Introduction:
The Mid-20th century was a major period in terms of space exploration and space research. it is not surprising therefore that another theory for the accretion of the Moon - the Coaccretion Theory - was first described in a scientific journal (in 1959). This concept has been developed and gained some scientific backing over the decades mainly because unlike many of the other theories of formation of the Moon, it does not rely on one, or multiple uncertain events happening, eg. a massive impact or the orbit of a large body coming close enough to the Earth for the body to be captured. Instead the coaccretion model is based on principles of planetary and lunar formation that we are confident happen throughout the universe [23].
The Co-accretion Theory:
This theory suggests that the Earth and Moon formed independently of each other but from the same 'circum-terrestrial disc', or 'protolunar swarm' as it is sometimes known (see Figure 1), in our solar system. Proponents of this theory suggest the following theory as to how this protolunar swarm (PLS) formed. As the early Earth was accreting, approaching planetisimals that came within the gravitational field of the Earth would change their trajectory relative to the amount of gravitational attraction acting on them. Because of the chaotic nature of the early Universe some planetisimals would pass Earth in a prograde direction and some in a retrograde direction. Inevitably there was collision between the planetisimals leaving debris from the collision in orbit around the Earth. Once there was some material surrounding the Earth the probability of capture of incoming material was greatly increased because the chance of collision was much higher [23].
The Mid-20th century was a major period in terms of space exploration and space research. it is not surprising therefore that another theory for the accretion of the Moon - the Coaccretion Theory - was first described in a scientific journal (in 1959). This concept has been developed and gained some scientific backing over the decades mainly because unlike many of the other theories of formation of the Moon, it does not rely on one, or multiple uncertain events happening, eg. a massive impact or the orbit of a large body coming close enough to the Earth for the body to be captured. Instead the coaccretion model is based on principles of planetary and lunar formation that we are confident happen throughout the universe [23].
The Co-accretion Theory:
This theory suggests that the Earth and Moon formed independently of each other but from the same 'circum-terrestrial disc', or 'protolunar swarm' as it is sometimes known (see Figure 1), in our solar system. Proponents of this theory suggest the following theory as to how this protolunar swarm (PLS) formed. As the early Earth was accreting, approaching planetisimals that came within the gravitational field of the Earth would change their trajectory relative to the amount of gravitational attraction acting on them. Because of the chaotic nature of the early Universe some planetisimals would pass Earth in a prograde direction and some in a retrograde direction. Inevitably there was collision between the planetisimals leaving debris from the collision in orbit around the Earth. Once there was some material surrounding the Earth the probability of capture of incoming material was greatly increased because the chance of collision was much higher [23].
The formation of the Moon from this PLS was described in detail in the early 1970s. Because of the large amount of orbiting debris and the differences in size between the material there was a lot of gravitational instability within the disc. this caused much of the material to accrete into a few, or several, 'proto-Moons'. Studies in the 1970s show that it is likely there were two or three of the proto-Moons surrounding an Earth with approximately half its present-day mass [24]. As they grew they were able to attract larger and faster moving material and eventually they themselves coalesced into the Moon. There is a problem with the hypothesis described above and that is that is states almost all material in the disc was 'swept up'. If this is the case we could comfortably assume that the Moon and Earth was have similar relative abundances of common elements such as Iron. We know this is not the case however as analysis of lunar material and calculations based on lunar observations show that the Moon is deficient in iron (we know this because the uncompressed density of the Moon is 3.34g/cm^3 compared to 4.45g/cm^3 for the Earth [25]). A possibility is that because of the relative high density of iron, the smaller proto-Moons did not have a high enough gravitational attraction to pull in lots of iron rich material whereas the larger Earth could.
Analysis of isotope ratios of oxygen in lunar and Earth rocks show that they have identical ratios. This therefore means that the Earth and Moon must have formed in the same planetary 'feeding zone' which has been worked out as being 1 Astronomical Unit (the current average distance from the Earth to the Sun) or closer to the Sun. This premise must be satisfied by whichever the favoured explanation of Moon formation is. In the case of the coaccretion theory this satisfies this constraint [25].
In addition we known that the Moon is deficient by a number of orders of magnitudes of volatile elements and siderophile elements. We believe that accretion of the Moon did not end until around the same time as accretion of the Earth ended. This would make sense as there would still be material in the PLS. If this is the case then the Moon would have finished forming sometime around 50-100 million years after the beginning of the Universe. This would provide sufficient time for the exposed debris in the disk to lose volatiles.
We know from analysis of the Moon that the surface was once a magma ocean. For a theory of Moon formation to be credited it must allow for a magma ocean to form. The coaccretion theory allows for this because towards the end of accretion of the Moon, collision of the larger proto-Moons would go a long way to providing the heat necessary to form a magma ocean [23].
In conclusion it seems that the coaccretion model is one that holds a lot of merit, especially as it explains the chemical similarities between the Earth and Moon and we know that it is possible for planet-moon systems to form in this way. The major downfall of this theory is that it does nothing to explain the observed angular momentum of the Earth and the Moon whereas the Impact Theory can account for the high angular momentum compared to the rest of the planets in our solar system.