Where Did the Moon Come From?

Coaccretion is the first of three older sets of ideas that describe how the Moon was formed. This theory posits that the Moon and Earth were formed at the same time from a primordial accretion disk—a disklike flow of gas, plasma, dust, or particles around an astronomical object that slowly collapses inward—which would help explain the geological similarities between the two. Gas from the cloud condensed into material and debris that were pulled in to attach to one or the other of these bodies. Earth happened to pull in more material and increased its mass. Of the two bodies, Earth’s mass allowed it to develop the dominant gravitational pull, and the Moon began to orbit Earth. However, critics noted that this model failed to explain the current angular momentum of the Moon around Earth.

In another set of early Moon-formation theories—one of which sprung from the mind of Sir George Darwin, English astronomer and son of naturalist Charles Darwin—Earth was thought to have once spun so rapidly that chunks of material flew from its surface. This material was thought to have later condensed into the Moon. Although fission theories appeared convincing—since the composition of Earth’s mantle and the Moon were similar—they fizzled over time because no one was able to discover the right combination of properties for a spinning proto-Earth that would generate the right kind of proto-Moon. More specifically, scientists simply did not believe that Earth could ever spin fast enough to throw off pieces of itself. In addition, so far no evidence of such a rapid spinning event on Earth or the Moon has been found.

A third set of older theories posited that the Moon could have formed somewhere else within the solar system but outside Earth’s gravitational influence. Some scientists think that the Moon may have even been in the thrall of another planet for awhile before it broke free. As the theories go, the Moon passed close to Earth sometime later. The path was so close that Earth was able to capture it within its orbit. Although other planets, such as Mars, are thought to have captured small asteroids which have since become de facto moons, scientists have yet to figure out the mechanics behind how Earth could have captured the Moon and forced the Moon’s velocity to brake hard enough for it to remain in orbit. In addition, capture theories fell out of favor after it was discovered that Earth and the Moon were geologically similar to one another.

The first of three theories that rely on the violent collision of a Mars-sized planet called Theia with Earth, this variation presupposes that Theia was made up of different, possibly weaker, material than Earth. When Theia struck, Earth remained relatively intact. Theia, however, broke apart, and the pieces that were left eventually coalesced into the Moon. Although this theory was compelling, it broke down because Earth and the Moon are made up of similar elements (silicon and oxygen, in particular) in similar concentrations.

What if Theia struck the young proto-Earth with such force that both bodies were vaporized? Some scientists propose that a strange rotating bagel-shaped cloud called a synestia could have been created by the impact. They argue that this structure could have acted like a kind of rotating mixing bowl, which blended the chemical elements found within each body evenly. Over time, the material on the very outside of the synestia coalesced into the Moon, while the rest of the material coalesced into Earth.

In this scenario, Theia still strikes Earth, but vaporization did not result, and the debris from the impact still coalesced into the Moon. What’s unique about this theory is that in it the material that makes up Theia happens to be the same stuff that makes up Earth. No harm, no foul, right? So the question becomes: how did Theia form? Perhaps both Theia and Earth formed on opposite sides of the same accretion disk (whose material was spread evenly throughout). Later, something disturbed Theia’s orbit around the Sun and caused it to drift away from its original location, which ultimately resulted in Theia crashing into Earth.

In this theory the early Earth is thought to have been pummeled not by one but by several impacts. Each strike is thought to have created a debris field that eventually coalesced into a tiny moonlet. Later, these smaller moonlets merged with one another to form the Moon. This hypothesis is unique in that it doesn’t rely on one single “smoking gun.” It allows for several events to have grown the Moon incrementally. The model notes that a disk of material would form within hours of each strike and that this material would condense into a single moonlet over the course of a few hundred years. Israeli scientists proposed this idea in early 2017 and argued that the aggregated effect of multiple high-velocity impacts could have produced enough material to form the Moon. They also said, however, that the mechanisms that explain how each of these individual moonlets got together into one larger body have yet to be described.