The question of the origin of the Moon has fascinated man since at least the time that Kant first hypothesized that lunar tides should impact the motions of the Moon. Where did the Moon come from? Many people from many scientific fields have weighed in on the subject. It was hoped that the Apollo missions would answer this long standing mystery. But alas the failure to do so may have been the greatest scientific disappointment of the Apollo effort. As you may well be aware, there are four major categories of lunar origin: the binary model, fission model, collision model, and capture model.
The Binary Model
The binary model, in some respects, is the simplest and least problematic model and thus is usually the default model. Binary formation is what would be expected in the standard solar system formation process with the Moon forming as part of the Earth’s nebular formation. Satellites which originate in this manor are called regular satellites. Satellites which have unusual orbits typified by a “backward” revolution about their primary are considered irregular satellites. This model does not require “special circumstance” for the formation of the Moon. The situation of the Earth-Moon system however, is generally believed to be unique. This is why the other three models, the fission model, collision model, and capture model, have been developed to answer these unique conditions in the Earth-Moon system.
The Fission Model
The fission model was the model taught in grade school during the Apollo era. Interestingly, at the time of Apollo, it was still taught that the Pacific Ocean was formed by the fission of the Moon from the Earth’s crust. Thus the model of plate tectonics and mid ocean ridges was still a hypothesis while man was walking on the Moon.
The fission model was first presented by George Darwin, the son of Charles Darwin. This was really the first scientific and comprehensive model developed to explain the origin of the Moon. Darwin’s work was done on rapidly spinning viscous bodies. He calculated that the Earth would have about a 2.5 hour day in order to have the centrifugal force to spin the Moon off. It was later determined that the fission model is not dynamically possible for two major reasons. First, the bulge in the Earth which ultimately would form and be flung off would cause instability in the Earth’s spin leading to a slowing of the Earth’s spin. This negative feedback would dampen the spin of the Earth before it could reach a fission speed. Second, it is not believed that the Earth could slow from a 2.5 hour day to a 24 hour day in the 4+ billion years which have elapsed since lunar formation. The classic fission model seems not to be possible but many of its consequences have been reborn in the newer collision model.
Collision vs. Capture
The collision model is currently clearly the darling of the scientific community, but the capture model was once the prized son. The collision model, like the two previous models, has the Earth and Moon sharing matter, thus chemical evidence of a common origin is an important constraint. The capture model on the other hand has the Earth and Moon forming as totally independent bodies, sharing only a gravitational bond. Both models have weaknesses and strengths.
The Collision Model
The collision model achieves wide acceptance.
The collision model, developed in the 1980s, originally had the same main objective as the fission model; to explain the observed similarity in density of the Earth’s mantle and the Moon. The original idea was that the Moon formed as a result of a collision between the protoearth and another free roaming planetary body.
The similarity in the density between the Moon and the Earth’s mantle had long attracted scientific interest. In the beginning it was thought that the collision model would account for this similarity in density, just as it had been thought that the fission of the Earth’s mantle would account for the similarity. Ironically, further investigation led to the understanding that in the case of collision, the Moon would actually be composed primarily of the impacting protomoon, not the protoearth.
A strong selling point of the collision model is that computer simulation of a protomoon impact into the Earth can produce the desirable outcome of creation of a moon. These simulations however, are quite simple approximations of reality.
Trouble in paradise.
The collision model still has some difficulties which need to be overcome. One serious problem is that it appears that the lunar material has not been subject to temperatures in excess of 1200K (Taylor, lunar science status report). Thus, the Moon seems not to have been vaporized. It is generally believed that the collision of the protomoon and protoearth would have vaporized at least the impactor indicating that the Moon should show signs of vaporization in its chemistry. This appears not to be the case.
The Moon appears to also lack any water bearing minerals. Given the aqueous nature of the Earth it is unusual that the Moon would be so dry. It now appears that water does exist on the Moon, however, it is believed to be more of a surface dusting.
Another potential problem with the collision model results from the finding that in order to generate a Moon such as ours in a collision origin, the initial proto-spin of the Earth must must have been around 2.5 hours. (45) The rapid spin rate would have to have slowed to our current 24 hour day, and this may be difficult to achieve. This same result was a blow to Darwin’s fission model and must be dealt with in proving the collision model.
The Capture Model
The observation that our Moon is a rather unusual partner for the Earth was the strongest selling point for the capture model. The Moon’s unusually large size, its non-equatorial orbit and its tidally locked orientation all suggested a possible capture origin. (NASA’S evolution of the Solar System)
The capture model was a favorite model up until the Kona conference held in 1984. The greatest difficulty with the capture model is the dynamics of the capture event itself. This event is typically imagined as a freely moving moon nearing the Earth and the gravitational power of the Earth literally slowing the Moon to the point that it becomes permanently “captured” into a geocentric or Earth centered orbit. The difficulty with this proposition is primarily the relative size of the Moon compared with the Earth. The kinetic energy of the Moon which would be required to be dissipated in order to facilitate a captured Moon is immense. It is felt that the capture event window would be too brief to allow this amount of energy to be dissipated from the lunar motion.
One solution to this slowing problem was what is called the gas drag model. In this capture variation it was postulated that the early Earth was enveloped in a thick gas cloud which would contribute to the slowing of the Moon as it approached the Earth. Even with this added dissipation mechanism, capture was still found to be too energetic of a process to be very likely.
It is important to recognize that at the time of the Kona conference the concept of tidal heating in the solar system was still poorly understood, just having been postulated and proven in the previous five years. Another major problem with the capture model was thought to be that oxygen isotope evidence proved a similar place of origin for the Earth-Moon system in the solar nebula. For the Earth and Moon this was seen as conclusive evidence that the capture model was not the correct model. This oxygen isotope evidence has since been found to be unrelated to the origin argument. Thus what was once thought to be one of the few strikes against capture was later called into question.
Finally, the findings of the isotopic dating of the lunar mare, using moon rocks from the Apollo mission, led scientists to believe that the lunar mare are much older than the time frame being considered for this model. However, new evidence throughout the solar system brings the isotopic date of the lunar mare into question as recent cratering information from the Mars Surveyor reveals a much younger Martian surface than previously thought. Since the lunar mare isotopic dating information was used indirectly (via crater counting) to date Mars’ surface, these recent findings bring into question the isotopic date of the lunar mare. The release of the constraint of an ancient lunar mare combined with new evidence of tidal heating in the solar system have reinvigorated the argument surrounding the origin of Earth’s only Moon.
All of the work that was done on the capture model remains relevant but the distinction with this model is that the Moon wasn’t captured from somewhere else but instead had a temporary close approach or episode within its original orbital evolution. The gravitational dynamics of The Capture Model and the Close Approach Lunar Model are similar. Isotopic dating of the lunar mare is very important to the CALM because this model postulates that tidal heating of a close approach is evident on both the Earth and Moon. Tying the lunar mare basalt episode to the Earth’s continental flood basalts in geologic time is a critical proof for the CALM.