide a far more robust fit with our observations from V isotopes. Our modeling also places constraints on the fraction of Theia (assumed to have a chondritic composition) that ends up in the Moon ( φ M-T ), revealing that only a very small fraction of the total impactor (<14%) is incorporated into the Moon (Table  1 ). These may be important constraints that could guide future numerical simulations of the canonical giant impact.

Results of mass balance calculations for giant impacts where Theia’s mass (MTheia) represents 0.8 or 1.2 times that of Mars (MMars) ( a and b ) or 0.45 Earth masses (Mearth) ( c and d ). It is assumed that Theia was chondritic in composition (δ 51 V = −1.089 ± 0.031; 2SE) and that the silicate portions of proto-Earth and Theia had identical V concentrations. The mass fractions (i) of the Moon deriving from Theia (φ M ) and (ii) of Theia that is incorporated into the Moon (φ T-M  = φ M * M Moon / M Theia ), are shown as a function of Δ 51 V BSE-Moon (Table  1 ). The mass fraction of the present-day Earth that originates from Theia (φ E ) is also displayed in ( b and d ). Details of these mass balance calculations are reported in the Supplementary Note  3 . Given the observed Δ 51 V BSE-Moon  = −0.181 ± 0.035‰ (95% c.i.), it can be seen that the minimum fraction of Theia in the Moon is ~60%, with any value up to 100% being allowed. The range of typical φ M values derived from the canonical giant impact model is 72–88%. The best estimate for φ M (when Δ 51 V BSE-Moon  = −0.181) ranges from 79% when the impactor is 0.8*M Mars to 87% when the impactor is 0.45*M Earth (Table  1 ), in excellent agreement with predictions from the canonical giant impact scenario.

Origin of material in Theia

The characteristic that sets V isotopes apart from other isotopic tracers of the giant impact is the invariant pre-irradiation V isotope composition of all chondrites (i.e., absence of V nucleosynthetic anomalies). Therefore, the Earth-Moon difference for V isotopes, which was most likely established during core-formation processes predating the Moon-forming giant impact 14 , could be explained by Theia containing any type of chondritic material (Fig.  4 ). As emphasized previously 14 , additional experimental investigations of V metal-silicate partitioning at high pressure-temperature conditions, for variable oxygen fugacities and/or chemical compositions will be essential to shed light on V isotopic fractionation processes during terrestrial core formation. However, the fact remains that Earth and the Moon are isotopically very similar for the majority of elements investigated to date 5 , 6 , 7 , 39 . The explanation of the V isotope difference between the Earth and Moon in the framework of the canonical giant impact refutes the possibility of post-impact Earth-Moon equilibration processes (e.g., a synestia model or through alternative impact geometries) 2 , 10 , 11 , 12 . In such scenarios, the V isotope compositions of the Earth and Moon should have been the same, which is not the case. This constraint from V isotopes also implies that the W isotope compositions of the Earth and Moon were most likely not identical in the aftermath of the Giant Impact as recently proposed 41 , although even under these circumstances Monte-Carlo simulations still predict that canonical Giant Impact mixing processes would more likely have produced larger W isotope offsets between Earth and the Moon than what is observed 42 . As such, the small W isotope difference between Earth and the Moon invariably implies that the compositions of Theia and proto-Earth, perhaps somewhat fortuitously 43 , were more similar for W isotopes than most other differentiated planetary bodies in the solar system. In that sense, one reason why elucidating some of the chemical and isotopic characteristics that resulted from the Moon-forming giant impact has proven so difficult may be that it indeed corresponds to a low-probability event, which cannot be readily predicted from a statistical modeling approach. Lastly, we note that the indistinguishable Si isotope compositions of Earth and Moon 29 would most likely reflect similar planetary formation processes for proto-Earth and Theia 44 , 45 , rather than post impact equilibration between the Earth and Moon 29 . We conclude that the most likely explanation for the Earth-Moon isotopic similarity for other isotope systems than V is that their primordial building blocks originated from a common accretionary reservoir in the inner solar system, therefore comprising broadly similar mixtures of chondritic materials 13 . In particular, enstatite chondrites (and aubrite meteorites) represent our best analogue to Earth’s building blocks for many isotope systems such as Ti, O, Cr, and Zr 5 , 6 , 9 , 39 , 46 , and so potentially our best analogue to Theia’s composition as well 13 , 39 . Such a conclusion is also consistent with the recent finding that enstatite-like materials could have been major contributors of terrestrial volatiles 47 , 48 .

