Maybe you've never looked up into the night sky and asked yourself, "How did the Moon form?" After all, from the human perspective, it’s always been hanging in the sky. While you may take it for granted, the truth is that the Moon hasn’t always been there. Over the centuries there have been many theories about how the Moon formed, but scientists don't have definitive proof any of these scenarios are correct.
Some experts hoped that trips to the Moon would solidify or debunk a few Moon formation theories, but analysis of lunar material brought back from NASA’s excursions has introduced as many questions as it has answered. The Moon shares much of the same material as Earth, but it contains mystery stuff as well. Maybe a giant object impacted the planet as it was forming, breaking off a piece that became the Moon. Or perhaps the Moon formed separately and somehow found its way into Earth’s orbit.
People may never know for certain where the Moon comes from, or why the Earth only has one satellite, but it's fascinating to consider these theories all the same.
The Giant Impact Theory
Scientists agree that the Earth was formed around 4.5 billion years ago, and it’s generally believed that the Moon was formed not too long after. Perhaps the most popular theory about how the Moon was formed involves a massive collision. The two bodies in question were protoplanets, or worlds that were still in the process of forming. One was an early Earth, and the other a Mars-sized object referred to as Theia. The two crashed into each other, and the resulting debris eventually formed the Moon.
Analysis of lunar rocks tends to support this theory, as they contain similar oxygen isotopes to those on Earth. The outlying chemical compounds give scientists a possible look into the composition of Theia.
The Fission Theory
In the 19th century, Charles Darwin’s son George theorized that the Moon was created by ejected material flung from the Earth’s superheated spinning core. The material cooled in space and coalesced into an orbiting body - the Moon.
Most experts discount the theory, as it’s generally agreed that the Earth couldn’t have been spinning fast enough to break that much material away.
The Co-Formation Theory
Instead of the Moon being a dislodged chunk of Earth, this theory posits that both the Earth and the Moon were formed at the same time from the same material. According to this hypothesis, while the Earth was being formed by cosmic dust, the nearby Moon was being formed by that same dust in a different place. This accounts for the position of the Moon in relation to the Earth. It also explains why both the Earth and the Moon have similar material. However, the lunar material that’s different from the Earth's is not accounted for.
The Capture Theory
One of the theories about how Earth got its Moon actually has little to do with the formation of the Moon itself. This theory states that the Moon formed elsewhere and was eventually captured by Earth’s gravity until it fell into a stable orbit. There are several moons of this sort in the solar system; the moons of Mars, Phobos and Deimos, as well as Neptune’s moon Triton, are all thought to be captured satellites.
However, there are several holes in this theory. First and foremost, captured objects usually aren’t spherical like the Moon. The second and most concrete strike against the theory is that the Moon and Earth are both made of nearly identical material, which means the Moon most likely came from the Earth.