Giant impact hypothesis facts for kids

The giant impact hypothesis is a big idea about how our Moon was made. It suggests that the Moon formed from the leftover pieces after a huge crash. This crash happened between the young Earth and another planet-like body, about the size of Mars. Scientists call this other body Theia. This idea is currently the most popular scientific explanation for how the Moon came to be.

Scientists have found several clues that support this idea:

• The Moon's path around Earth and Earth's own spin are very similar. They both tilt at a similar angle to the ecliptic plane, which is the flat plane where most planets orbit the Sun.
• Rocks from the Moon and Earth have almost the same chemical fingerprints, called isotope ratios. This suggests they came from the same original material.
• The Earth and Moon together have a lot of angular momentum, which is like the amount of "spinning energy" they have. This is much more than other rocky planets. A giant impact could have given them this extra spin.
• Moon samples show that the Moon was once completely melted, like a magma ocean. This would have needed a huge amount of energy, more than just from the Moon forming slowly. A giant crash could provide that energy.
• The Moon has a very small iron core, which means it's not as dense as Earth. Computer models show that if a Mars-sized body hit Earth, its core would likely sink into Earth's core. This would leave the Moon to form from lighter pieces of Earth's outer layers (the crust and mantle).
• The Moon has fewer "volatile" elements than Earth. These are elements that easily turn into gas. A high-energy event like a giant impact could have boiled them away. The Moon's weaker gravity couldn't hold onto them, while Earth's stronger gravity could.
• Scientists have seen signs of similar huge collisions in other star systems, where they form rings of debris.
• Giant collisions fit well with the main theory of how our entire Solar System formed.

There are still some things scientists are studying about this idea. For example, the oxygen isotopes in Moon rocks are very similar to Earth's. This makes it hard to see if any material from another body contributed. Also, some elements in Moon samples aren't exactly what was expected.

How the Moon Formed

Our Moon is quite special because it's very large compared to Earth. During the Apollo program, astronauts brought rocks back from the Moon. By studying these rocks, scientists found that the Moon is about 4.527 billion years old. This makes it a bit younger than other parts of our solar system. More recent studies suggest it might be even younger, forming about 4.48 billion years ago.

Another interesting thing about the Moon is its low density. This means it doesn't have a large metallic core like other rocky planets. The Moon's overall makeup is very similar to Earth's outer layers, the mantle and crust, but without Earth's heavy core. These facts led to the giant impact hypothesis.

The Collision

The object that hit Earth, sometimes called Theia, is thought to have been a bit smaller than the planet Mars. This huge collision happened about 4.533 billion years ago. Computer models show that when Theia hit the early Earth at a low angle and a relatively slow speed (about 8 to 20 kilometers per second), a lot of material was blasted into space. This material came from the outer layers of both Earth and Theia.

Much of this blasted material stayed in orbit around Earth. Over time, these pieces came together to form the Moon. The heavy metallic core of Theia would have sunk deep into Earth and joined with Earth's own core. This explains why the Moon has so little metal. The ejected material orbiting Earth could have quickly gathered into a single body within just a few weeks. Then, its own gravity pulled it into a round shape, creating the Moon we see today.

Scientists believe Earth already had its own core and outer layers formed at least 10 million years before the impact. This means that when the crash happened, mostly material from Earth's outer layers was thrown into space, leaving Earth's heavy core untouched.

What Happened After the Impact

The giant impact had huge effects on the young Earth. It released an enormous amount of energy, so much that both Earth and the newly forming Moon became completely melted. Right after the crash, Earth's outer layer was a swirling magma ocean. Earth's first atmosphere was likely completely blown away by the massive energy released.

The impact is also thought to have changed Earth’s axial tilt. This tilt is now about 23.5 degrees and is what gives us our seasons. Without this impact, Earth might have had no tilt and no clear seasons. The collision may also have made Earth spin much faster.

New Discoveries

Scientists have continued to study Moon rocks brought back by Apollo astronauts. Recent analysis of these rocks seems to show tiny traces of Theia. Researchers say this finding further supports the idea that the Moon was formed by this huge collision. Some scientists are surprised that the difference between the Theia material found in Moon rocks and Earth's material is so small.

Another clue comes from comparing the zinc in Moon rocks with zinc in Earth and Mars rocks. Zinc behaves differently when it evaporates from rocks. Moon rocks have more heavy zinc and less zinc overall than Earth or Mars rocks. This fits the idea that zinc was lost from the Moon through evaporation during a high-energy event like the giant impact.

Scientists have also looked at how pieces blasted from Earth might have hit asteroids. By studying these impacts, they've estimated the giant impact happened about 4.47 billion years ago. This date matches what they found using other methods.

Most recently, in November 2023, scientists shared new computer simulations. These simulations suggest that parts of Theia might still be hidden deep inside Earth. They could be the two giant, unusual blobs of material found in Earth's mantle.

Images for kids

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  A simple drawing showing the giant-impact hypothesis.

See also

In Spanish: Teoría del gran impacto para niños