Summary
Highlights
The Moon is unusually large relative to Earth, about one-quarter of Earth's size and one-eightieth of its mass, which is uncommon compared to other planetary moons. Its composition is also strange, being mostly rock with very little metal, unlike other objects of similar size in the solar system. These oddities raise questions about its formation.
Before the Apollo missions, three main hypotheses existed: the Capture Theory, suggesting the Moon formed elsewhere and was captured by Earth's gravity, which is unlikely due to compositional differences and the Moon's size; the Twin Formation Theory, proposing the Earth and Moon formed simultaneously from the same material, which is debunked by their differing compositions (Earth has more metals); and the Fission Theory, which posited the Moon spun off a rapidly rotating Earth, but this is disproven by physics.
After data from the Apollo missions, the Large Impact Hypothesis became the leading theory. Around 4.4-4.5 billion years ago, a Mars-sized object collided with a young Earth. This massive impact ejected material, primarily from Earth's crust and mantle (rocky material), into orbit, forming a disk that eventually coalesced into the Moon.
Computer simulations support the physics of this collision, demonstrating how a moon could form. This theory explains the Moon's rock-heavy composition (due to being formed from Earth's mantle and crust). It also accounts for Earth's axial tilt of 23.5 degrees, as an off-center impact could have caused this. Additionally, the large impact would have vaporized volatile elements like water, explaining their scarcity on the Moon, and clarifies why the Moon's orbit is tilted relative to the Earth's equator.