The Moon has been a prime example of regularity and stability for as long as humans have had the notion to note patterns in the sky. It always presents the same face to the Earth, and its revolutions and phases cycle around and around like clockwork. However, despite this appearance of placidity, the Moon’s history isn’t entirely one of peace. In fact, our satellite’s birth was precipitated by an event of such violence that, if it were to happen again today, all life on earth would be extinguished instantly.
Before detailing the leading theory for the Moon’s formation, we ought to examine some of the evidence that led scientists to this conclusion. The Earth-Moon system is rather unusual when compared with the rest of the Solar System. In the vast majority of cases, the proportional difference in mass between natural satellites and the bodies they orbit is much larger than that of the Earth and the Moon. Often, these satellites are unrelated to the planets they orbit and were captured by the gravitational well generated by larger bodies, so their orbits are likely to be eccentric and don’t necessarily fall in the plane of rotation of their captors, and their isotopic compositions are generally distinct. However, the Earth’s spin and the plane of revolution for the Moon are very closely aligned, and their isotopic compositions are nearly identical. In addition, the Moon’s internal structure is highly differentiated, consisting of a thin crust, a thick outer mantle, a partially melted inner mantle, a liquefied outer core, and a solid inner core. This indicates that, at some point in the distant past, the Moon must have reached incredibly high temperatures, and the energy required to generate such heat is hard to imagine. All of this information must be accounted for when proposing a hypothesis to explain the Moon’s formation, and, as of now, it appears that only one such hypothesis does an adequate job.
The early solar system was a chaotic place. Planetesimals at varying stages of accretion were flying around willy-nilly, often crashing together and releasing huge amounts of kinetic energy as heat. As the Moon appears to have formed only a little while after the Earth, this is the backdrop against which its creation took place , and this tendency toward catastrophe appears to have played a key role. It’s believed that in one of the many, many collisions that happened during the solar system’s childhood, a Mars-sized object (dubbed Theia, who happened to be the Titan mother of the Greek Moon goddess, Selena ) crashed into Earth. This intersection of trajectories was so energetic that our planet’s crust was entirely liquefied, and huge quantities of material were spewed into space. It’s thought that this Earth-matter then gradually coalesced under its own gravitational pull into the Moon! The giant, super-heated satellite then underwent differentiation as the heavier minerals sank through the liquid to the center of the sphere, forming the nested structure we see today. This hypothesis also neatly explains the other anomalies noted in the previous paragraph: the alignment of Earth’s rotation and the Moon’s orbital plane is a consequence of conservation of angular momentum; the unusual mass ratio of the Earth-Moon system is explained by the sheer energy of the impact, which drove huge quantities of matter into space; and the close similarity in isotopic composition is explained by the fact that the matter in the Moon used to be part of the Earth! Go science – the universe is crazy!