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The idea that Earth simply captured a passing Moon runs into which specific problem?
AA captured body would be much denser
BA captured body could not be round
CA captured body would leave no rocks
DA captured body needs to lose energy
Answer & Solution
Correct answer: D. A captured body needs to lose energy
1. An object falling past a planet keeps enough speed to swing away again.
2. To be trapped it must shed a large amount of energy of motion.
3. No one knows a way for the young Earth to have drained that much energy from a passing body.
4. Even if capture had worked, the orbit would be very stretched rather than nearly circular.
5. The Moon's orbit is in fact nearly circular, which argues against capture again.
6. Earth and Moon also share an identical fraction of the major oxygen isotopes.
7. That shared chemistry points to a common origin, not to a body formed elsewhere.
8. Density is a problem for the fission idea rather than the specific barrier to capture.
_Source: OpenStax Astronomy (CC BY 4.0), section 9.4 The Origin of the Moon_
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