Giant-impact hypothesis facts for kids

Artist's picture of a huge crash between two space objects. A similar crash between early Earth and a planet the size of Mars probably created our Moon.

The giant-impact hypothesis, also called the Big Splash or Theia Impact, is a leading scientific idea about how the Moon was formed. It suggests that the Moon was made from the leftover pieces of a massive crash. This crash happened between the early Earth and a Mars-sized planet about 4.5 billion years ago. This was during the Hadean eon, roughly 20 to 100 million years after our Solar System first came together.

The planet that crashed into Earth is sometimes called Theia. This name comes from a mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Recent studies of lunar rocks, published in 2016, suggest that the impact might have been a direct hit. This powerful collision would have broken up and thoroughly mixed both planets.

The giant-impact hypothesis is currently the most popular scientific hypothesis for how the Moon came to be. There's a lot of evidence that supports this idea:

• Earth's spin and the Moon's orbit around Earth are lined up in similar ways.
• The Earth–Moon system has a very high amount of angular momentum. This is the energy stored in how Earth spins, how the Moon spins, and how the Moon goes around Earth. This amount is much higher than for other rocky planets. A giant impact could have given the system this extra energy.
• Samples from the Moon show that it was once completely melted, like a sea of hot liquid rock. This would have needed a lot of energy, more than what's usually expected from a body the Moon's size forming. A super energetic event, like a giant impact, could provide this heat.
• The Moon has a small iron core. This makes the Moon less dense than Earth. Computer models of a Mars-sized body hitting Earth show that the impactor's core would likely sink into Earth's core. This would leave the Moon, formed from the scattered pieces, with less iron than other planets.
• The Moon has fewer volatile elements (elements that easily turn into gas) compared to Earth. These elements would have turned into gas and escaped during a high-energy event. The Moon's smaller gravity couldn't pull them back, while Earth's stronger gravity could.
• Scientists have found signs of similar collisions in other star systems. These collisions have created rings of debris.
• Giant collisions fit well with the main theory of how the Solar System formed and changed.
• The amounts of different isotopes in Moon rocks and Earth rocks are almost exactly the same. This suggests they came from the same original material.

However, there are still some questions about the best models of the giant-impact hypothesis. For example, such a huge impact should have heated Earth so much that it would have become a global magma ocean. We have evidence that heavier materials sank into Earth's middle layer (the mantle) as it cooled. But there isn't a perfect model that shows how the debris from the impact turned into a single moon. Other questions include when the Moon lost its easily vaporized elements and why Venus, which also had big impacts when it formed, doesn't have a moon like ours.

How We Learned About the Moon's Birth

In 1898, a scientist named George Darwin suggested that Earth and the Moon were once a single body. Darwin thought that the Moon spun off from a molten Earth due to strong spinning forces. This idea became the main explanation for a while. He calculated that the Moon used to orbit much closer to Earth and was slowly moving away. This movement away from Earth was later confirmed by American and Soviet experiments using lasers aimed at targets left on the Moon.

However, Darwin's math couldn't fully explain how the Moon could have separated from Earth's surface. In 1946, Reginald Aldworth Daly from Harvard University challenged Darwin's idea. He suggested that the Moon was created by an impact, not just spinning forces. Not many people paid attention to Professor Daly's idea until a conference in 1974. There, the idea was brought up again and later published in the journal Icarus in 1975 by William K. Hartmann and Donald R. Davis.

Their models suggested that at the end of the planet-forming period, several planet-sized objects had formed. These objects could either crash into planets or be captured by them. They proposed that one of these objects might have hit Earth. This collision would have thrown out dust that was low in easily vaporized materials. This dust then came together to form the Moon. This crash could explain the Moon's special geological and chemical features.

Another similar idea came from Canadian astronomer Alastair G. W. Cameron and American astronomer William R. Ward. They suggested that the Moon formed from a glancing blow to Earth by a Mars-sized body. They thought that most of the outer rocky parts of the colliding body would turn into gas. But its metal core would not. So, most of the material sent into orbit would be rocky, leaving the forming Moon low in iron. The easily vaporized materials from the crash probably escaped the Solar System. The rocky materials would tend to clump together.

Before a conference in October 1969, scientists Bill Hartmann, Roger Phillips, and Jeff Taylor challenged other Moon scientists. They told them to use their Apollo mission data and computer models to figure out the Moon's origin. At the 1969 conference in Kona, Hawaii, the giant-impact hypothesis became the most popular idea. Before the conference, scientists were divided among three older theories. But after the conference, most scientists either supported the giant impact idea or were still unsure.

Theia: The Missing Planet

The name of the planet that supposedly crashed into Earth is Theia. It comes from the mythical Greek titan Theia, who was the mother of the Moon goddess Selene. This name was first suggested by the English geochemist Alex N. Halliday in 2000. It is now widely accepted by scientists.

