Introduction:
When major research went into studying the Moon in the 1950s and 1960s the theory of intact capture was favoured by many scientists but since then as simulations of planet capture have been created and analysis of lunar characteristics have been undertaken, interest in the intact capture theory has slipped. More recently, however the theory of disintegrative capture has been pioneered and this has attracted some attention [37].
Intact Capture:
The Intact Capture Theory was favoured initially because its ease of explaining the differences in composition between the Moon and the Earth - in particular the difference in iron content. The chances of two objects, both formed in different parts of the solar system having similar chemical abundances is highly unlikely. However the physics of actually capturing the Moon as it travels past Earth is almost impossible and this is the downfall of the intact capture theory.
The problem with this theory is that the ratio of size of the Moon to the Earth is actually anomalously high for our solar system - only Pluto and its moon Charon can come close to matching it. Also because the encounter would happen at relatively high velocities its is extremely unlikely that the gravitational attraction of the Earth could alter the course of the Moon in such a large way that it remained in orbit around the Earth for the next 4.5 billion years or so. If the intact capture theory is to be believed then it is estimated that the Moon originated about 1 astronomical unit away from the Earth (the average distance of the Earth to the Sun) [38].
There is a possibility that a small body could have been temporarily captured by the Earth if it had a very specific velocity compared to the Earth. If this is the case then the Earth would be surrounded by a 'zero velocity zone surface' through which the Moon cannot pass, except for a small door known as the liberation point (see Figure 1). The probability of this occurring based on the numerous specific parameters is incredibly small and indeed simulations have shown that this can only temporarily capture a body until the body's trajectory takes it back out of the liberation point and away from the Earth. It has been suggested that tidal forces associated with the incoming body could close the liberation point in some way [39] but there has been no real evidence to back this up.
When major research went into studying the Moon in the 1950s and 1960s the theory of intact capture was favoured by many scientists but since then as simulations of planet capture have been created and analysis of lunar characteristics have been undertaken, interest in the intact capture theory has slipped. More recently, however the theory of disintegrative capture has been pioneered and this has attracted some attention [37].
Intact Capture:
The Intact Capture Theory was favoured initially because its ease of explaining the differences in composition between the Moon and the Earth - in particular the difference in iron content. The chances of two objects, both formed in different parts of the solar system having similar chemical abundances is highly unlikely. However the physics of actually capturing the Moon as it travels past Earth is almost impossible and this is the downfall of the intact capture theory.
The problem with this theory is that the ratio of size of the Moon to the Earth is actually anomalously high for our solar system - only Pluto and its moon Charon can come close to matching it. Also because the encounter would happen at relatively high velocities its is extremely unlikely that the gravitational attraction of the Earth could alter the course of the Moon in such a large way that it remained in orbit around the Earth for the next 4.5 billion years or so. If the intact capture theory is to be believed then it is estimated that the Moon originated about 1 astronomical unit away from the Earth (the average distance of the Earth to the Sun) [38].
There is a possibility that a small body could have been temporarily captured by the Earth if it had a very specific velocity compared to the Earth. If this is the case then the Earth would be surrounded by a 'zero velocity zone surface' through which the Moon cannot pass, except for a small door known as the liberation point (see Figure 1). The probability of this occurring based on the numerous specific parameters is incredibly small and indeed simulations have shown that this can only temporarily capture a body until the body's trajectory takes it back out of the liberation point and away from the Earth. It has been suggested that tidal forces associated with the incoming body could close the liberation point in some way [39] but there has been no real evidence to back this up.
To conclude it is very unlikely that the Moon formed by intact capture, and even if it did that does not answer the question of how the Moon formed, only how the Moon got to its present day position.
Disintegrative Capture:
Disintegrative capture theory says that if a planetisimal of independent orbit to the Earth passed close enough to the Earth, tidal forces exerted on the planetisimal would decelerate it and bring part of the planetisimal within the Earth's Roche limit. Although the velocity of the material closest to the Earth and the material in the center of the planetisimal would be equal, the fact that the outer material was close to the Earth means it would have a higher escape velocity and so would be ripped off of the planet (see figure). The slower the body is moving relative to the Earth the easier this is to happen because there would be more time to tidally disrupt the planetisimal. It has been predicted that if the body is moving at less than 2km/s then up to 50% of the mass of the passing body may be ripped off and go into geocentric orbit around the Earth in a similar fashion to the PLS described in Coaccretion Theory [40].
Disintegrative Capture Theory also gives evidence for the compositional differences between the Earth and the Moon. During disintegration of the passing planetisimal, if it has already undergone core differentiation then the majority of the disintegrated material would be iron-poor mantle material. Alternatively if the body has not undergone core differentiation then as it is disintegrated, because of the high density of the iron-rich material, the iron-rich material would have higher velocity and would be more likely to leave the geocentric orbit of Earth and be lost to space [41]
The main advantages of this disintegrative capture model are that it explains the differences in chemical element abundances in the Earth and the Moon and it explains how the Moon developed its prograde orbit. Disintegrative Capture (if it occurs) is perhaps more likely to happen even than the Impact Theory because the probability of a planetisimal coming close to the Earth is greater than a small planetisimal actually colliding with the Earth [37].
An apparent downfall of this hypothesis is that scientists simulating the passing of viscous, molten planetisimals past Earth (within the Roche Limit), concluded that the amount of disruption required to rip the edge of a planetisimal off would not be enough, if any at all because the viscosity of the material on the edge of the passing body and the relatively short passing time of the object would mean that the stresses would not be great enough to distort the planetisimal [42].
To conclude it is very unlikely that intact capture can explain the presence of our Moon and it certainly cannot explain the formation of our Moon. Disintegrative Capture offers a more insightful look into the formation of our Moon although more research is needed to establish whether disintegrative capture is a phenomenon that can actually happen.
The main advantages of this disintegrative capture model are that it explains the differences in chemical element abundances in the Earth and the Moon and it explains how the Moon developed its prograde orbit. Disintegrative Capture (if it occurs) is perhaps more likely to happen even than the Impact Theory because the probability of a planetisimal coming close to the Earth is greater than a small planetisimal actually colliding with the Earth [37].
An apparent downfall of this hypothesis is that scientists simulating the passing of viscous, molten planetisimals past Earth (within the Roche Limit), concluded that the amount of disruption required to rip the edge of a planetisimal off would not be enough, if any at all because the viscosity of the material on the edge of the passing body and the relatively short passing time of the object would mean that the stresses would not be great enough to distort the planetisimal [42].
To conclude it is very unlikely that intact capture can explain the presence of our Moon and it certainly cannot explain the formation of our Moon. Disintegrative Capture offers a more insightful look into the formation of our Moon although more research is needed to establish whether disintegrative capture is a phenomenon that can actually happen.