what is the great impact hypothesis

We have completed maintenance on Astronomy.com and action may be required on your account. Learn More

• Login/Register
• Solar System
• Exotic Objects
• Upcoming Events
• Deep-Sky Objects
• Observing Basics
• Telescopes and Equipment
• Astrophotography
• Space Exploration
• Human Spaceflight
• Robotic Spaceflight
• The Magazine

Giant Impact Hypothesis: An evolving legacy of Apollo

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.

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.

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.

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.

Ancient star streams, named Shiva and Shakti, may be Milky Way’s earliest building blocks

Spectacular meteor lights up the skies over Portugal and Spain 

2024 Full Moon calendar: Dates, times, types, and names

New exoplanet discovered in habitable zone of a multi-star system 

JWST discovers black holes merging near the dawn of the cosmos

Found: An Earth-sized exoplanet named SPECULOOS-3 b

NASA’s asteroid Bennu samples have rocks unlike any meteorite ever found

‘Hairy’ black holes may get a massive glow-up from the LISA spacecrafts

How old is each planet in our solar system?

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.

Gear-obsessed editors choose every product we review. We may earn commission if you buy from a link. Why Trust Us?

• 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.

.css-cuqpxl:before{padding-right:0.3125rem;content:'//';display:inline;} Moon and Mars .css-xtujxj:before{padding-left:0.3125rem;content:'//';display:inline;}

An Unsettling New Image Shows ‘Spiders’ on Mars

Human-Made Space Debris Could Crash on Mars

How to View the Solar Eclipse

How a Lunar Supercollider Could Upend Physics

The Moon Is Going Nuclear

Private Company to Mine Helium-3 from the Moon

3 Moons Emerged From Around Neptune and Uranus

4 People Are Spending a Year In a Martian World.

America Has Planted Its Feet on the Moon Once More

The Strange Origin of the Hollow Moon Conspiracy

The Moon Is Shrinking Where We're Trying to Land

• Giant Impact Hypothesis: The Theory of Moon’s Origin

Alex Bennet

Table of Contents

Understanding the Giant Impact Hypothesis

The  Giant Impact Hypothesis  is a fascinating theory that has been proposed to explain the origin of our Moon. This hypothesis suggests that approximately 4.5 billion years ago, a Mars-sized body named Theia collided with the young Earth in a cataclysmic event. The impact was so powerful that it ejected a significant amount of debris into space. This debris, composed of parts of both Theia and Earth’s mantle, eventually coalesced to form the Moon. The Giant Impact Hypothesis is currently the most widely accepted explanation for the Moon’s origin due to the compelling evidence that supports it. It provides a captivating insight into the violent and chaotic conditions of the early Solar System .

The Cataclysmic Event: Theia’s Collision

The Giant Impact Hypothesis proposes a cataclysmic event that forever changed the face of our planet and led to the creation of our Moon. This event, known as Theia’s Collision, is believed to have occurred approximately 4.5 billion years ago, during the early stages of the Solar System’s formation.

Theia, named after the Titan goddess of sight and the shining ether of the bright, blue sky in Greek mythology, was a Mars-sized body that likely formed in the inner Solar System. Due to gravitational interactions, Theia was set on a collision course with Earth. This wasn’t a slow, gradual event – it was a high-speed impact, with Theia crashing into Earth at a velocity of several kilometers per second.

The collision was not a direct hit but a glancing blow, which had significant implications for the outcome. A direct hit might have resulted in the complete destruction of both bodies, but the glancing blow allowed both Theia and a significant portion of Earth’s mantle to be ejected into space. This ejected material did not simply disperse into the void of space; instead, it went into orbit around the Earth, forming a ring of debris.

Here’s a step-by-step breakdown of the event:

• The Approach:  Theia, likely formed in the inner Solar System, was set on a collision course with Earth due to gravitational interactions.
• The Collision:  Theia struck the Earth in a glancing blow, causing a significant portion of Earth’s mantle and Theia to be ejected into space.
• The Aftermath:  The ejected material formed a debris disk around Earth.

