The Birth of Earth's Moon

Harvest moon in Japan 2013.
The Moon's origin is still a very active area of study for planetary scientists.

The Moon has been a presence in our lives as long as we've existed on this Earth. However, a simple question about this spectacular object went unanswered until fairly recently: how was the Moon made? The answer lies in our understanding of conditions in the early solar system. That's when our Earth and other planets were formed. 

The answer to this question hasn't been without controversy. Until the last 50 years or so every proposed idea about how the Moon came into being has been faced many problems.

Co-Creation Theory

One idea says the Earth and Moon formed side-by side out of the same dust and gas. Over time, their close proximity might have caused the Moon to fall into orbit around Earth.

The main problem with this theory is the composition of the Moon's rocks. While Earth rocks contain significant amounts of metals and heavier elements, particularly below its surface, the Moon is decidedly metal poor. Its rocks just don't match Earth rocks, and that's a problem if you think they formed from the same piles of material in the early solar system.

If both were created out of the same set of material, their compositions would 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 formed at the same time but ended up with such vast differences in composition is pretty small.

Lunar Fission Theory

So what other possible ways could the Moon have come about? There's the fission theory, which suggests that the Moon was spun out of Earth early in the solar system's history.

While the Moon doesn't have the same composition as the entire Earth, it does bear 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's a problem with that idea, too. Earth doesn't spin nearly fast enough to spit anything out and likely didn't early in its history. Or, at least, not fast enough to hurl a baby Moon out to space. 

Large Impact Theory

So, if the Moon wasn't "spun" out of the Earth and didn't form from the same set of material as Earth, how else could it have formed?

The large impact theory may be the best one yet. It suggests that 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, which planetary scientists have called Theia, is thought to have collided with the infant Earth early in its evolution (which is why we don't see much evidence of the impact in our terrain). Material from the Earth's outer layers was sent hurtling into space. It didn't get far though, as Earth's gravity kept it close by. The still-hot matter began to orbit about the infant Earth, colliding with itself and eventually coming together like putty. Eventually, after cooling, the Moon evolved to the form that we are all familiar with today.

Two Moons?

While the large impact theory is widely accepted as by far the most likely explanation for the Moon's birth, 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 all know today. This idea may explain some aspects of the Moon that other theories do not, but much work needs to be done to prove that it could have happened, using evidence from the Moon itself. 

Edited and updated by Carolyn Collins Petersen.