According to modern theories of how planets form, Theia was one of many Mars-sized objects in the Solar System 4.5 billion years ago. One cool thing about the giant-impact hypothesis is that it connects the formation of the Moon and Earth. Scientists believe that Earth had many collisions with planet-sized bodies as it formed. The Moon-forming collision would have been just one of these "giant impacts," but it was likely the last really big one. The Late Heavy Bombardment, which involved much smaller asteroids, happened later, about 3.9 billion years ago.

How the Collision Happened

Simplistic picture of the giant-impact hypothesis.

Astronomers believe the crash between Earth and Theia happened about 4.4 to 4.45 billion years ago. This was about 100 million years after the Solar System started to form. In space terms, the impact would have been at a medium speed. Scientists think Theia hit Earth at an angle when Earth was almost fully formed.

Computer models of this "late-impact" idea suggest Theia hit Earth at an angle of about 45 degrees. Its speed would have been over 9.3 kilometers per second (about 5.8 miles per second) at impact. However, the amounts of oxygen isotopes in lunar rocks suggest a "strong mixing" of Theia and Earth. This points to a steeper impact angle.

Theia's iron core would have sunk into the young Earth's core. Most of Theia's rocky middle layer (the mantle) would have joined Earth's mantle. But a lot of the mantle material from both Theia and Earth would have been thrown into orbit around Earth. This happened if the pieces were ejected at speeds between what's needed to orbit and what's needed to escape Earth's gravity.

Models suggest that the material orbiting Earth might have come together to form the Moon in three steps. First, pieces outside Earth's Roche limit (a distance where gravity pulls things apart) gathered. These pieces helped keep the inner disk material inside the Roche limit. Then, the inner disk slowly spread out to Earth's Roche limit again. It pushed outer pieces along through gravitational pushes. After several decades, the disk spread beyond the Roche limit. It started making new objects that helped the Moon grow. This continued until the inner disk ran out of material after several centuries.

Pieces in stable orbits around the Sun would likely hit the Earth–Moon system later. Computer simulations suggest that about 20% of Theia's original mass would have ended up as a ring of debris around Earth. About half of this material then came together to form the Moon. Earth would have gained a lot of angular momentum and mass from this collision. After the impact, Earth's day would have been about five hours long. Earth's equator and the Moon's orbit would have been in the same flat plane.

Not all the ring material was gathered up right away. The thicker crust on the Moon's far side suggests that a second moon, about 1,000 kilometers (620 miles) wide, might have formed. This smaller moon could have stayed in orbit for tens of millions of years. As the two moons moved away from Earth, the Sun's gravity would have made the smaller moon's orbit unstable. This would have led to a slow collision that "flattened" the smaller moon onto what is now the Moon's far side. This added material to its crust.

The Moon's far side has a thick crust, so hot liquid rock (magma) cannot easily break through it. This is why there are fewer dark, flat areas called lunar maria there. The near side has a thin crust, which is why we see large maria from Earth.

New studies from 2022 show that giant impacts can immediately put a satellite into orbit far outside Earth's Roche limit. This satellite would have a similar mass and iron content to the Moon. Even satellites that first pass inside the Roche limit can survive. They might lose some material but then move into wider, stable orbits. Also, the outer layers of these directly formed satellites are melted over cooler insides. They are made of about 60% material from early Earth. This could help explain why the Moon's chemical makeup is so similar to Earth's. It also offers a simpler, one-step way for the Moon to form.

What the Moon is Made Of

In 2001, a team at the Carnegie Institution of Washington found something important. Rocks brought back by the Apollo program had a chemical fingerprint (an isotopic signature) that was exactly the same as rocks from Earth. This was different from almost all other objects in the Solar System.

In 2014, a team in Germany reported that the Apollo samples had a slightly different isotopic signature than Earth rocks. The difference was small but important. One idea is that Theia formed very close to Earth.

This data, showing how similar the compositions are, can only be explained by the standard giant-impact hypothesis. It's very unlikely that two separate bodies had such similar compositions before a collision.

How They Became Similar

In 2007, scientists from the California Institute of Technology showed that the chance of Theia having the exact same isotopic signature as Earth was very small (less than 1%). They suggested that after the giant impact, Earth and the disk of material that would form the Moon were molten and vaporized. These two pools of material were connected by a shared atmosphere of hot rock vapor. The Earth–Moon system became mixed evenly by currents while it was a continuous fluid. This "equilibration" between the post-impact Earth and the proto-lunar disk is the only idea that explains the isotopic similarities. For this to work, the proto-lunar disk would have to last for about 100 years. Scientists are still working to see if this is possible.