The energy released during this colossal impact was immense, generating heat and melting a part of Earth’s crust. The Earth itself was set spinning at a much faster rate than today, its day lasting only a few hours. Theia, on the other hand, was completely destroyed, its remnants forming part of the debris disk.

Theia’s Collision was a defining moment in Earth’s history. It was a violent and chaotic event, but one that ultimately led to the formation of our Moon. The Giant Impact Hypothesis provides a captivating insight into the violent and chaotic conditions of the early Solar System. It underscores the dynamic and often violent processes that can lead to the formation of celestial bodies. As we continue to explore the mysteries of our universe, our understanding of the Moon’s origin and the processes that shaped our Solar System continues to evolve.

Aftermath of the Impact

The aftermath of the cataclysmic event proposed by the Giant Impact Hypothesis led to the formation of our Moon. Here’s a detailed breakdown of how this process unfolded:

• Formation of the Debris Disk:  The impact resulted in a ring of debris around the Earth, primarily composed of material from both Theia and Earth’s mantle. This debris did not simply disperse into the void of space; instead, it went into orbit around the Earth, forming a debris disk.
• Accretion of the Moon:  Over time, the debris in the disk began to coalesce and form larger bodies. This process, known as accretion, eventually led to the formation of the Moon. The Moon initially formed close to the Earth and has been gradually moving away ever since.
• Cooling and Differentiation:  After its formation, the Moon was likely covered in a “ magma ocean ,” which eventually cooled and solidified. During this cooling process, the Moon underwent a process called differentiation, where heavier materials sank to the core while lighter materials rose to the surface, forming the Moon’s crust.
• Late Heavy Bombardment:  About 500 million years after the Moon’s formation, it experienced a period known as the Late Heavy Bombardment, where a large number of asteroids impacted the Moon, creating many of its largest craters and basins.
• Evolution of the Lunar Surface:  Over billions of years, the Moon’s surface has been shaped by processes such as impact cratering, volcanic activity, and tectonic movements . The lunar surface we see today is a snapshot of its long and complex geological history.

The Giant Impact Hypothesis not only provides a plausible explanation for the Moon’s formation but also offers insights into the violent and chaotic conditions of the early Solar System. It underscores the dynamic and often violent processes that can lead to the formation of celestial bodies. As we continue to explore the mysteries of our universe, our understanding of the Moon’s origin and the processes that shaped our Solar System continues to evolve.

Comparisons with Other Theories: How Does the Giant Impact Hypothesis Stand Out?

The Giant Impact Hypothesis has significant implications for our understanding of planetary science. It provides a captivating insight into the violent and chaotic conditions of the early Solar System and underscores the dynamic and often violent processes that can lead to the formation of celestial bodies.

• Insights into the Early Solar System:  The hypothesis provides insights into the violent and chaotic conditions of the early Solar System. The proposed collision between Earth and Theia gives us a glimpse into the tumultuous environment that characterized the early stages of our Solar System’s formation. This was a time when celestial bodies were frequently colliding, leading to the formation of planets and their satellites.
• Understanding Planetary Formation:  The Giant Impact Hypothesis offers a case study of how planetary bodies can form through impacts, a process known as accretion. The formation of the Moon from the debris of the Earth-Theia collision is a prime example of this process. This helps us understand how other celestial bodies in our Solar System and beyond might have formed.
• Astronomy and Public Interest:  The Giant Impact Hypothesis also has implications for the field of astronomy and public interest in space science. The dramatic scenario of a giant impact leading to the formation of the Moon can spark interest and curiosity, encouraging more people to learn about astronomy. For those interested in observing the Moon and other celestial bodies, there are many  best telescopes for beginners  available in the market. These telescopes can provide a closer look at the Moon’s surface, allowing beginners to connect more deeply with the concepts of planetary science.
• Educational Implications:  The Giant Impact Hypothesis is often taught in schools as part of the curriculum in Earth Science and Astronomy classes. Understanding this hypothesis and the evidence supporting it can help students develop critical thinking skills. It encourages them to understand how scientists use observations and theoretical models to explain natural phenomena.
• Future Research:  The Giant Impact Hypothesis continues to be a subject of intense research. As we develop more sophisticated technology and embark on missions to return to the Moon, we may be able to gather more evidence to support or refine this hypothesis. This ongoing research not only helps us understand our Moon’s history but also sheds light on the broader processes that govern the evolution of our Solar System.

