### Models of the Formation of the Moon
How the Moon formed is a fundamental question in planetary science. Scientists have proposed several models to explain its origin, chemical composition, and orbital dynamics.
Model 1 (Giant Impact Model)
This model proposes that approximately 4.5 billion years ago, a Mars-sized protoplanet named Theia collided with the proto-Earth. The collision released immense energy, vaporizing the outer layers of both bodies. This vaporized rock was ejected into orbit around Earth, cooled, and accreted to form the Moon. Because volatile elements (such as water, sodium, and potassium) vaporize easily and were lost to space during the high-temperature event, the Moon is highly depleted in these volatiles. The model also accounts for the Moon's small iron core, as the iron from the impactor sunk to Earth's core.
Model 2 (Capture Model)
This model proposes that the Moon formed independently in another region of the solar system, with its own distinct chemical signature. As the Moon passed close to the proto-Earth, Earth's gravitational pull captured it into a permanent orbit. Because a passing body typically accelerates and escapes gravity, this model requires a mechanism to slow the Moon down during its encounter. Proponents suggest that tidal forces, gravitational interactions with other protoplanets, or friction from a thick, primordial atmosphere dissipated the Moon's kinetic energy, allowing capture.
Model 3 (Fission Model)
This model proposes that the early Earth rotated so rapidly on its axis that centrifugal force exceeded gravitational force at the equator. A large portion of Earth's mantle was thrown off, eventually accreting in orbit to form the Moon. Since the Moon would be composed entirely of Earth's mantle material, this model explains why the Moon's bulk density is similar to that of Earth's mantle. However, the model requires an exceptionally high initial angular momentum that is difficult to reconcile with the current Earth-Moon system's dynamics.
Based on the passage, match each model with the physical mechanism, assumption, or prediction that is unique to that model.
- Model 1 (Giant Impact Model)Predicts that the Moon would be depleted in volatile elements due to vaporization during a high-energy collision.
- Model 2 (Capture Model)Requires a mechanism, such as atmospheric drag or tidal friction, to dissipate kinetic energy during a close planetary encounter.
- Model 3 (Fission Model)Relies on the centrifugal force of a rapidly spinning proto-Earth to separate mantle material into orbit.