Direct Collision Idea

Research from 2012, based on computer simulations at the University of Bern, offers another way to explain the similar compositions of Earth and the Moon. Physicist Andreas Reufer and his team suggested that Theia hit Earth directly, instead of just grazing it. The collision speed might have been faster than first thought. This higher speed could have completely destroyed Theia. In this changed idea, Theia's makeup isn't as limited. It could have been up to 50% water ice.

Synestia Idea

In 2018, scientists tried to explain how the collision's products became so similar. They suggested that early Earth was spinning much faster before the collision. This way, more material from early Earth would be thrown off to form the Moon. Further computer models showed that this could happen if the pre-Earth body was spinning extremely fast. It would have formed a new, temporary object called a 'synestia'. This is an unstable state that could have been created by another collision to get the rotation fast enough. Models of this temporary structure show that the early Earth, shaped like a doughnut (the synestia), existed for about a century. Then it cooled down and formed Earth and the Moon.

Magma Ocean Idea

Another model from 2019 explains the similarity of Earth and the Moon's compositions. It suggests that shortly after Earth formed, it was covered by a sea of hot liquid rock. The object that hit it was likely solid. Models suggest that this would heat the magma much more than the solid impactor. This would cause more material to be thrown out from early Earth. So, about 80% of the Moon-forming debris would have come from early Earth. Many older models suggested that 80% of the Moon came from the impactor.

Proof for the Idea

Indirect proof for the giant impact idea comes from rocks collected during the Apollo Moon landings. These rocks show oxygen isotope ratios that are almost exactly the same as Earth's. The Moon's crust is very rich in a rock called anorthosite. Also, there are Moon rock samples rich in KREEP (a mix of elements). These facts suggest that a large part of the Moon was once melted. A giant impact could easily have provided the energy needed to form such a magma ocean.

Several pieces of evidence show that if the Moon has an iron-rich core, it must be a small one. The Moon's average density, how it spins, and how it reacts to magnetic fields all suggest its core is small. Its radius is less than about 25% of the Moon's total radius. This is different from most other rocky planets, where the core is about 50% of the radius. The right impact conditions that match the Earth–Moon system's angular momentum would create a Moon mostly from the rocky outer layers of Earth and the impactor. The impactor's core would join Earth's core. Earth has the highest density of all the planets in the Solar System. The idea that Earth absorbed the impactor's core helps explain this.

Comparing the zinc isotopic makeup of Moon samples with Earth and Mars rocks gives more proof for the impact idea. Zinc changes a lot when it turns into gas in planetary rocks. But it doesn't change much during normal rock-forming processes. So, zinc amounts and isotopic makeup can tell us about these processes. Moon rocks have more heavy isotopes of zinc and less zinc overall than similar Earth or Mars rocks. This fits with zinc being lost from the Moon through evaporation, which is what we'd expect from a giant impact.

Collisions between pieces escaping Earth's gravity and asteroids would have left heat marks in stony meteorites. Scientists have used this idea to estimate the impact happened 4.47 billion years ago. This matches dates found by other methods.

Warm, silica-rich dust and lots of SiO gas have been found around other young stars. These are products of very fast impacts (over 10 kilometers per second) between rocky bodies. The Spitzer Space Telescope detected these around the star HD 172555. A belt of warm dust around the young star HD 23514 in the Pleiades cluster looks similar to what's predicted after Theia's collision with early Earth. This is thought to be from planet-sized objects crashing into each other. A similar belt of warm dust was also found around the star BD+20°307.

Challenges and Unanswered Questions

This idea about the Moon's origin still has some problems that need to be solved. For example, the giant-impact hypothesis suggests that a surface ocean of melted rock would have formed after the impact. But there's no clear proof that Earth ever had such a magma ocean. It's likely that some material on Earth has never been processed in a magma ocean.

Chemical Differences

Several differences in chemical makeup need to be explained:

• The amounts of easily vaporized elements on the Moon are not fully explained by the giant-impact hypothesis. If the hypothesis is right, these amounts must be due to something else.
• It's harder to explain the presence of easily vaporized materials like water trapped in Moon rocks and carbon coming from the Moon's surface if the Moon was formed by a high-temperature impact.
• The Moon's iron oxide (FeO) content (13%) is between that of Mars (18%) and Earth's mantle (8%). This rules out most of the Moon's material coming from Earth's mantle.
• If most of the Moon's material came from the impactor, the Moon should have more "siderophilic" elements (elements that like to mix with iron). But it actually has less of them.
• The Moon's oxygen isotopic ratios are almost exactly the same as Earth's. Oxygen isotopic ratios are very precise and give a unique fingerprint for each Solar System body. If a separate planet Theia had existed, it probably would have had a different oxygen isotopic fingerprint than Earth. The mixed material thrown out would also be different.
• The Moon's titanium isotope ratio (⁵⁰Ti/⁴⁷Ti) is so close to Earth's (within 4 parts per million). This suggests that very little, if any, of the colliding body's mass could have become part of the Moon.

