User:RobDumi/sandbox

Theories on the Formation of Earth's Moon

The formation of Earth's moon has been a widely pondered and researched topic for many years now. Prior to the Apollo missions, there had been a few theories that were quite prominent concerning how Earth's moon came to be. However, the Giant Impact Hypothesis, which involves a proto-planet dubbed Theia colliding into Earth during its formation, causing debris to coalesce and form a satellite in orbit around Earth, has since been the subject of scientific consensus. This is still a question that lacks a concrete answer, as there is still much discussion regarding any theory's viability.

Pre-Apollo Theories

Capture Theory

The idea that Earth captured the Moon as it approached was a widely popular one for quite some time. It is not unusual for planets to have captured satellites, as we have seen with many of the planets residing in the outer solar system and even with Mars' moons, Phobos and Deimos. This theory was also attractive at the time due to its solution of competing theories' primary downfalls: composition. Since, in the scope of capture theory, the Moon would have been initially formed in a different region of the solar system, the fact that the Moon's density is significantly lower than Earth's is not detrimental to this hypothesis. However, the question about the specific mechanics regarding Earth capturing the Moon remains and proves to be this theory's ultimate undoing. Given the relative mass of the Earth and its moon, it would be highly unlikely that Earth's gravity alone could have countered the kinetic energy possessed by the Moon.

Co-Formation Theory

The Co-Formation Theory proposes that the Earth and its Moon formed simultaneously in approximately the same relative locations as they are at now. Many moons do indeed form in this manner, and this theory would not need to explain how the Moon got to its current location. However, a moon formed in this fashion would have a nearly identical composition to its planet, which implies a nearly identical density. This is not the case for Earth's moon, as its density is far too low to possess an iron core like Earth's, rendering this hypothesis highly unlikely.

Fission Theory

This theory involves the idea that Earth had not yet hardened at the time the Moon had formed, leaving the mantle vulnerable to separation from the Earth due to potentially high centrifugal force generated by Earth's rotation. It was theorized that this event could have created the basin where the Pacific Ocean now resides. The Fission Theory additionally has the benefit of describing why the Moon does not possess an iron core. However, there are a few problems with this hypothesis that dismiss its viability. Firstly, the Earth would likely have not been rotating quickly enough for this to occur. And secondly, if the Earth had been rotating quickly enough, it would not have been able to slow down to its current rate during the time elapsed since the fission would have taken place.

Giant Impact Hypothesis

The crux of the Giant Impact Hypothesis is that, during Earth's formation, a hypothetical proto-planet, Theia, collided with Earth sending debris into its orbit. This debris would ultimately gather and form the Moon. If the Moon were to have formed in this fashion, it would answer many of the previous theories' problems. For instance, this would explain why the Moon's density is lower than that of the Earth, because all of the particles ejected into orbit from the collision would have originated from the outer layers of the Earth, which are far less dense than those closer to the center. There is evidence that the Moon's surface had been extremely hot at some point during its life, and this hypothesis would explain why that is the case, as an impact of this scale would generate an immense amount of heat and much of the material would be molten during the Moon's formation. A major setback for this hypothesis is that we do not yet understand what Theia's composition might have been. Over 60% of the Moon's material should have originated from Theia according to models, yet its composition is almost identical to that of Earth's outer layers, which indicates that Theia's composition would have also had to be quite similar to Earth's, which seems like an unlikely event unless the models are wrong or Theia formed at a similar distance from the Sun as Earth.

Synestia Theory

An artists rendition of what a synestia might look like

It is possible to consider the Synestia Theory a derivative of the Giant Impact Hypothesis due to the underlying event being a large impact. However, the two are mechanically quite different. The Synestia Theory proposes that the Earth and its Moon both formed from the same structure, called a synestia. This hypothetical structure, composed of both molten and vaporized material, is the result of a high angular momentum collision between two large bodies. As the synestia cools, lunar seeds can form outside of the Roche Limit after which, material begins to get caught onto the seeds, ultimately forming moons. The rest of the material condenses into a planet, producing a planet with moons from the same material. This theory explains why the Moon does not have an iron core like Earth's (iron would be at the center of the synestia, inside the Roche Limit) as well as why the Moon does not contain any trace of Theia.