Theia and proto-Earth would have mainly accreted from a common accretionary reservoir in the inner solar system (which is required to account for their similar nucleosynthetic inheritance) 5 , 6 , 9 , 39 . Subsequently, the proto-Earth experienced its main phase of core-formation at high pressure and high temperature, which caused the V isotope composition of the BSE to be enriched in 51 V relative to its originally chondritic composition 14 . A canonical giant impact between Theia (chondritic V isotope composition) and proto-Earth would then have produced the present-day Earth-Moon system, with ~80% of the lunar accretionary material deriving from the impactor and essentially no difference in the nucleosynthetic assemblages of the Earth and Moon.

Samples of both lunar meteorite and Apollo mission rocks were dissolved as either 100 mg chips (meteorites), <40 μm fines, or whole rock powders (Apollo samples) using double distilled concentrated mineral acids such as HF, HNO 3 , HCl. Vanadium was separated from the sample matrix using a four-step cation/anion exchange chromatography procedure 15 , 49 . Mass spectrometry to measure V isotope ratios was performed using a Neptune multi-collector inductively coupled plasma mass spectrometer, housed at the Plasma Mass Spectrometry Facility of the Woods Hole Oceanographic Institution (WHOI). Isotope compositions were calculated using standard-sample bracketing with the Alfa Aesar standard that is defined as δ 51 V AA  = 0‰. Each unknown sample was interspersed with a pure V reference solution from BDH Chemicals that has now been measured in four different labs with the identical result of δ 51 V = −1.20‰ 15 , 49 , 50 , 51 . The mass spectrometer was operated in medium resolution mode, which ensured that all significant isobaric interferences in the mass spectrum (48–53 atomic mass units) were resolved from the isotopes of interest: 48 Ti, 49 Ti, 50 V, 51 V, 52 Cr and 53 Cr 51 , 52 . We collected 51 V using a Faraday cup equipped with a 10 10 Ω resistor, whereas Faraday cups with conventional 10 11 Ω resistors were used for all other masses collected. Samples and standards were measured at a concentration of 800 ng/ml V, which produced an ion beam of ~2 nA on 51 V and ~0.005 nA on 50 V. Precision and accuracy of the V isotope measurements was assessed by measuring the BDH standard throughout the study (δ 51 V = −1.21 ± 0.07; n  = 160, 2 SD) and by processing USGS reference materials AGV-2, BCR-2 and BHVO-2 that have previously been analyzed by multiple different laboratories 15 . The vanadium isotope compositions and external reproducibility of these measurements during our study were δ 51 V AGV-2  = −0.77 ± 0.07 ( n  = 12, 2 SD); δ 51 V BHVO-2  = −0.87 ± 0.12 ( n  = 10, 2 SD); δ 51 V BCR-2  = −0.79 ± 0.08 ( n  = 5, 2 SD), which are all in excellent agreement with previous studies 15 , 51 , 53 , 54 , 55 . Blanks were monitored with each batch of samples and were always <2 ng, which is insignificant compared with the 1000 ng minimum amount of V processed.

Elemental concentrations were determined for all samples using a ThermoFinnigan iCap quadrupole ICP-MS (Supplementary Table  1 ), also situated at the WHOI Plasma Facility. Concentrations were calculated via reference to ion beam intensities obtained from a five-point calibration curve constructed from serial dilutions of a gravimetrically-prepared multi-element standard. Drift was monitored and corrected via normalization to indium intensities. Accuracy and precision were determined to be better than ±7% (SD) based on the correspondence of secondary USGS reference materials AGV-2, BCR-2, and BHVO-2 concentrations determined during the same analytical sessions as the lunar rocks.

Data availability

The authors declare that all data supporting the findings of this study are available within the paper and in Supplementary Table  1 .

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Acknowledgements

This study was funded by NASA Emerging Worlds grant NNX16AD36G to S.G.N. We thank NASA-JSC, Tony Irving, and Thorsten Kleine for access to meteorite and Apollo mission samples. US Antarctic meteorite samples are recovered by the Antarctic Search for Meteorites (ANSMET) program, which has been funded by NSF and NASA, and characterized and curated by the Astromaterials Curation Office at NASA Johnson Space Center and the Department of Mineral Sciences of the Smithsonian Institution. J. Blusztajn is thanked for help with mass spectrometry support at WHOI. We also thank Thorsten Kleine, Stephane Le Roux, Rainer Wieler, and Michael Broadley for helpful discussions.