Why No Moon for Venus?

If the Moon was formed by such a big impact, it's possible that other inner planets also had similar impacts. A moon that formed around Venus from this process would likely not have escaped. If such a moon-forming event happened there, one idea why Venus doesn't have a moon is that a second collision happened. This second collision might have canceled out the spinning energy from the first impact. Another idea is that the Sun's strong gravity would tend to make the orbits of moons around close-in planets unstable. So, if Venus started spinning slowly early in its history, any moons larger than a few kilometers would likely have spiraled inward and crashed into Venus.

Computer models of the chaotic period when rocky planets formed suggest that impacts like the one thought to have formed the Moon were common. For typical rocky planets, such an impact usually creates a single moon that is 4% of the host planet's mass. The tilt of the resulting moon's orbit is random. But this tilt affects how the system changes over time. For example, some orbits might cause the moon to spiral back into the planet. Also, how close the planet is to its star affects the moon's orbit. The overall result is that moons created by impacts are more likely to survive if they orbit farther from their star and are lined up with the planet's orbit.

Where Theia Might Have Come From

One possible path for the Big Splash, seen from the south pole (not to scale).

In 2004, Princeton University mathematician Edward Belbruno and astrophysicist J. Richard Gott III suggested that Theia came together at a special point in space. This was either the L₄ or L₅ Lagrangian point relative to Earth. These points are in about the same orbit as Earth, but about 60 degrees ahead or behind it. This is similar to how Trojan asteroids orbit Jupiter.

Two-dimensional computer models suggest that Theia's stable orbit would have been affected when its growing mass became more than about 10% of Earth's mass (the mass of Mars). In this idea, gravitational pushes from other small planets caused Theia to leave its stable spot. Later interactions with early Earth led to a collision between the two bodies.

In 2008, evidence suggested that the collision might have happened later than the accepted time of 4.53 billion years ago. It might have been around 4.48 billion years ago. A 2014 study compared computer simulations with measurements of elements in Earth's mantle. It suggested the collision happened about 95 million years after the Solar System formed.

Some scientists have suggested that other important objects might have been created by the impact. These objects could have stayed in orbit between Earth and the Moon, stuck in Lagrangian points. Such objects might have stayed in the Earth–Moon system for as long as 100 million years. Then, the gravitational pulls of other planets made the system unstable enough to free these objects. A study from 2011 suggested that a later collision between the Moon and one of these smaller bodies caused the big differences between the Moon's two sides. Computer models supported this idea. This collision would have been slow enough not to form a crater. Instead, the material from the smaller body would have spread across the Moon (on what would become its far side). This added a thick layer of high-up crust. The resulting uneven mass would then cause the Moon to become tidally locked. This means that today, only the near side of the Moon is visible from Earth. However, mapping by the GRAIL mission has shown this idea is unlikely.

In 2019, a team at the University of Münster reported something interesting about the molybdenum isotopes in Earth's early mantle. They found that these isotopes came from the outer Solar System. This hints at where Earth's water came from. One possible idea is that Theia came from the outer Solar System.

Other Ideas for the Moon's Origin

Other ideas have been suggested over time for how the Moon formed:

• The Moon spun off from Earth's melted surface due to strong spinning forces.
• The Moon formed somewhere else and was later captured by Earth's gravity.
• Earth and the Moon formed at the same time and place from the same disk of material.

None of these ideas can explain the high amount of angular momentum in the Earth–Moon system.

Another idea suggests that a large asteroid hit Earth much later than previously thought. This impact created the Moon mostly from pieces of Earth. In this idea, the Moon formed 60–140 million years after the Solar System began. Before, the Moon's age was thought to be 4.527 billion years. The impact in this idea would have created a magma ocean on Earth and the early Moon. Both bodies would have shared a common atmosphere of hot metal vapor. This shared connection would have allowed material from Earth and the early Moon to mix and become more similar in composition.

Yet another idea proposes that the Moon and Earth formed together, instead of separately as the giant-impact hypothesis suggests. This model, published in 2012 by Robin M. Canup, suggests that the Moon and Earth formed from a massive collision of two planetary bodies, each larger than Mars. These two bodies then crashed into each other again to form what is now called Earth. After this second collision, Earth was surrounded by a disk of material. This material then came together to form the Moon. This idea could explain evidence that other theories cannot.

Images for kids

• 

  Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients) (October 1, 2014).

• 

  Ancient rift valleys – context.

• 

  Ancient rift valleys – closeup (artist's concept).

See also

In Spanish: Teoría del gran impacto para niños