Introduction:
The Fission Theory of the formation of the Moon was proposed much earlier than the Impact Theory and was proposed originally by George Darwin (son of Charles Darwin) in 1879 [27]. Darwin's calculations of how much angular momentum the Earth needed to have to fission off an object the size of the Moon were very accurate however his theory was flawed on the grounds that he had little scientific evidence to back up his hypothesis. Subsequent experiments in the 1930s disproved Darwin's theory saying that based on his calculations, the amount of tidal distortion that resulted from the angular momentum he proposed was not significant enough to fission a Moon sized object. Follow up investigations in the 1960s however, revived the idea of fission as a legitimate theory for Moon formation.
Fission Theory was revived in the 1960s because scientists investigated rotational instability as a mechanism for fission of the Moon [28, 29]whereas Darwin had envisaged the Earth having a constant fast spin.
Fission Theory:
At the most basic level Fission Theory states that very early in Earth history the Earth was spinning many times faster than it does today. in its early molten or semi-molten state the rapid rotation distorted the shape of the planet and part of the planet was thrown off and then accreted to form the Moon (see Figure 1).
The Fission Theory of rotational instability was based on a model that estimated the early Earth to have a rotational period of 2.6 hours (just inside the rotational stability field) and a uniform density throughout the planet [27]. As differentiation of the core and mantle occurred there was a decrease in the Earth's moment of inertia because there was more mass focused towards the center of the planet. Because moment of inertia is proportional to angular acceleration this caused the Earth to rotate faster (with a rotational period of about 2.1 hours) and this made the Earth rotationally unstable. As a result the shape of the Earth became increasingly distorted until part of it fissioned off. The remaining Earth material had a much lower mass and became more stable but because its speed of rotation was comparably high to the orbital speed of the new Moon, tidal forces would have caused the orbital distance of the Moon to increase.
The Fission Theory of the formation of the Moon was proposed much earlier than the Impact Theory and was proposed originally by George Darwin (son of Charles Darwin) in 1879 [27]. Darwin's calculations of how much angular momentum the Earth needed to have to fission off an object the size of the Moon were very accurate however his theory was flawed on the grounds that he had little scientific evidence to back up his hypothesis. Subsequent experiments in the 1930s disproved Darwin's theory saying that based on his calculations, the amount of tidal distortion that resulted from the angular momentum he proposed was not significant enough to fission a Moon sized object. Follow up investigations in the 1960s however, revived the idea of fission as a legitimate theory for Moon formation.
Fission Theory was revived in the 1960s because scientists investigated rotational instability as a mechanism for fission of the Moon [28, 29]whereas Darwin had envisaged the Earth having a constant fast spin.
Fission Theory:
At the most basic level Fission Theory states that very early in Earth history the Earth was spinning many times faster than it does today. in its early molten or semi-molten state the rapid rotation distorted the shape of the planet and part of the planet was thrown off and then accreted to form the Moon (see Figure 1).
The Fission Theory of rotational instability was based on a model that estimated the early Earth to have a rotational period of 2.6 hours (just inside the rotational stability field) and a uniform density throughout the planet [27]. As differentiation of the core and mantle occurred there was a decrease in the Earth's moment of inertia because there was more mass focused towards the center of the planet. Because moment of inertia is proportional to angular acceleration this caused the Earth to rotate faster (with a rotational period of about 2.1 hours) and this made the Earth rotationally unstable. As a result the shape of the Earth became increasingly distorted until part of it fissioned off. The remaining Earth material had a much lower mass and became more stable but because its speed of rotation was comparably high to the orbital speed of the new Moon, tidal forces would have caused the orbital distance of the Moon to increase.