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Study was conceived by S.G.N. Sample processing and V isotope measurements by S.G.N. and M.A. S.G.N. and D.B. interpreted the data and wrote the paper with input from M.A.

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Nielsen, S.G., Bekaert, D.V. & Auro, M. Isotopic evidence for the formation of the Moon in a canonical giant impact. Nat Commun 12 , 1817 (2021). https://doi.org/10.1038/s41467-021-22155-7

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Why won’t this debate about an ancient cold snap die.

Despite mainstream opposition, a controversial comet impact hypothesis persists

WHERE’D THEY GO? About 13,000 years ago, during the Pleistocene Epoch, bison, mammoths (illustrated) and other large mammals roamed North America. Researchers continue to argue over what caused their extinction.

Victor O. Leshyk

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By Carolyn Gramling

June 26, 2018 at 2:00 pm

Around 13,000 years ago, Earth was emerging from its last great ice age. The vast frozen sheets that had covered much of North America, Europe and Asia for thousands of years were retreating. Giant mammals — steppe bison, woolly mammoths and saber-toothed cats — grazed or hunted across tundra and grasslands. A Paleo-Indian group of hunter-gatherers who eventually gave rise to the Clovis people had crossed a land bridge from Asia hundreds of years earlier and were now spread across North America, hunting mammoth with distinctive spears.

Then, at about 12,800 years ago, something strange happened. Earth was abruptly plunged back into a deep chill. Temperatures in parts of the Northern Hemisphere plunged to as much as 8 degrees Celsius colder than today. The cold snap lasted only about 1,200 years — a mere blip, in geologic time. Then, just as abruptly, Earth began to warm again. But many of the giant mammals were dying out. And the Clovis people had apparently vanished.

Geologists call this blip of frigid conditions the Younger Dryas, and its cause is a mystery. Most researchers suspect that a large pulse of freshwater from a melting ice sheet and glacial lakes flooded into the ocean, briefly interfering with Earth’s heat-transporting ocean currents. However, geologists have not yet found firm evidence of how and where this happened, such as traces of the path that this ancient flood traveled to reach the sea ( SN: 12/29/12, p. 11 ).

But for more than a decade, one group of researchers has stirred up controversy by suggesting a cosmic cause for the sudden deep freeze. About 12,800 years ago, these researchers say, a comet — or perhaps its remnants — hit or exploded over the Laurentide Ice Sheet that once covered much of North America ( SN: 6/2/07, p. 339 ).

Pieces of the comet most likely exploded in Earth’s atmosphere, the researchers suggest, triggering wildfires across North America . Those fires would have produced enough soot and other compounds to block out the sun and cool the planet. Most scientists think that a similar aboveground explosion, known as an airburst, happened on a far smaller scale in 1908 over Siberia’s Tunguska region. That event produced as much energy as 1,000 Hiroshima bombs ( SN Online: 7/28/09 ). A similar but even larger cataclysm at the onset of the Younger Dryas, according to the hypothesis’s proponents, would neatly solve several prehistoric puzzles, including what caused the extinctions of large animals and what happened to the Clovis people.

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For more than a decade, scientific journals have been the battleground for skirmishes over this impact hypothesis. The idea has drawn opponents from a spectrum of scientific fields, including paleoclimatology, physics and archaeology. The critics contend that there is little to no reproducible or incontrovertible evidence for many of the key arguments of the hypothesis.

“Over and over and over, there are these things that are claimed to be proxies for an impact,” says Vance Holliday, an archaeologist and geologist at the University of Arizona in Tucson. “And they’re all debatable, every single one.”

Allen West, a retired geophysicist who owned GeoScience Consulting in Dewey, Ariz., has long been a lead proponent of the impact hypothesis. West acknowledges that the hypothesis has been battered on all sides. “We have different battles with different disciplines,” he says. He compares these battles to the fights that raged in the 1980s over whether an asteroid struck Earth 66 million years ago, killing off all dinosaurs except birds — an idea that he notes is now widely accepted.

“There were just vicious, nasty attacks for nearly a decade on that,” West says. “People said it just couldn’t have happened, and then they found the crater. That’s probably what it would take with us, too.”

Indeed, no craters have been found dating to the Younger Dryas, and the landscape of North America — the likely ground zero for such an impact, proponents say — has been pretty thoroughly checked out. In the absence of direct evidence of an impact, West and colleagues have turned to indirect evidence, releasing a steady stream of papers outlining numerous possible signs of an impact, all dating to about 12,800 years ago.

