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Why is the capture theory of lunar formation considered unlikely?
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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 capture theory, which suggests the Moon formed elsewhere in the solar system and was later ensnared by Earth’s gravity, is considered unlikely for two primary reasons:
- Mechanical Implausibility: Scientists have struggled to identify the specific celestial mechanics required to "brake" a passing body effectively. For the Moon to remain in a stable orbit rather than simply swinging past Earth, it would have needed to lose a significant amount of velocity at exactly the right moment, a scenario that remains highly improbable.12
- Geological Evidence: Capture theories lost favor after it was discovered that Earth and the Moon share significant geological similarities. If the Moon had formed independently in a different part of the solar system, it would not be expected to possess such a similar composition to Earth.1