As we continue to explore the mysteries of our universe, our understanding of the Moon’s origin and the processes that shaped our Solar System continues to evolve. The Giant Impact Hypothesis serves as a reminder of the dynamic and sometimes violent nature of these processes. It invites us to keep questioning, keep exploring, and keep looking up at the night sky with wonder and curiosity.

In conclusion, the Giant Impact Hypothesis offers a fascinating glimpse into the violent and dynamic processes that shaped our Solar System. It underscores the importance of continual exploration and study of  astronomical phenomena  to enhance our understanding of the universe. As we continue to probe the mysteries of space, theories like the Giant Impact Hypothesis serve as crucial guides in our quest for knowledge. They remind us of the dynamic nature of the cosmos and inspire us to keep looking up and wondering about the celestial dance that has been unfolding for billions of years. Keep exploring, the universe awaits! 

What Is the Giant Impact Hypothesis?

The Giant Impact Hypothesis is a theory that suggests the Moon was formed from the debris left over after a colossal collision between Earth and a Mars-sized body named Theia.

Who Proposed the Giant Impact Hypothesis and When?

The Giant Impact Hypothesis was first proposed in the mid-1970s by a group of scientists including William K. Hartmann, Roger J. Phillips, G. Jeffrey Taylor, and others.

What Evidence Supports the Giant Impact Hypothesis?

Several pieces of evidence support the Giant Impact Hypothesis. These include the Moon’s orbital characteristics, the similarities and differences in the composition of the Earth and Moon, and computer simulations that show how a giant impact could have led to the current Earth-Moon system.

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Latest Posts

• What Can You Do With an Astronomy Degree? (Comprehensive Guide)
• Astronomical Phenomena: Unraveling the Secrets of the Cosmos
• 10 Best Telescopes for Beginners in 2023
• What is Zenith in Astronomy? Exploring the Highest Point in the Sky

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.

Leave a Comment Cancel reply

Privacy overview.

'Giant impact' theory of moon's formation gets another boost

Scientists have found yet more differences between Earth and moon rocks.

Scientists have found fresh evidence in lunar rocks showing that the moon was likely formed after a Mars-sized planet crashed into the proto-Earth more than 4 billion years ago.

A NASA-led team examined moon rocks brought back to Earth by Apollo astronauts more than 50 years ago. Investigating the samples with advanced tools not available to researchers in the 1960s and 1970s, the team found further evidence of the "giant impact theory" by focusing on the amount and type of chlorine in the rocks, a new study reports.

The researchers discovered the moon has a higher concentration of "heavy" chlorine compared to Earth , which sports more "light" chlorine. The terms "heavy" and "light" refer to versions of the chlorine atom, known as isotopes, that contain different numbers of neutrons in their nuclei.

Related: How the moon formed: 5 wild lunar theories

Shortly after the mammoth collision occurred, Earth was just able to stay together while pieces of both planets that were blasted into space coalesced to form the moon . Both of these blobby bodies had a mix of light and heavy chlorine isotopes at first, but that mix began to change as Earth's gravity pulled on the newly forming moon.

As the cosmic bodies continued taking new shape after the crash, Earth tugged away the lighter chlorine toward itself, leaving the harder-to-move heavy chlorine on the moon. This left the moon depleted of lighter chlorine compared to the heavier isotope.

"There’s a huge difference between the modern elemental makeup of the Earth and moon, and we wanted to know why," study co-author Justin Simon, a NASA planetary scientist, said in a statement . "Now, we know that the moon was very different from the start, and it's probably because of the 'giant impact' theory."