The latest salvo came in March, when West and more than two dozen researchers published a pair of papers in the Journal of Geology . The papers include data from ice cores as well as sediment cores from land and sea . The cores contain signatures of giant wildfires that support the idea of a widespread burning event about 12,800 years ago, West says.

The papers promptly elicited exasperation from some opponents, including Holliday. “We have 10 years of this we have to deal with. They keep building on their past record, ignoring the critiques,” he says. “It just drives me crazy.”

Birth of a hypothesis

The first formal description of the Younger Dryas impact hypothesis came in 2007, when four researchers sat in front of a gaggle of reporters at the American Geophysical Union’s spring meeting in Acapulco, Mexico. The researchers, including West, had taken a close look at about two dozen sites across North America showing a “boundary layer” of sediments dating to the onset of the Younger Dryas. Half a dozen of the sites also have thin layers of organic-rich sediments called “black mats” immediately overlying the boundary layer. Several of those sites show signs of occupation by the Clovis people.

A line in the clay

The mats apparently mark the line between occupation and absence at the Clovis sites: For example, a black mat at a site called Murray Springs, in Arizona, sits above a trove of Clovis artifacts, a fire pit and an almost fully articulated skeleton of an adult mammoth. But above the mat, there are no such artifacts; at Murray Springs and elsewhere, the fluted stone spearpoints made by the Clovis culture disappear from the archaeological record, leading to speculation that the people mysteriously and abruptly vanished.

Those Younger Dryas boundary layers, West and colleagues reported in 2007, contain a variety of intriguing items, including tiny round magnetized grains called microspherules, other magnetized grains of sediment, little spherules of carbon, hollow round carbon molecules called fullerenes and nanodiamonds. Chemical analyses also revealed spikes in iridium and nickel concentrations and in charcoal and soot.

Taken alone, these items may or may not be signs of an extraterrestrial impact: Microspherules, for instance, form when a material heats up and then rapidly cools. They can form during a volcanic eruption, from industrial pollution or as a result of an extraterrestrial impact.

But when taken together, such a suite of markers could point only to an extraterrestrial impact, the researchers concluded: Something struck Earth and exploded in its atmosphere at the onset of the Younger Dryas, about 12,800 years ago. The soot and charcoal suggested that the impact triggered widespread burning that blocked out the sun and brought about a thousand years of near-glacial temperatures in the Northern Hemisphere.

Because no impact crater dating to this time has been found, the researchers suggested that the impactor was probably already fragmented when it entered Earth’s atmosphere. Smaller fragments would have done plenty of damage as they exploded in the atmosphere over the retreating Laurentide Ice Sheet, but they wouldn’t have left much of a smoking hole in the ground.

The news made a splash — and scientists were intrigued ( SN: 6/2/07, p. 339 ). The prospect of layers rich in impact markers found scattered across a continent was definitely worth investigating further. Mark Boslough, a physicist at the University of New Mexico in Albuquerque, says that initially, he took the data at face value. “I thought, ‘they’re on to something interesting.’ ”

Frustrations mount

Then scientists, Boslough included, began to do their own independent analyses. And questions arose. Some researchers claimed that they couldn’t find strong evidence of some of the purported impact markers , such as the microspherules and nanodiamonds. Others questioned the precision of the dating at many of the Younger Dryas boundary layer sites , which would undermine the idea that one event affected them all simultaneously.

Boslough says he took issue with the physics of the proposed impact mechanisms, which have ranged from a single object striking the ice sheet to multiple fragments exploding in the atmosphere. But none of the scenarios make sense, Boslough says. Either the pieces of a fragmented comet would have been too small to generate much energy or they would have been too large not to leave craters , he wrote in 2012.

Holliday, meanwhile, says that when it comes to the apparent disappearance of the Clovis culture, the Younger Dryas impact hypothesis is a solution to an archaeological problem that doesn’t exist. Hunter-gatherers like the Paleo-Indian people who made Clovis points didn’t stay at one site for long; it’s no surprise that they would have moved on, Holliday says.

More important, he adds, “there is no mysterious ‘gap’ in the archaeological record following the time that Clovis artifacts were made.” Immediately following the Clovis period, a different style of projectile points, called Folsom points, appeared. Paleo-Indian peoples probably just changed their spear technology due to a shift from hunting mammoth and mastodon to bison, Holliday says.

As for those large Ice Age animals such as mammoths, he adds, they were in decline, but their disappearance wasn’t that sudden. “All these animals running around and then, boom, at 12,800 years ago they just go away? That’s just not the case,” Holliday says. “These extinctions were global and happened at different times around the world.”

