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. It has been around our planet much longer, practically since Earth was formed. However, one simple question about this spectacular object went unanswered until fairly recently: how was the Moon made? The answer requires a deep understanding of conditions in the early solar system and how they worked during the formation of the planets.

The answer to this question hasn't been without controversy. Until the last fifty years or so every proposed idea about how the Moon came into being has had problems, either with technical aspects, or plagued by scientists' own lack of information about the materials that make up the Moon.

Co-creation Theory

One idea says the Earth and Moon formed side-by-side out of the same cloud of dust and gas. That makes sense, given that the entire solar system arose from actions within that cloud, called a protoplanetary disk.

Over time, their close proximity might have caused the Moon to fall into orbit around Earth. The main problem with this theory is in 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 for a theory that suggests they both formed from the same piles of material in the early solar system.

The Sun and planets formed in a cloud of gas and dust called a protoplanetary disk some 4.5 billion years ago. The Moon formed about the same time as Earth, but could have been made during a collision event, rather than co-formed with Earth. NASA 

If they did form at the same time, their compositions should be very similar or close to identical. 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. So, that raises some doubt about the "co-forming" theory.

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 wasn't spinning fast enough to do it early in its history. Or, at least, not fast enough to hurl a baby Moon out to space. 

One idea of the Moon's formation.
The best theory about the formation of the Moon says that the infant Earth and a Mars-sized body called Theia collided early in the history of the solar system. The remnants were blasted to space and eventually coalesced to form the Moon. NASA/JPL-Caltech 


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. 

As with all science, theories are strengthened by additional data. In the case of the Moon, further studies of rocks from various places on and beneath the surface will help fill in the tale of our neighbor satellite's formation and evolution.

Edited and updated by Carolyn Collins Petersen.

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Your Citation
Millis, John P., Ph.D. "The Birth of Earth's Moon." ThoughtCo, Aug. 27, 2020, Millis, John P., Ph.D. (2020, August 27). The Birth of Earth's Moon. Retrieved from Millis, John P., Ph.D. "The Birth of Earth's Moon." ThoughtCo. (accessed March 22, 2023).