Get the Space.com Newsletter

Breaking space news, the latest updates on rocket launches, skywatching events and more!

The scientists also checked their understanding by looking at other elements that are halogens, in the same chemical family as chlorine. Other "light" halogens are also less abundant on the moon, and the team could not see any pattern that would suggest a later event caused the loss. 

The new study was published this month in the Proceedings of the National Academy of Sciences. It was led by Anthony Gargano, a graduate fellow at NASA's astromaterials research and exploration science division at the Johnson Space Center in Houston. 

The research adds to a growing mountain of chemical evidence to support the giant impact hypothesis, which was first suggested decades ago. For example, a study released in March of this year used high-precision measurements of oxygen isotopes to show that Earth and moon rocks are probably even more different from each other than previously thought.

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, " Why Am I Taller ?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

Science and music festival Starmus VII is about to rock Bratislava with a stellar lineup

China's Chang'e 6 mission to collect samples of the far side of the moon enters lunar orbit (video)

Space Trash Signs project creates debris 'constellations' to highlight space junk problem (video)

• rod The space.com report states "Shortly after the mammoth collision occurred, Earth was just able to stay together while pieces of both planets that were blasted into space coalesced to form the moon. Both of these blobby bodies had a mix of light and heavy chlorine isotopes at first, but that mix began to change as Earth's gravity pulled on the newly forming moon." My observation, a great deal of Theia original chemical composition and the proto-Earth is required to be known in the giant impact model to explain such chemical differences. Oxygen isotopes must be explained that Theia and the proto-Earth had a similar chemical composition when they formed in the protoplanetary disks, Oxygen Isotopes and the Moon-Forming Giant Impact, https://ui.adsabs.harvard.edu/abs/2001Sci...294..345W/abstract, October 2001. Theia elements are not yet identified in lunar rocks, Identification of the giant impactor Theia in lunar rocks, https://ui.adsabs.harvard.edu/abs/2014Sci...344.1146H/abstract, June 2014 Carbon in the Moon is a problem too for Theia impact, "The findings by the researchers suggest that the moon has a large amount of ancient carbon beneath its surface, and it has likely been there since the moon was formed. How it could have persisted on a very hot early moon remains a mystery.", Carbon emissions on the moon put theory of moon birth in doubt, https://phys.org/news/2020-05-carbon-emissions-moon-theory-birth.html Metal content differences cause problems too, Higher concentration of metal in Moon's craters provides new insights to its origin, https://phys.org/news/2020-07-higher-metal-moon-craters-insights.html The giant impact model is the only game in town now it seems but studies continue. I note that the Moon forms after Theia impact near 3 earth radii and moves out to some 10 earth radii over a short period of time. However, testing and showing the Moon actually orbited Earth so close has not been done. Reply
• Geomartian I doubt if more than 2% of the recovered moonrocks are more than 3 billion years old. Likely about 50% of Houston’s moon rocks are around 300 million years old or less. Ordinary surface bombardment will liberate a higher number of lighter isotopes than heavier isotopes. Chlorine vaporizes at a low temperature and the lighter isotopes will have a higher velocity so they will have a higher probability of reaching lunar escape velocity. This study is a nothing burger. The surface material is pretty worthless for determining the moon’s bulk isotopic properties. When will Houston publish the Uranium content and isotopic composition of the Lunar KREEP component? Houston does not publish this data since it would look a lot like the Uranium signature in black shale (the source rock for oil). The heavier uranium isotopes found in black shale are a marker for a very high energy event which blasted the lighter uranium isotopes from the Earth’s surface. That event was likely an interstellar asteroid impact since higher impact velocities equate to higher energies and impact temperatures. Reply
• drzurf I am so thankful for our moon. Reply
• Lovethrust It’s really hard to see any other way it could have happened. The details still need hashed out but as of now this theory is the only game in town. Reply
• View All 4 Comments

Most Popular

• 2 Enchanting new Hubble Telescope image reveals an infant star's sparkle
• 3 Blue Origin launches 1st Black astronaut candidate, age 90, and 5 space tourists on New Shepard rocket (video)
• 4 'Star Wars: The Phantom Menace' at 25: Who are the angels on the moons of Iego?
• 5 Everything we know about 'A Quiet Place: Day One'

Subscribe or renew today

Every print subscription comes with full digital access

Science News

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

Share this:

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.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

Thank you for signing up!

