Blog #4: How did the Moon form?


Representation of Giant Impactor Theory

For a long time, we were generally unsure of how Earth’s moon originally formed, and there were a few popular theories which were later disproven. Below are original theories about the formation of the Moon and how they were disproven:

  1. Fission Theory:

This theory is based on the fact that the chemical composition of the Moon is somewhat similar to that of Earth’s mantle. The theory proposes that in the early days of the solar system, the Earth was spinning so quickly that a piece of it was flung off of one of its outer layers, and eventually went into orbit around the Earth. This theory does not hold up because there is no “fossil evidence” of the rapid spinning at the basis of this theory.

  1. Capture Theory:

This theory suggests that the Moon was a particular object flying through space that got captured by the Earth’s gravitational field, thus sending the Moon into orbit around the Earth. This theory does not hold up given the likelihood that the Moon was formed in some way from the Earth (similar chemical composition) and because it is unlikely that the Moon slowed down enough to become captured by Earth’s gravitational field.

  1. Condensation Theory:

This theory suggests that the Earth and Moon were formed individually and at the same time, from the nebula that formed the solar system. This seems to make sense given that the Earth and the Moon have similar composition, however, they would have to have a far more similar composition for this to be possible (the Moon should have an iron core, but it does not).


The theory which is widely accepted today is called the Giant Impactor Theory, and suggests that the Moon was formed through a collision between the newly formed Earth and a rock about the size of Mars. Astrophysicist Mastrobuono-Battisti simulated collisions in the early solar system and found that most simulations produced 3 to 4 rocky planets comparable to Earth. Around 30% of the time, the composition of one planet was very similar to that of the last planet that collided with it, providing additional evidence in favor of the Giant Impactor Theory.



How the Moon Formed: Violent Cosmic Crash Theory Gets Double Boost

StarChild Question of the Month for October 2001


One thought on “Blog #4: How did the Moon form?

  1. Wait, so I’m having trouble understanding the Giant Impactor Theory. Is this saying that the Earth and some rock the size of Mars collided, and then the rock got smaller and became the Moon while the rest of the rock became scattered through the solar system as debris? Or is it saying that a piece of the debris became the Moon? Because if the latter is the case, I wonder what happened to the rest of the rock following the collision.


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