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Why did the coaccretion theory fail to explain the Earth-Moon system?
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Moon
(Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.
The coaccretion theory, which posits that the Moon and Earth formed simultaneously from the same primordial cloud of gas and dust, failed to explain the Earth-Moon system primarily because it could not account for the system's large angular momentum.12
Additionally, while the theory was initially proposed to explain geological similarities between the two bodies, later scientific analysis revealed significant chemical differencesâspecifically that the Moon is much less dense than Earth and contains significantly less ironâwhich argued against a common origin.2