Earth and the Moon, with an arrowed trajectory suggesting that the Moon was once a wandering body (like an asteroid) in space, the Moon was then captured by Earth's gravitational pull and settled into orbit around it.
The Capture theory is a hypothesis regarding the origin of Earth's Moon, proposing that the Moon was formed elsewhere in the solar system and was later captured by Earth's gravitational influence. This theory is one of several hypotheses that aim to explain the Moon's formation, alongside the Giant-impact hypothesis, the Co-formation Theory, and the Fission Theory. Although once a prominent explanation, the Capture Theory has faced significant criticism and is now considered less likely compared to the Giant-impact hypothesis.
Overview[]
According to the Capture Theory, the Moon was originally a celestial body formed independently, possibly as a planetesimal or a protoplanet, in a region of the solar system distinct from Earth's orbit. During a close encounter with Earth, the Moon entered Earth's gravitational field, which led to its capture into orbit around the planet. Proponents of this theory argue that such a process could account for the Moon's orbital characteristics and its physical composition.
Historical Context[]
The Capture Theory was proposed in the mid-20th century as scientists sought to explain discrepancies in the Moon's composition compared to Earth's. It emerged as an alternative to the Fission Theory and Co-formation Theory, which struggled to account for certain geochemical and orbital data.
Supporting Arguments[]
Orbital Mechanics[]
Advocates of the theory suggest that the Moon's slightly elliptical orbit and its inclination to Earth's equator are consistent with the capture of an external body.
Composition Variances[]
Early analyses of lunar rocks showed differences in isotopic composition between the Earth and the Moon, leading some to speculate that the Moon may have originated elsewhere.
Planetary Capture in the Solar System[]
The capture of moons by planets is not unprecedented; irregular moons of the gas giants, such as Triton (captured by Neptune), provide examples of this phenomenon.
Criticism and Challenges[]
The Capture Theory faces several challenges that undermine its plausibility:
Orbital Dynamics[]
For the Moon to be captured by Earth's gravity, it would have required a significant loss of kinetic energy during the encounter. This would necessitate an interaction with a third body or the presence of a dense atmosphere around Earth at the time, both of which are considered unlikely.
Geochemical Similarities[]
While early studies highlighted differences in isotopic composition, later, more precise analyses revealed striking similarities in oxygen isotopes between Earth and the Moon. These similarities suggest a shared origin rather than an independent formation.
Lack of Evidence for a Capture Mechanism[]
No conclusive evidence has been found for the mechanisms required to achieve such a gravitational capture. Simulations often fail to replicate the Moon's current orbital parameters under the constraints of the Capture Theory.
Modern Consensus[]
The Capture Theory has been largely supplanted by the Giant-impact hypothesis, which posits that the Moon formed from the debris ejected following a collision between Earth and a Mars-sized body, often referred to as Theia. This hypothesis better accounts for the isotopic similarities, the angular momentum of the Earth-Moon system, and the Moon's overall mass and composition.
Related Theories[]
- Giant Impact Hypothesis: The prevailing explanation for the Moon's origin, suggesting a cataclysmic collision.
- Co-formation Theory: The hypothesis that the Earth and Moon formed simultaneously from the same protoplanetary disk material.
- Fission Theory: The idea that the Moon was ejected from a rapidly spinning proto-Earth.
See Also[]
References[]
- Canup, R. M., & Asphaug, E. (2001). Origin of the Moon in a giant impact near the end of the Earth's formation.
- Stevenson, D. J. (1987). Origin of the Moon—The collision hypothesis.
- Melosh, H. J. (1990). Giant impacts and the thermal state of the early Earth.
- Taylor, S. R. (1987). The Origin of the Moon: Collision Hypotheses.
- Wiechert, U., et al. (2001). Oxygen isotopes and the Moon-forming giant impact.
- Hartmann, W. K., & Davis, D. R. (1975). Satellite-sized planetesimals and lunar origin.
- NASA. (2021). The Moon: Our Natural Satellite.
| Models of the Solar System | ||
|---|---|---|
| Solar Formation | Nebular hypothesis | |
| Dynamical Evolution | ||
| Origin of the Moon | ||
| Early Development | Grand tack hypothesis • Fission theory • Co-formation theory • Capture theory • Migration of Neptune • Jumping-Jupiter scenario | |
| Later Development | Nice model (Nice 2 model • Five-planet Nice model) | |
| Planetary Formation | Giant-impact hypothesis | |
| Placement | Heliocentric model • Geocentric model | |