How Earth was blessed with a Moon was anybody’s guess 50 years ago. In our solar system it’s a relatively big moon. At slightly over 1,000 miles (1,700 km) in diameter it’s about half the size of planet Mars and there are only four moons (Ganymede, Titan, Callisto, and Io) that are bigger than Earth’s Moon.

Pre-Apollo Mission Theories
So how did Earth end up with a natural satellite that rivals the biggest moons of  the massive planets Jupiter and Saturn? And why just one?

Prior to the Apollo missions to the Moon, there were three main theories of the origin of the Moon. First, was the adopted daughter theory. It proposed that our Moon wandered into Earth’s gravitational pull and was captured. Second, was the mother/daughter theory that suggested the Moon was spun off from the Earth when it was still a molten blob of spinning material. The final theory was the sister theory where both bodies that formed side by side.

Hard Evidence: The Destroyer of Theories
When the Apollo missions came back to Earth with Moon rocks the three existing theories took a big hit. Had the Moon rocks matched the composition of Earth rocks then scientists could dismiss the adopted daughter theory because a wandering Moon wouldn’t likely have rocks similar to Earth’s. If the composition of the Moon rocks were different then they could dismiss the other two theories. What no one saw coming was the idea that the Moon rocks would match the composition of our Earth rocks, except for a lack of iron. The rocks were the same, but different.

None of the theories really met the evidence in hand, but now the geologists had a vital clue. When Earth first formed all the elements were mixed throughout the molten mass that would become our planet. As time passed most of the iron sank deeper into the mass to become Earth’s core. The evidence suggested that the Moon must have formed from Earth’s material after the iron sank into the core.

The New Theory: Impact Earth
It was clear the material for the Moon had to come from Earth, but the transfer of material had to occur after the most of the iron was not mixed in with the shallow layers of molten Earth. Enter the Impact Theory.

Scientists proposed that the Earth must have been hit with a large object (about the size of Mars) that pushed out shallow molten material into a near Earth orbit to create our Moon. The theory assumes the object was absorbed into Earth’s mass and didn’t significantly change the composition of the Earth or the Moon. Those are big assumptions.

The Little Bang Theory
Recently scientists are suggesting a new theory for the formation of the Moon that proposes a natural nuclear explosion blew off part of Earth’s shallow material after most of the iron sank into the core. The idea of natural nuclear reactions are not new. Radioactive material will always start a chain reaction upon reaching its critical mass and we have evidence that Earth has had multiple natural nuclear explosions in the past. It simply requires enough radioactive material to consolidate in close proximity to start a chain reaction. If it is radioactive and it reaches critical mass, BANG! A nuclear explosion.

So is this it? Do we now know how the Moon was formed? Not Exactly. Scientific knowledge is like the work of a detective. Learn something new and you can rule out certain possibilities, but it takes decades, sometimes centuries, to understand enough of what ‘couldn’t have happened’ in order to understand what did happen. It is likely we won’t have many more answers coming until we have scientists working on the Moon again. It’s hard to gather the evidence when you aren’t at the scene of the crime.