The March papers focus mainly on the wildfires, a long-standing aspect of the original hypothesis. Greenland ice cores show peaks in ammonium dating to the onset of the Younger Dryas, which the researchers say, suggests large-scale biomass burning. These data were previously presented in 2010 by astrophysicist Adrian Melott of the University of Kansas in Lawrence and colleagues. They suggested that the ammonium ions in those ice cores could be best explained by an extraterrestrial impact. A similar spike dating to 1908 — the year of the airburst over Siberia — had also been found in those same cores. The papers also describe finding peaks in charcoal that date to the start of the cold snap.

“The big thing here is a careful comparison of [many possible impact markers], normalized to the same dating method,” says Melott, one of the authors on the new impact papers. Those markers, including previously described evidence of microspherules, iridium and platinum dust, are consistent with having been caused by the same event, he says.

However, Jennifer Marlon, a paleoecologist and paleoclimatologist at Yale University and an expert on biomass burning, has taken her own look at sediments in North America dated to between 15,000 and 10,000 years ago. She sees no evidence for continent-wide fires dating specifically to the onset of the Younger Dryas.

“I’ve studied charcoal records for many years now,” Marlon says. In 2009, she and colleagues reported data on charcoal and pollen in lake sediments across North America. Importantly, the sediment records in her study encompassed not only the years of the Younger Dryas cold episode, but also a few thousand years before and after.

Her team found multiple small peaks of wildfires, but none of them were near the beginning of the Younger Dryas. “Forests burn in North America all the time,” she says. “You can’t find a cubic centimeter of sediment in any lake on this continent that doesn’t have charcoal in it.”

Missing peak

Charcoal records from 15 lake sediment cores from across North America show how often fires occurred at each site over 5,000 years. The records show no peak in burning about 12,800 years ago, as would be expected if there were continent-scale fires. 

Such fires could be triggered by rapid climate change, when ecosystems are quickly reorganizing and more dead fuel might be available. “That can cause major vegetation changes and fires,” Marlon says. “We don’t need to invoke a comet.”

The problem with the data in the recent papers, Marlon says, is that the researchers look only at a narrow time period, making it difficult to evaluate how large or unusual the signals really were. From her data, there appeared to have been more burning toward the end of the Younger Dryas, when the planet began to warm abruptly again.

“That speaks to my fundamental problem with the biomass burning part of the papers,” Marlon says. “I don’t understand why they’re zooming in. It’s what makes me skeptical.”

Holliday echoes that criticism. “Most of the time they sample only around this time interval,” he says. What would be more convincing, he says, are data from cores that span 15,000 to 20,000 years, sampled every five centimeters or so. “If this is a unique event, then we shouldn’t see anything like it in the last 15,000 years.”

West says that other peaks are irrelevant, because the impact hypothesis doesn’t imply that there was only one wildfire, just that one occurred around 12,800 years ago. He adds that the new papers suggest that Marlon and her colleagues didn’t correctly calibrate the radiocarbon dates for their samples. When done correctly, he says, one spike in fires that Marlon estimated at around 13,200 years ago actually occurred several hundred years later — right around 12,800 years ago.

Radiocarbon dating for such old events is challenging regardless of calibration, Marlon says. That’s why she analyzed and compared her sites in several different ways, yet still found no unusual peak at 12,800 years. In fact, she says, many of the sites show no signs of burning at that time.

As for whether the impact hypothesis proponents have ignored scientific criticisms, West rejects this. “We have directly rebutted those criticisms multiple times,” he says. An upcoming paper he and others are preparing will describe in detail those rebuttals, such as errors he says previous critics made in properly reproducing the analyses West and his colleagues used to identify a key impact marker, the magnetic spherules.

Yet critics of the Younger Dryas impact hypothesis say that too many questions remain unanswered. Holliday says he and others are preparing a response to the Journal of Geology papers, outlining numerous points of contention.

“Confronting and dealing with critical reviews and contradictory data is a significant problem in this debate,” Holliday says. None of the rebuttals have dealt with various criticisms, he adds, such as the proper dating of rock layers and soils, and the contradictory data over animal extinctions and Paleo-Indian archaeology.

“We all love a good debate,” Marlon says, “but I know there’s a lot of frustration in the community” that this hypothesis persists. Like many opponents of the impact hypothesis, she says that the data presented in the new papers have done nothing to change her mind about the comet strike. “It didn’t happen.”

