The moon may have made life as we know it possible here on Earth, but it's also full of mysteries. We don't even know its exact origins.
Wondering about the moon is a pastime that has been enjoyed by scientists, philosophers and artists throughout history. Galileo was the first scientist to point out that the moon has a landscape similar to Earth's.
Over time, other scientists have posited a variety of theories about what the moon is and where it came from. From mostly debunked hypotheses to the current prevailing theory, scientists have debated several scenarios, each of which might explain our moon, but none of which are without flaws.
1. Fission theory
In the 1800s, George Darwin, the son of Charles Darwin, suggested that the moon looked so similar to the Earth because at one point in Earth's history, Earth might have been spinning so fast that part of our planet spun off into space but was kept tethered by Earth's gravity. Fission theorists posit that the Pacific Ocean might be the site where the would-be moon material came off of Earth. However, after moon rocks were analyzed and introduced to the equation, they largely debunked this theory because the moon rock compositions differed from those in the Pacific Ocean. In short, the Pacific Ocean is too young to be the source of the moon.
2. Capture theory
According to the capture theory, the moon traveled about the solar system before getting stuck in Earth's gravitational pull. (Photo: NASA/JPL/Wikimedia Commons)
The capture theory suggests that the moon originated elsewhere in the Milky Way, completely independent of Earth. Then, while traveling past Earth, the moon got trapped in our planet's gravity. The holes in this theory range from suggestions that the moon would have eventually broken free from Earth's gravity because Earth's gravity would have been massively altered by catching the moon. Also, chemical components of both the Earth and the moon suggest they formed at around the same time.
3. Co-accretion theory
This composite image shows the Earth, the moon and a black hole. The co-accretion theory posits that both the Earth and the moon formed together while orbiting a black hole. (Photo: janez volmajer/Shutterstock)
Also known as the condensation theory, this hypothesis offers that the moon and the Earth formed together while orbiting a black hole. However, this theory neglects an explanation of why the moon orbits the Earth, nor does it explain the difference in densities between the moon and Earth.
4. Giant impact hypothesis
An artist's illustration of two celestial objects colliding in a way that some scientists think the moon came to be. (Photo: NASA/JPL-Caltech)
The reigning theory is that a Mars-sized object impacted with a very young, still-forming Earth about 4.5 billion years ago. The planetary object that impacted Earth has been dubbed "Theia" by scientists because in Greek mythology, Theia was the mother of the moon goddess Selene. When Theia hit Earth, a portion of the planet came off and eventually hardened into the moon. This theory does a better job than others of explaining the similarities in chemical compositions of the Earth and the moon, however it doesn't explain why the moon and the Earth are chemically identical. Scientists have suggested that, among other alternatives, Theia could have been made of ice, or that Theia could have melted into Earth, leaving no separate trace of its own on the Earth or the moon; or Theia could have shared a close chemical composition to Earth. Until we can determine how large Theia was, at what angle it hit the Earth and precisely what it was made of, the giant impact hypothesis will have to remain just that — a hypothesis.
New lunar findings will inform the continued discussion of the origins of the moon. (Too bad we can't simply ask the man in the moon how he got there.)