There was a problem signing you up.

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.”

More Stories from Science News on Climate

‘The High Seas’ tells of the many ways humans are laying claim to the ocean

As the Arctic tundra warms, soil microbes likely will ramp up CO 2 production

A new approach to fighting wildfires combines local knowledge and AI

A ruinous hailstorm in Spain may have been supercharged by warming seas

Three reasons why the ocean’s record-breaking hot streak is devastating

Will stashing more CO 2 in the ocean help slow climate change?

A rapid shift in ocean currents could imperil the world’s largest ice shelf

A new U.S. tool maps where heat will be dangerous for your health

Subscribers, enter your e-mail address for full access to the Science News archives and digital editions.

Not a subscriber? Become one now .

Giant Impact Hypothesis

• Living reference work entry
• First Online: 13 July 2017
• Cite this living reference work entry

• Hidenori Genda 4  

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

111 Accesses

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...

This is a preview of subscription content, log in via an institution to check access.

Access this chapter

Institutional subscriptions

Agnor CB, Canup RM, Levison HF (1999) On the character and consequences of large impacts in the late stage of terrestrial planet formation. Icarus 142:219–237

Article   Google Scholar  

Benz W, Anic A, Horner J, Whitby JA (2007) The origin of mercury. Space Sci Rev 132:189–202

Boss AP (1986) The origin of the moon. Science 231:341–345

Cameron AGW, Ward WR (1976) The origin of the moon. Lunar and Planetary Science Conference, Houston , pp 120–122

Google Scholar  

Canup RM (2004) Dynamics of lunar formation. Annu Rev Astron Astrophys 42:441–475

Canup RM (2005) A giant impact origin of Pluto-Charon. Science 307:546–550

Canup RM (2012) Forming a moon with an earth-like composition via a Giant impact. Science 338:1052–1055

Chambers JE, Wetherill GW (1998) Making the terrestrial planets: N -body integrations of planetary embryos in three dimensions. Icarus 136:304–327

Citron RI, Genda H, Ida S (2015) Formation of Phobos and Deimos via a giant impact. Icarus 252:334–338

Clayton RN (1993) Oxygen isotopes in meteorites. Annu Rev Earth Planet Sci 21:115–149

Ćuk M, Stewart ST (2012) Making the moon from a fast-spinning earth: a giant impact followed by resonant despinning. Science 338:1047–1052

Hartmann WK, Davis DR (1975) Satellite-sized planetesimals and lunar origin. Icarus 24:504–514

Hartmann WK, Phillips RJ, Taylor GJ (eds) (1986) Origin of the moon. Houston, Lunar Planetary Science Institute, 781pp

Hayashi C, Nakazawa K, Nakagawa Y (1985) Formation of the solar system. In: Black DC, Matthews MS (eds) Protostars and planets II. University of Arizona Press, Tucson, pp 1100–1153

Hui H, Peslier AH, Zhang Y, Neal CR (2013) Water in lunar anorthosites and evidence for a wet early moon. Nat Geosci 6:177–180

Ida S, Canup RM, Stewart GR (1997) Lunar accretion from an impact-generated disk. Nature 389:353–357

Kleine T, Touboul M, Bourdon B, Nimmo F, Mezger K, Palme H, Jacobsen SB, Yin Q-Z, Halliday AN (2009) Hf–W chronology of the accretion and early evolution of asteroids and terrestrial planets. Geochim Cosmochim Acta 73:5150–5188

Kokubo E, Genda H (2010) Formation of terrestrial planets from protoplanets under a realistic accretion condition. Astrophys J 714:L21–L25