All the same, Marlon understands the allure. “I have had pet theories, too. We are pattern-seekers. We tend to see things that look like a signal and so many times they’re not. A comet is simpler and more visually compelling — more appealing, in a way — than trying to sort out what Earthbound trigger might have caused such an abrupt climate change.

“I wish the evidence were stronger for [an impact],” she says. “It’s not as much fun when it turns out to be a more complicated, nuanced story.”

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Researchers have discovered new evidence to support the Younger Dryas Impact Hypothesis, which postulates that a fragmented comet slammed into the Earth close to 12,800 years ago.

When James Kennett and his colleagues set out years ago to examine signs of a major cosmic impact toward the end of the Pleistocene epoch, little did they know just how far-reaching the projected climatic effect would be.

“It’s much more extreme than I ever thought when I started this work,” notes Kennett, a professor of geology at the University of California, Santa Barbara. “The more work that has been done, the more extreme it seems.”

The hypothesis maintains that the impact caused rapid climatic changes, megafaunal extinctions, sudden human population decrease and cultural shifts, and widespread wildfires (biomass burning). The hypothesis suggests a possible triggering mechanism for the abrupt changes in climate at that time, in particular a rapid cooling in the Northern Hemisphere, called the Younger Dryas, amid a general global trend of natural warming and ice sheet melting that show changes in the fossil and sediment record.

Controversial from the time scientists proposed it, those who prefer to attribute the end-Pleistocene reversal in warming entirely to terrestrial causes continue to contest the hypothesis even now. But Kennett and fellow stalwarts of the Younger Dryas Boundary (YDB) Impact Hypothesis, as it is also known, have recently received a major boost: the discovery of a very young, 31-kilometer-wide (19-mile-wide) impact crater beneath the Greenland ice sheet, which they believe may have been one of the many comet fragments that struck Earth at the onset of the Younger Dryas.

Now, in a paper in the journal Nature Scientific Reports , the researchers present further evidence of a cosmic impact, this time far south of the equator, that likely led to biomass burning, climate change, and megafaunal extinctions nearly 13,000 years ago.

‘A major expansion’

“We have identified the YDB layer at high latitudes in the Southern Hemisphere at near 41 degrees south, close to the tip of South America,” Kennett says. This is a major expansion of the extent of the YDB event.” The vast majority of evidence to date, he adds, has been found in the Northern Hemisphere.

This discovery began several years ago, according to Kennett, when a group of Chilean scientists studying sediment layers at a well-known Quaternary paleontological and archaeological site, Pilauco Bajo, recognized changes known to be associated with YDB impact event. They included a “black mat” layer, 12,800 years in age, that coincided with the disappearance of South American Pleistocene megafauna fossils, an abrupt shift in regional vegetation and a disappearance of human artifacts.

“Because the sequencing of these events looked like what had already been described in the YDB papers for North America and Western Europe, the group decided to run analyses of impact-related proxies in search of the YDB layer,” Kennett says. This yielded the presence of microscopic spherules that researchers interpreted as forming by melting due to the extremely high temperatures associated with impact. The layer containing these spherules also show peak concentrations of platinum and gold, and native iron particles rarely found in nature.

“Among the most important spherules are those that are chromium-rich,” Kennett explains. The Pilauco site spherules contain an unusual level of chromium, an element from South America, not in Northern Hemisphere YDB impact spherules.

“It turns out that volcanic rocks in the southern Andes can be rich in chromium, and these rocks provided a local source for this chromium,” he adds. “Thus, the cometary objects must have hit South America as well.”

Other evidence, which, Kennett notes, is consistent with previous and ongoing documentation of the region by Chilean scientists, pointed to a “very large environmental disruption at about 40 degrees south.” These included a large biomass burning event that, among other things, micro-charcoal and signs of burning in pollen samples researchers collected at the impact layer shown.

“It’s by far the biggest burn event in this region we see in the record that spans thousands of years,” Kennett says. Furthermore, he continues, the burning coincides with the timing of major YDB-related burning events in North America and western Europe.

Climate shifts

The sedimentary layers at Pilauco contain a valuable record of pollen and seeds that show change in character of regional vegetation—evidence of a shifting climate. However, in contrast to the Northern Hemisphere, where conditions became colder and wetter at the onset of the Younger Dryas, the opposite occurred in the Southern Hemisphere.

“The plant assemblages indicate that there was an abrupt and major shift in the vegetation from wet, cold conditions at Pilauco to warm, dry conditions,” Kennett says.

