Four theories about how the Moon formed

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Scientists still fail to provide an official explanation on how the Moon was formed. The “mystery” was a subject of wonder for scientists, philosophers and artists throughout history.


Galileo was the first scientist to point out that the moon has a landscape similar to Earth’s. Over time, other scientists have posited a variety of theories about what the moon is and where it came from. Scientists have given several possible explanations, but none of which are without flaws. From all the versions, four are most known.

1. Fission theory about how the Moon formed

In the 1800s, George Darwin, the son of Charles Darwin, at one point in Earth’s history, Earth might have been spinning so fast that part of our planet spun off into space but was kept tethered by Earth’s gravity. That is why he believed that the moon looked so similar to the Earth.

But the theory was pretty much debunked since the moon rock compositions differs from those in the Pacific Ocean, which is too young to be the source of the moon.

2. Capture theory on the birth of Earth’s natural satellite

The capture theory suggests that the moon originated elsewhere in the Milky Way, completely independent of Earth. Then, while traveling past Earth, the moon got trapped in our planet’s gravity.

What comes against this theory is the fact that the moon would have eventually broken free from Earth’s gravity because Earth’s gravity would have been massively altered by catching the moon. Also, chemical components of both the Earth and the moon suggest they formed at around the same time.


3. Co-accretion theory: A Moon spinning around a black hole

Co-accretion theory or the condensation theory suggests that the moon and the Earth formed together while orbiting a black hole. However, this theory neglects an explanation of why the moon orbits the Earth, nor does it explain the difference in densities between the moon and Earth.

4. Giant impact hypothesis: The Theia impact

The Moon is millions of years older than scientists previously thought

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According to this theory, a Mars-sized object impacted with a very young, still-forming Earth about 4.5 billion years ago. The planetary object that impacted Earth has been dubbed “Theia”, inspired from Greek mythology, where Theia was the mother of the moon goddess Selene. When Theia hit Earth, a portion of the planet came off and eventually hardened into the moon.

However, it doesn’t explain why the moon and the Earth are chemically identical.

A new study, published in Nature Geoscience in 2017, says that multiple moon- to Mars-sized objects struck Earth, and the debris from these collisions formed disks around the Earth before forming into moonlets that drifted away from Earth and merged to create the moon we know today.

This multi-impact hypothesis helps to explain the chemical composition similarities, the authors of the study say. If multiple objects collided with Earth, the chemical signatures between those objects and Earth would even out more as the moon formed than if it had just been a single impact event.