December 12, 2024

Bradley Truong

12th Grade

Fountain Valley High School

Looking up at night, some may be curious as to how the brightest object in the sky came to be, and how it ended up in Earth’s orbit. Astronomers and other scientists were no exception, as throughout the years, many different scientific hypotheses were formed to explain how the Moon came to be. The main hypotheses involve fission, capture, accretion, and giant impact, though the last of these is the most widely accepted.

The fission hypothesis was proposed by George Darwin, son of Charles Darwin. It states that what would become the Moon was expelled from the Earth due to the latter’s rapid rotation. However, a splitting in this form has been considered implausible to have occurred, and it does not explain how the Moon is made up of different materials from the Earth.

The capture hypothesis states that the Moon was a lone body until it was captured by the Earth and brought into the latter’s orbit, similarly to many other moons in the solar system, like those of Mars. However, there doesn’t appear to be a plausible way that the Earth would have managed to capture a moon with such a size, and captured moons tend to have orbital paths that don’t line up with the planet, instead of the Moon’s lined-up, circular orbit. It also does not explain the Moon’s chemical similarities with the Earth.

The accretion hypothesis, also known as the sister hypothesis or the co-formation hypothesis, suggests that the Earth and Moon simply formed at the same time and place. Similarly with the fission hypothesis, it does not explain differences in lunar material compared to the Earth, along with the fact that the Moon is much less dense than the Earth.

The problems of the previous hypotheses, including similarities and differences in composition, are mostly solved by the giant-impact hypothesis, in which a Mars-sized object, nicknamed Theia, collided with the Earth about 4.5 billion years ago. Debris from the explosion was sent into Earth’s orbit, forming a ring around the Earth, and it eventually collapsed into the Moon.

More evidence for the giant-impact hypothesis can be found in Moon rocks from meteorites and Apollo missions, in which said rocks have similar chemistry to the Earth’s, while also having smaller differences due to materials from Theia. Furthermore, analysis of the mineral composition of the lunar surface suggests that it once had an ocean of magma, which would have been caused by such a massive collision.

Other variations of the giant-impact hypothesis involve either multiple impacts or a much greater impact that vaporized and liquefied the Earth, forming a cloud of vaporized and molten rock known as a synestia, which eventually cooled and formed the Moon alongside a new Earth. 

Despite the giant-impact hypothesis’ acceptance, there are existing issues with the model, including how a large collision would cause the Moon’s orbit, rotation, and gravitational interactions with the Earth. There are also other oddities, including radioactive elements on the near side, a thicker crust on the far side, and the Moon’s history of volcanism. Such discrepancies will likely be smoothed out as we learn even more about the Earth’s closest companion.