According to Kennett, the atmospheric zonal climatic belts shifted “like a seesaw,” with a synergistic mechanism, bringing warming to the Southern Hemisphere even as the Northern Hemisphere experienced cooling and expanding sea ice. The rapidity—within a few years—with which the climate shifted is best attributed to impact-related shifts in atmospheric systems, rather than to the slower oceanic processes, Kennett says.

Meanwhile, the impact with its associated major environmental effects, including burning, is thought to have contributed to the extinction of local South American Pleistocene megafauna—including giant ground sloths, sabretooth cats, mammoths, and elephant-like gomphotheres—as well as the termination of the culture similar to the Clovis culture in the north, he adds. The amount of bones, artifacts, and megafauna-associated fungi that were relatively abundant in the soil at the Pilauco site declined precipitously at the impact layer, indicating a major local disruption.

The distance of this recently identified YDB site—about 6,000 kilometers (around 3728 miles) from the closest well-studied site in South America—and its correlation with the many Northern Hemispheric sites “greatly expands the extent of the YDB impact event,” Kennett says.

The sedimentary and paleo-vegetative evidence researchers gathered at the Pilauco site is in line with previous, separate studies Chilean scientists conducted that indicate a widespread burn and sudden major climate shifts in the region at about YDB onset. This new study further bolsters the hypothesis that a cosmic impact triggered the atmospheric and oceanic conditions of the Younger Dryas, he says.

“This is further evidence that the Younger Dryas climatic onset is an extreme global event, with major consequences on the animal life and the human life at the time,” Kennett says. “And this Pilauco section is consistent with that.”

Additional researchers contributing to the work came from Universidad Austral de Chile, Universidad Católica de Temuco, Elizabeth City State University, University of South Carolina, Northern Arizona University, DePaul University, Comet Research Group.

Source: UC Santa Barbara

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Evidence behind Giant Impact hypthesis

The giant impact hypothesis is a theory to explain how the moon was formed. What evidence is there that supports the theory and led us to find the current theory?

• 1 $\begingroup$ I don't think the answer which answered that question would sufficiently answer this question, as there is no explanation as to why the things mentioned are evidence for the theory. $\endgroup$ –  Mitch Goshorn Commented Oct 23, 2014 at 12:53
• $\begingroup$ I'm seconding Mitch here. The only answer there is entirely based on theory (which isn't a bad thing, but that means that it doesn't cover this question). By the way, why was a) the question answered by the asker and b) asked and answered on the community wiki? $\endgroup$ –  HDE 226868 ♦ Commented Oct 23, 2014 at 21:58
• $\begingroup$ Fair enough, will withdraw my close vote $\endgroup$ –  user2449 Commented Oct 24, 2014 at 6:47

The smoking guns:

• The ratio of oxygen isotopes of lunar rocks are almost identical to those of Earth.
• Lunar recession due to the tides which causes the Earth's rotation to slow down, means that just after the Earth formed, the Moon was very close to the Earth and the Earth was rotating very fast. This situation can be reached due to an oblique impact that transfers a lot of angular momentum to the Earth.
• The formation of the planets is known to have proceeded via small objects growing larger by colliding with each other, this means that the last stages of planet formation would have involved collisions between large objects. It is to be expected that around the time the Earth was taking shape, there could also be a proto-planet at the Lagrange point 60 degrees away from Earth in the same orbit. On the long term this is an unstable situation which ends with a collision that according to simulations is of the sort that typically leads to the formation of the Moon.
• The asymmetry in the geology between the far side and the near side of the Moon. (there are many more maria on the near side compared to the far side). If the Moon had indeed been very close to the Earth after it formed, then the near side would have stayed very hot due to the Earth's surface still being molten and the radiant heat affecting the near side of the Moon. An impact on the near side of the Moon would far more easily be able to penetrate the solid crust and cause magma to flow to the surface compared to the far side of the Moon.

• 1 $\begingroup$ @HDE226868 I agree that point #2 is potentially the most compelling bit of evidence, but I also think associated figures and references shouldn't be omitted for this point. $\endgroup$ –  David H Commented Oct 25, 2014 at 22:33
• $\begingroup$ @DavidH Ah, shoot. I didn't mean that no references were needed - they certainly are. I didn't stop to think that there weren't any. $\endgroup$ –  HDE 226868 ♦ Commented Oct 25, 2014 at 22:36
• 1 $\begingroup$ #4: Surely the moon wasn't tidally locked while the Earth was cooling, was it? At what point in Earth's history do we believe it became tidally locked? $\endgroup$ –  Scottie Commented Oct 27, 2014 at 13:59

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New Evidence Is Forcing Scientists to Reconsider How The Moon Was Formed

For decades, scientists have been debating what it would have looked like when a chunk of Earth broke off and formed our Moon some 4.5 billion years ago. 

