The Formation of the Moon 

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Competing Theories:

• Capture - The Earth captures a moon adrift in space

 
• Pros: well, it's not impossible
• Cons:
• it's very unlikely
• kinetic energy must be carried away by a 3rd body
• Accretion - The Earth accretes planetesimals in early solar system to form Moon.


• Pros: Lots of material around in the early solar system to accrete
• Cons:
• Why would the Earth and Moon have different compositions (ie why is the moon deficient in Iron)?
• Earth-Moon system has too much angular momentum compared to other planets.
• Fission - A fast spinning Earth "calves" the Moon.


• Pros: Density of Moon similar to that of the outer layers of the Earth
• Cons:
• Moon should be orbiting along Earth's equator; it's not.
• Composition of Moon rocks dissimilar to that of the Earth's surface.
• Earth would have to have been spinning extremely fast (HW problem).
• Impact - A large planetesimal hits Earth, ejecting material which forms the Moon

• Pros:
• Collisions happen (but this is a whopper: Mars sized or bigger!)
• Explains lack of volatiles in Moon
• Explains lack of an Iron core
• Simulations confirm the possibility
Animation
• Bonus: explains tip of the Earth's axis!
• Cons:
• To get material past the Earth's Roche limit, the impactor needs more angular momentum than is now in the Earth-Moon system.
• Why just one moon?