Kokubo E, Ida S (1998) Oligarchic growth of protoplanets. Icarus 131:171–178

Lissauer JJ (1987) Timescales for planetary accretion and the structure of the protoplanetary disk. Icarus 69:249–265

Lugmair GW, Shukolyukov A (1998) Early solar system timescales according to 53 Mn- 53 Cr systematics. Geochim Cosmochim Acta 62:2863–2886

Melosh HJ (2014) New approaches to the Moon’s isotopic crisis. Phil Trans R Soc A 372:20130168

O’Brien DP, Walsh KJ, Morbidelli A, Raymond SN, Mandell AM (2014) Water delivery and giant impacts in the “grand tack” scenario. Icarus 239:74–84

Ogihara M, Ida S (2009) N-body simulations of planetary accretion around M dwarf stars. Astrophys J 699:824–838

Pahlevan K, Stevenson DJ (2007) Equilibration in the aftermath of the lunar-forming giant impact. Earth Planet Sci Lett 262:438–449

Rosenblatt P, Charnoz S, Dunseath KM, Terao-Dunseath M, Trinh A, Hyodo R, Genda H, Toupin S (2016) Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons. Nat Geosci 9:581–583

Safronov VS (1969) Evolution of the protoplanetary cloud and formation of the Earth and Planets. Moscow. (Translation, 1972, NASA TT F-677)

Sekine Y, Genda H, Kamata S, Funatsu T (2017) The Charon-forming giant impact as a source of Pluto’s dark equatorial regions. Nature Astronomy 1:0031

Stevenson DJ (1987) Origin of the moon – the collision hypothesis. Annu Rev Earth Planet Sci 15:271–315

Touboul M, Kleine T, Bourdon B, Palme H, Wieler R (2007) Late formation and prolonged differentiation of the moon inferred from W isotopes in lunar metals. Nature 450:1206–1209

Touma J, Wisdom J (1998) Resonances in the early evolution of the earth-moon system. Astron J 115:1653–1663

Warren PH (1985) The magma ocean concept and lunar evolution. Annu Rev Earth Planet Sci 13:201–240

Wetherill GW (1985) Occurrence of giant impacts during the growth of the terrestrial planets. Science 228:877–879

Wiechert U, Halliday AN, Lee D-C, Snyder GA, Taylor LA, Rumble D (2001) Oxygen isotopes and the moon-forming giant impact. Science 294:345–348

Wisdom J, Tian Z (2015) Early evolution of the earth-moon system with a fast– spinning earth. Icarus 256:138–146

Wood JA (1986) Moon over Mauna Loa: a review of hypotheses of formation of Earth’s moon. In: Hartmann WK (ed) Origin of the moon. Lunar & Planetary Institute, Houston, pp 17–55

Young ED, Kohl IE, Warren PH, Rubie DC, Jacobson SA, Morbidelli A (2016) Oxygen isotopic evidence for vigorous mixing during the moon-forming giant impact. Science 351:493–496

Zhang J, Dauphas N, Davis AM, Leya I, Fedkin A (2012) The proto-earth as a significant source of lunar material. Nat Geosci 5:251–255

Download references

Author information

Authors and affiliations.

Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Tokyo, Japan

Hidenori Genda

You can also search for this author in PubMed   Google Scholar

Corresponding author

Correspondence to Hidenori Genda .

Editor information

Editors and affiliations.

Department of Earth, Cornell University Department of Earth, Ithaca, New York, USA

William M. White

Rights and permissions

Reprints and permissions

Copyright information

© 2017 Springer International Publishing AG

About this entry

Cite this entry.

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

Download citation

DOI : https://doi.org/10.1007/978-3-319-39193-9_338-1

Received : 22 August 2016

Accepted : 18 June 2017

Published : 13 July 2017

Publisher Name : Springer, Cham

Print ISBN : 978-3-319-39193-9

Online ISBN : 978-3-319-39193-9

eBook Packages : Springer Reference Earth and Environm. Science Reference Module Physical and Materials Science Reference Module Earth and Environmental Sciences

• Publish with us

Policies and ethics

• Find a journal
• Track your research