And now new chemical evidence suggests that things were way more violent than we'd assumed, with researchers suggesting that the impact that set our Moon free was "like a sledgehammer hitting a watermelon".

It's well-established that the Moon was once a part of Earth before it was sloughed off the side and thrown into our orbit, but the circumstances in which this 'great uncoupling' occurred has been a topic of heated debate.

Until recently, the most widely accepted hypothesis for how the Moon was formed suggested that a Mars-sized object (sometimes called Theia) once collided with the still-developing Earth, about 20 to 100 million years after the Solar System first came together.

While our young planet appears to have come out of the collision fairly unscathed, the impact would have caused Theia's core and most of its mantle to sink into and merge with Earth's own core and mantle. 

Of the remaining dust and debris that were ejected into Earth's orbit, a small accretion disc was formed, and from this, our Moon eventually took shape.

While this encounter might sound pretty violent, the consensus among scientists for almost five decades has been that Theia made a fairly low-energy graze across the surface of Earth.

This hypothesis, known as 'the giant impact' , went on to explain all kinds of other things - such as the large size of the Moon relative to Earth, and their separate rotation rates - and there's a whole lot of evidence to support it.

But there was always one big problem with this hypothesis. It would make sense that a large portion of the material that makes up the Moon would have come from Theia, but chemical analyses on samples brought back by the Apollo missions in the 1970s indicated that Earth and lunar rocks were nearly identical.

Simulation after simulation of the impact predicted that most of the material ( 60 to 80 percent ) that formed the Moon would have come from the impactor, rather than from Earth, and it was extremely unlikely that Earth and Theia had the same chemical make-up.

Fast-forward to now, and geochemists from Harvard and Washington University are reporting that a new, more detailed analysis of seven Moon rocks and eight Earth rocks didn't clear things up like they were expecting - it actually blew the giant impact hypothesis right out of the water.

"We're still remeasuring the old Apollo samples from the '70s, because the tech has been developing in recent years," one of the team, Kun Wang from Washington University, told Ria Misra from Gizmodo .

"We can measure much smaller differences between Earth and the Moon, so we found a lot of things we didn't find in the 1970s. The old models just could not explain the new observations."

In fact, not only did the new analysis find no new evidence of materials that could have come from something other than Earth - it actually suggested that the origins of these Moon rocks were even more tightly bound to Earth than we thought.

And there was another neat little detail in there. Every single isotopic signature in the chemical analysis matched up to both Earth and the Moon, except for one: heavy-potassium isotope in the lunar samples.

In order for this heavy-potassium isotope to appear separately in the lunar rocks, they must have sustained some incredibly hot temperatures , and from this, the team suggests that the Moon-forming collision was a whole lot more violent than we could ever have imagined.

As Loren Grush explains over at The Verge:

"The collision that formed the Moon wasn't low energy at all, [Wang] argues. Instead, the impact was extremely violent, pulverising most of Earth and the impactor, and turning them into a vapour. In this scenario, the vaporised Earth and impactor mix together into a giant dense atmosphere. This atmosphere then cools and condenses into our planet and its satellite."

It's an incredibly bold claim, because not only does it suggest we were wrong about how our own Moon formed, but it paints a picture of a far more violent and volatile early Solar System than we thought. 

While no one's come out to dispute the claims outright, the onus is now on Wang and his team to make their hypothesis more convincing and weighted in evidence than the one we've been carrying around for almost 50 years.

And that involves demonstrating how seven lunar samples high in heavy-potassium isotope can accurately represent the Moon's overall potassium composition.

"I'm very pleased overall with what they have done, I just wish they had used better samples," Munir Humayun, a geologist at Florida State University who was not involved in the study,  told The Verge,  adding that there's not enough data to support the hypothesis just yet. 

Wang himself doesn't seem too fazed by the criticism, saying every new hypothesis takes time to settle in and become accepted as the evidence mounts around it. 

"It took people decades to accept this giant-impact hypothesis,"  he says . "Now we're saying that [the] giant impact hypothesis is not right, so it may take 10 to 20 years to accept the new model."

Only time will tell if his version of the Moon origin story will hold up to scrutiny.

The study has been published in  Nature .