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Did the Earth Once Have Two Moons?

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Did the Earth Once Have Two Moons?

Image of the far side of the Moon.

Image Credit: NASA

Ask any elementary school aged child what that bright object is in the night sky and they will, undoubtedly, reply that it is the Moon.

This grayish orb has been studied and scrutinized for millennia by astronomers across the globe. Over the years we have come to understand more and more about our nearest neighbor, and that knowledge took a large leap forward during the Apollo Moon landings when samples were gathered and brought back to Earth for analysis.

There have also been unmanned missions to orbit the Moon, to analyze both what we see (the familiar side of the Moon that always faces the Earth) as well as the far side of the Moon which most of us never observe -- except perhaps in pictures.

Even after all of this study there are still properies of the Moon that baffle scientists. And perhaps the biggest mystery of them all is why the far side of the Moon appears so different than the side that we see every day.

The Conventional Wisdom

The problem that scientists have sought to answer for so many years, why the far side of the Moon has a vastly different terrain - specifically how the upper crust is thicker and more mountainous - has had many proposed solutions.

Most commonly researchers argue for the gravitational tidal forces theory. Simply, this proposition states that the Earth's gravitational force, which varies as the distance an object is away, that the Moon feels is different (more powerful) than the side that is farther away.

The result is that the closer side gets thinned out as the inner bulk of the Moon is pulled ever so slightly toward earth, allowing the far side to remain thicker.

This makes sense, and explains quite well why the upper crust of the Moon is thicker than the nearer side, but there are still questions that remained, like how the terrain itself became so different.

Some have argued that since the nearer side of the Moon protected by Earth it is more difficult for asteroids and comets to impact it, making it less jagged. However, this doesn't quite make sense, because the bumpy terrain on the far side can't simply be explained by impacts. Besides, the nearside of the Moon is fairly well cratered.

Two Moon Theory

A new theory has emerged that suggests that perhaps the reason the Moon appears so differently from one side to the next is because at one time it, in fact, was two separate objects.

Essentially the theory argues that the two moons formed out of a large impact with Earth. Material lifted out of our still forming planet was captured by our own gravity and held in a ring like shape in orbit around the Earth.

As the material cooled it also clumped together. At this point, conventional theory would suggest that the matter simply came together into a single spheroidal object that we now call our Moon. But scientists now think that it may have instead initially formed two separate objects.

These objects then individually orbited the Earth on a slow collision course toward each other. Still cooling, these mini-moons (one would have been considerably larger than the other however) eventually collided and smooched together.

While I singular Moon was created from the collision, evidence of their individuality remains in the vast differences between the two sides of the grayish orb in the night sky.

But is it True?

That is the million dollar question. There are no obvious flaws in the theory, other than it can't be directly proven; at least at this point.

The largest hurdle this theory faces is that, while it explains well how the Moon could have formed the way that it did, it doesn't necessarily provide any conclusive evidence for universal acceptance.

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