Alternate Theories of Moon Formation

Throughout history scientist have dealt with the question of how our Moon was formed. There have been several proposed theories and hypotheses. They all at some point have been disfavored because evidence has shown that they couldn't possibly be the explanation of how our Moon formed. The first theory is the great capture theory, which states that the Earth captured the Moon, the second theory is the co-accretion theory it states that the Earth and Moon formed at the same time, the third theory is the fission theory which states that the Moon was part of the Earth and at some point in time it broke off. The last and final theory is the most commonly accepted by all scientists; it is the giant impact theory. This theory states that a Mars-sized object collided with our Earth and from the debris of the collision, our Moon formed becoming Earth's only natural satellite. The Apollo missions also had a great deal in the discovery of which one was the most accepted theory. Every time one of these missions was performed, evidence was brought back that seemed to prove one of the other three theories wrong.

The capture theory states that our Moon was captured by the Earth with the Moon forming somewhere else. This means that at some point in time the Moons own orbit got close enough to the Earth that the Earth's gravitational pull "captured" it. This theory is disfavored because it would have required that they pass each other at low speeds and that the Moon's orbit loses energy while coming close to Earth. It is also rejected because if it was captured it would have been released at some point in time because the Earth wouldn't have had enough energy to capture the Moon. This theory however would explain the difference in densities and composition of the Moon and Earth; but it would be difficult to explain why the Moon doesn’t have an iron core. 

The co-accretion theory, also known as the condensation theory states that the Moon was formed at the same time as planet Earth; therefore, that both were came from the original nebula that formed the solar system. The weakness of this theory is that is because of the Moon and Earth were condensed together the gravitational force of Earth would have caused for the Moon to become part of the planet and not a natural satellite. However, when the Moon was close to plant Earth a whole day would be completed in five hours instead of twenty-four how it is now. If the Moon and planet Earth had condensed together than the Moon would have the same amount of Iron (Fe) as Earth. They both came from the same cloud of material. But, in the end, the theory is disapproved because if they had been condensed together than they would share similar properties, such as gravity force, densities, inner cores, etc. 

The fission theory proposes the idea that that our Moon was once part of Earth and somehow separated from Earth early in the formation of the solar system. The Fission Theory states that the Moon broke from a rapidly spinning proto-Earth. The chunk of Earth that separated to form the Moon is believed to have come from the Pacific Ocean basin. This used to be the most acceptable theory, and it was thought possible since the Moon's composition resembles parts of the Earth’s mantle.  These objects are known as siderophiles found in Moon rocks.  Not only does the Fission theory provide an explanation for the similar siderophiles, but also explains the Moon’s iron core.  However, the Moon lacks "fossil evidence" of a rapid spin, and doesn’t orbit the Earth following the equatorial plane. Furthermore, in order for the Fission Theory to be true, the Earth would have had to be spinning so fast, that it is believed to be highly improbable. 

The main reason why the Apollo Missions were launched was to explore the three theories that NASA believed were how the Moon was formed.  Their goal was to choose the right theory, but not one of the three theories seemed to be consistent with the projects (Apollo Missions) on how the Moon or any of the other planets formed. One piece of evidence that proved that none of the theories were favored was that the Moon rocks were depleted in elements that vaporize quickly; therefore, the diminution in these elements cause rocks to be very dry. As the Moon heated up it lost almost all of its H2O. It got to a point where it heated up hotter than planet Earth. Since all three original theories were disfavored, the Apollo missions found some evidence that dispute these theories; therefore, another theory came along: giant impact theory. With the help of the Apollo missions amongst many others that have been launched today the most accepted theory is the giant impact theory. 

As this article shows science is never easy, there are always many theories and hypotheses that have to be meticulously studied until the right one is found. Who would think there is so much work that goes into finding out information about our only natural satellite? The Moon is something great that will never quite be understood and for now the only accepted explanation is the giant impact theory.

Works cited
StarChild Question of the Month for October 2001,The star child team, Web. 9 Feb 2010.
Theories of Formation for the Moon, Web. 9 Feb 2010.

Theories and Formation of the Moon