The Fission theory is one of the hypotheses proposed to explain the origin of Earth's Moon, suggesting that the Moon was formed from material ejected from Earth due to the planet's rapid rotation during its early history. According to this theory, Earth was once spinning at such a high speed that centrifugal forces caused a portion of its outer material to break off, eventually coalescing to form the Moon. This theory was first suggested in the late 19th century and remains one of the early explanations for the Moon's origin. However, the Fission Theory has been largely discredited in favor of other models, particularly the Giant-impact hypothesis, due to various scientific challenges and its inability to account for key features of the Earth-Moon system.
Overview[]
The Fission Theory proposes that during Earth's early formation, the planet's rotation was so fast that centrifugal forces overcame gravitational forces, leading to the ejection of a portion of Earth's material. This material would have eventually formed the Moon, which would have been in orbit around Earth from the start. The theory was supported by the observation that the Moon and Earth share similar compositions, particularly in terms of isotopic signatures. The Fission Theory suggests that the Moon’s formation was a natural consequence of Earth's own dynamics, requiring no external impact or capture event.
Historical Context[]
The Fission Theory was first proposed by George Darwin, the son of the famous Charles Darwin, in 1878. George Darwin suggested that the Moon formed from Earth due to the planet's rapid rotation, which caused a bulge in the Earth’s equator. This bulge eventually became unstable, resulting in the ejection of material that would coalesce into the Moon. The theory was a popular early explanation, as it was simple and relied on observable phenomena, such as the Earth’s rotation and the Moon's composition.
At the time, the Fission Theory was one of the few hypotheses available to explain the formation of the Moon, and it gained some support in the early 20th century. However, as scientific knowledge expanded, particularly with the advent of lunar exploration and more advanced geochemical analysis, the theory faced increasing criticism.
Supporting Arguments[]
Earth-Moon Composition Similarities[]
Proponents of the Fission Theory pointed to the compositional similarities between Earth and the Moon as evidence for a shared origin. The Moon's isotopic ratios, particularly in terms of oxygen and other elements, were found to be very similar to Earth's. This was interpreted as an indication that the Moon formed from material that was originally part of Earth.
High Angular Momentum[]
The theory was also supported by the argument that the Earth-Moon system has a relatively high angular momentum compared to other planet-moon systems in the solar system. Proponents of the Fission Theory suggested that the formation of the Moon in this way could explain the angular momentum of the Earth-Moon system, as the Moon’s orbit would be a natural result of material ejected during Earth's early rapid rotation.
The Earth’s Early Spin[]
The Fission Theory took into account the fact that the early Earth, during its formation, likely rotated much faster than it does today. The rapid spin would have been a result of the planet's accretion from the protoplanetary disk and the conservation of angular momentum. This could have caused a bulge at the Earth's equator, leading to the ejection of material that eventually formed the Moon.
Criticism and Challenges[]
While the Fission Theory provided an early and elegant explanation for the origin of the Moon, it has faced numerous criticisms over time.
Insufficient Rotation Speed[]
One of the most significant criticisms of the Fission Theory is that Earth’s rotation would have had to be impossibly fast for the material to break off. Calculations suggest that for Earth's rotation to have been fast enough to overcome the planet’s gravity, Earth would have had to be spinning at least five to ten times faster than it does today. Such a rotation speed would have been unstable, leading to the disintegration of Earth itself, rather than just the ejection of material.
Lack of a Clear Mechanism for Ejection[]
The Fission Theory lacks a detailed mechanism for how material could have been ejected from Earth and then stabilized into an orbit. If material was thrown off Earth, it would have needed a very specific set of conditions to come together and form the Moon. However, the theory does not adequately explain how this material would have coalesced in a stable orbit without external influence or disruption.
Composition Differences[]
While the Moon and Earth share some isotopic similarities, they also show significant differences in their composition. Notably, the Moon is deficient in certain volatile elements, such as water, that are present in Earth's crust. The Fission Theory struggles to explain this difference, as material ejected from Earth should, in theory, have similar volatile content.
Angular Momentum Issues[]
The angular momentum of the Earth-Moon system poses another challenge to the Fission Theory. The Moon’s angular momentum is higher than what would be expected if it were simply ejected from Earth. Simulations show that such a process would not result in the observed angular momentum, leading scientists to favor alternative models that involve larger-scale events like a giant impact.
Modern Consensus[]
Today, the Fission Theory is largely dismissed in favor of the Giant-impact hypothesis. The Giant-impact hypothesis proposes that the Moon formed from debris created by a collision between Earth and a Mars-sized body, often referred to as Theia. This hypothesis is better supported by current evidence, including the isotopic similarities between Earth and the Moon, the Moon's composition, and the angular momentum of the Earth-Moon system. The Fission Theory is now considered an outdated explanation, though it remains an important part of the historical development of lunar origin theories.
Related Theories[]
- Giant Impact Hypothesis: The prevailing theory for the Moon’s origin, suggesting that the Moon formed from debris created by a collision between Earth and a Mars-sized object.
- Co-Formation Theory: The hypothesis that the Earth and Moon formed simultaneously from the same protoplanetary disk.
- Capture Theory: The hypothesis that the Moon was a celestial body captured by Earth's gravity after forming elsewhere in the solar system.
See Also[]
References[]
- https://www.nhm.ac.uk/discover/how-did-the-moon-form.html#:~:text=Prior%20to%20the%20Apollo%20mission,began%20to%20orbit%20the%20planet.
- https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question38.html#:~:text=The%20Fission%20Theory:%20This%20theory,spin%20and%20it%20does%20not.
- https://study.com/academy/lesson/formation-of-the-moon-theories.html#:~:text=In%201878%2C%20George%20Howard%20Darwin,outer%20layers%20of%20the%20Earth.
- https://www.britannica.com/science/fission-hypothesis#:~:text=In%20fission%20theories%20a%20fluid,spinning%20proto%2DEarth%20that%20would%E2%80%A6
- https://www.pbs.org/wgbh/nova/tothemoon/origins.html#:~:text=Coaccretion%20led%20the%20pack%20through,the%20Earth%2Dmoon%20system%20today.
- https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/09%3A_Cratered_Worlds/9.04%3A_The_Origin_of_the_Moon#:~:text=The%20fission%20theory%E2%80%94the%20Moon,orbit%20our%20Moon%20occupies%20today.
| 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 | |