The Moon is such an important presence in our everyday lives that, for centuries, our sole natural satellite has been an object to fear, the marvel at and to strive for. But a simple question about this spectacular objet went unanswered until fairly recently. How was the Moon made?
The answer to this question hasn't been without controversy. Until the last 50 years or so every proposed solution has been faced with unconquerable problems.
Simply, this theory proposes that the Moon was created independently from the Earth in the early Solar System. Shortly after the Moon crossed paths with a young Earth and was captured by our planet's gravitational field.
With the presence of all the other planets in our Solar System, this seemed like an attractive idea. Only one problem: the math doesn't work.
While smaller than our world, the Moon is still a significant percentage of the Earth's mass. In order to slow down such a massive object and pull it into orbit would require conditions that are extremely unlikely to have existed in the early solar system.
So what if, instead of the Earth capturing a passing moon, the Earth and Moon were formed side-by side out of the same dust and gas? Eventually then, over time, their close proximity would cause the Moon to fall into orbit around the Earth.
The main problem with this theory is the composition of the Moon. While the Earth contains significant amounts of metals and heavier elements, particularly below its surface, the Moon is decidedly metal poor.
If the two orbs were indeed created out of the same set of material their compositions would need to be very similar. We see this as the case in other systems when multiple objects are created in close proximity for the same pool of material. The likelihood that the Moon and Earth could have co-formed but ended up with such vast differences in composition is virtually impossible.
Lunar Fission Theory
So capture theory fails because the Earth and Moon almost certainly would not have come together if created at great distance. But co-creation theory fails because their compositions indicate that they couldn't have formed close to each other. So what's left?
Seeking to overcome the short falls of the other two, fission theory suggests that the Moon was spun-out of the Earth.
While the Moon doesn't have the same composition as the entire Earth, it does bare a striking resemblance to the outer layers of our planet.
So what if the material for the Moon was spat out of the Earth as it spun around early in its development. Well, there is an obvious problem here: the Earth doesn't spin nearly fast enough.
Large Impact Theory
The revelation, and basis for, lunar fission theory is that the moon shares a basic composition with the outer layers of the Earth. This fact is too important to ignore. But if the Moon wasn't "spun" out of the Earth, how else could it have formed out of the outer layers?
Large impact theory takes the best of every theory and puts it into one. Instead of being spun-out of the Earth, the material that would become the Moon was instead ejected from the Earth during a massive impact.
An object, roughly the size of Mars is thought to have collided with the Earth early in its evolution (which is why we don't see much evidence of the impact in our terrain). The result was so significant that material from the Earth's outer layers were sent hurtling into space.
The material couldn't get far though, as Earth's gravity kept it close by. Still hot, the matter began to orbit about the Earth, colliding with itself and eventually coming together like putty.
Eventually, after cooling the Moon came to exist in the form that we are all familiar with today.
While the large impact theory is widely accepted as by far the most likely theory, there is still at least one question that the theory has difficulty in answering. Why is the far side of the Moon so different than the near side?
While the answer to this question is uncertain, one theory suggests that after the initial impact not one, but two moons formed around the Earth. However, over time these two spheres started a slow migration toward each other until, eventually, they collided.
The result was the single moon that we are all familiar with, but the differences in terrain and crustal structure leave behind evidence that they may have one day long ago, been two.