More recent simulations in the 1980s however have shown that the likelihood of a rapidly rotating Earth fissioning an entire Moon-sized body is almost zero. What is more likely is that a rapidly rotating early-Earth would have thrown off lots of dispersed material that would have formed a disc-like structure around the Earth [30] (see Figure 2).The material that was beyond the Earth's Roche limit would have come together under gravitational attraction and built up the Moon. Towards the end of this potentially short fissioning episode the Moon has a mass several fold more than it does today and had a surface temperature of around 2000-2500 degrees (high enough for devolatilize the moon). Because of the high temperature of the Moon, compounds with relatively low boiling points such as silicates were vaporized and formed a thick, extensive atmosphere around the Moon. This atmospheres is though to have at least in part extended within the Earth's Roche field because of the close proximity of the Moon soon after fissioning. Much of this atmospheric material was ripped apart and fell back to Earth but some is thought to have gone into orbit around the Earth and was subsequently 'blown' away by solar activity. This loss of mass is thought to account for the reduction in angular momentum seen today [31].
This is the best mechanism for explaining fission theory because as the material was orbiting the Earth in a hot disc it would have devolatized a lot more easily than it would have done if an intact Moon was produced immediately and this can therefore account for the lack of volatile elements in the Moon (See Figure 3).
Figure 3. The depletion of some of the siderophile elements in the mantles of the Earth and Moon. Data derived from analysis of basalts on both bodies. The Earth is represented by a circle with a cross in the middle and the Moon is represented by a crescent. Depletion factor is obtained by dividing the element abundance in the mantle over the element abundance in C1 meteorites. [36]
There is other evidence too that shows the Fission Theory has merit. More importantly the Iron depletion in the Moon relative to the Earth. If fission occurred after differentiation of the core and mantle then it is likely that Iron-deficient mantle material was fissioned off to form the Moon. In addition the common isotope ratios of oxygen and trace element abundances in the Earth's mantle and Moon invariably shows that they formed from the same material unless by some great coincidence [27].
Evidence against Fission of the Moon:
While Fission Theory does hold merit there are some strong cases of evidence against it that its advocates find difficult to explain. Firstly, if fission occurred after, or towards the end of, core segregation as should be expected by the low iron content of the Moon then core differentiation needs to be 97% complete before the Moon is fissioned [32]. If this is the case then why did it take until 97% of the core had formed before the Earth;s rotation became unstable. Surely core differentiation wouldn't go on so close to completion before destabilization occurred?
In addition, fission advocates find it difficult to explain how the Earth had such a high angular momentum and, indeed how that angular momentum has dissipated since. Some tenuous proposals have been given but each has been treated with much scrutiny. It is possible that the Earth gained its high angular momentum if the Earth itself accreted from numerous planetisimals that orbited along a flat plane and all impacted at similar off-center angles however this starts to enter the realms of the Impact Theory rather than traditional fission [27]. it has been suggested that a Moon formation may be analogous with binary stars where the two stars have too great an amount of angular momentum to occur as a single large star [31]. The problem with this is that binary stars have completely different parameters and interactions than Earth-Moon systems and therefore cannot be compared.
Some speculation has been given to addressing the question of where the angular momentum has gone, however these answers have received little credit. it has been suggested that the super-heated Earth (5000K) boiled silicates on the surface to form an early Earth atmosphere after fissioning of the Moon. Proposals have said that if the atmosphere rotated synchronously with the Earth then only 3.7% of the Earth's mass would need to be lost to the top of the atmosphere to reduce the angular momentum to what is seen today [33]. This has been widely dismissed because simulations have shown that the interaction between Earth and its upper atmosphere is not strong enough to reduce angular momentum by such a large amount. Another suggestion is that the original fissioned Moon has a mass much greater than it has today and at very high temperatures the majority of the Moon's mass was boiled away with the angular momentum lost [34]. However the loss of such a huge proportion of the Moon through boiling would leave a body today with a very different composition.
To conclude, although Fission Theory does hold value in that it explains many similarities we see between the Moon and the Earth's mantle today, it has some major downfalls and must therefore be treated with speculation.