How Did The Moon Form?

Our early solar system likely looked like a giant game of destructive billiards. Proto-asteroids, comets, and planets were colliding into each other. Sometimes these collisions caused complete disintegration of the “victims” and sometimes they caused bigger objects to form. Some proto-planets may even have been punted into the sun or out of our solar system altogether due to gravitational slingshots with planets like Jupiter, which may have wandered all over our solar system before settling into its current orbit.

Early theories included the idea that Earth and its Moon formed relatively peacefully, either together through a process called co-accretion or separately, with the Moon having formed elsewhere in our solar system and then captured by Earth’s gravity. Earth could have originally been a lot larger than it is now, but spun so fast that it created its own Moon through a process called fission.

The current prevailing theory indicates that the formation of the Moon may actually be a result of the early billiards game. About four and a half billion years ago, the Moon formed in a collision between a proto-Earth and another proto-planet that was about the size of Mars and the vaporized material from this collision eventually coalesced into our modern Earth and Moon.

What Evidence Do Scientists Have to Support This Theory?

Like most of the proto-planets, Earth was constantly bombarded with smaller objects early on, though we usually don’t see most of the results of this early bombardment due to the effects of erosion and plate tectonics. That means on Earth, there isn’t much evidence of an ancient collision between two planet-sized objects. Much of the evidence for this theory comes from the Apollo lunar landing missions in which astronauts like Neil Armstrong, Buzz Aldrin, Gene Cernan, and scientist-astronaut Harrison Schmitt brought back a total of 382 kilograms’ worth of samples of lunar rocks and dust.

It wasn’t all just plain gray rocks and soil, either. Apollo 17’s Harrison Schmitt was the first to observe copious amounts of orange “soil” on the Moon, for instance. The orange “soil”, along with yellow and green samples, turned out to be beads that indicated a history of volcanic activity on the Moon. The variety of samples taken on the Moon also included black basalts, lighter rocks called anorthosite, and breccias that were the result of meteorite impacts. Without a noticeable atmosphere, water cycle, or modern tectonic activity, the Moon could display the results of ancient impacts and past activity a lot longer than Earth could.

Chemical analyses of Apollo’s lunar samples revealed that material found on the readily accessible surface of the Moon resembles what scientists might expect if those samples had originated on Earth, but had been vaporized at some point in the distant past and re-condensed in the vacuum of space. The isotope ratios of many elements are nearly identical on Earth and in the samples returned by the Apollo missions. The differences in oxygen isotope ratios between Earth and Mars are 100 times greater than the difference between Earth and the Moon, for instance.

The lunar samples also lacked the expected concentrations of water and hydrogen, which could have easily vaporized and failed to condense. The anorthosite samples were especially lacking in heavier metals and most likely formed in an event that would have liquefied the entire contents of the Moon and allowed them to essentially float to the surface.

Who Came Up With This Theory?

This giant-impact hypothesis was first proposed by two pairs of scientists, William Hartmann and Donald Davis, and Alastair Cameron and William Ward, in 1974. None of the previously proposed theories would have explained how the entire moon would have been liquefied, but a glancing blow from another planetoid might. If some of Earth’s crust had been vaporized and thrown off by such a blow, it might explain why the Moon was chemically similar to Earth, larger in relation to its host planet than other moons in the solar system, formed later than other bodies in the solar system, and lacked a metallic core.

This theory did not gain much traction until models of planetary formation changed in the 1980s. Scientists now theorized that collisions between large bodies may have been more common than previously believed, especially once most of the smaller bodies had been absorbed by larger ones. The theoretical Mars-sized body that collided with Earth came to be known as Theia, the mother of the Moon in Greek mythology.

As the models were refined, flaws in the “glancing blow” model were revealed. If Theia had simply sideswiped a proto-Earth, between 70% and 90% of the material that ended up forming the Moon would have come from Theia’s mantle. However, the ratios of tungsten-182 to other elements in Theia’s upper layers is unlikely to have been identical to Earth’s. Scientists faced a choice between discarding or refining the giant-impact hypothesis.

A biochemist named Alberto Saal proposed going back to the drawing board – or, rather, back to the Apollo lunar samples to see if anything new could be found. Perhaps the measurement of volatile substances like water could be refined.

“They actually laughed. … It took me three years for them to say yes,” he recalled in an article for the magazine Sky & Telescope.

He received some of the glass beads that Harrison Schmitt had collected during Apollo 17. Those beads had come from a volcano that spewed magma from the lunar mantle. Tests revealed a minuscule amount of water in those beads that measured only 46 parts per million, but this was a significant departure from the consensus that the Moon had no water. Under the lunar surface, the magma may have contained much more dissolved water and gas that escaped during the volcanic eruption in an effect that Saal compared to “opening a can of soda”. Saal’s team estimated that the magma that formed his beads had probably contained about 750 parts per million of water.

This ratio was similar to the water content of the basalts erupting along Earth’s mid-ocean ridges. The Moon’s interior could be as wet as Earth’s. Work done by a research team led by Erik Hauri on melt inclusions, small bits of magma that had been trapped inside minerals that solidified instantly upon being ejected by the volcano before any substances in that magma was lost, verified the results by finding up to 1,410 ppm of water in the melt inclusions.

The discovery of higher levels of water on the Moon than previously thought would mean refining, or maybe scrapping, the Theia model. However, many scientists thought the giant-impact hypothesis was too good to discard out of hand.

Head-On Collision a Possibility

Proposals to refine the Theia collision model include the possibility of a super-heated protolunar disk that allowed the transfer of material from Earth into orbit through convection and turbulent mixing right after the collision. However, the flaw in this model – proposed by Kaveh Pahlevan and David Stevenson in 2007 – is that the transfer of material would have had to be amazingly efficient in order to hide Theia’s influence.

In 2012, Sarah Stewart and Matija Cuk suggested that the collision might not have been a glancing blow at all, but a head-on or “T-bone” style collision that could have launched more of Earth’s mantle into orbit. Angular momentum would have done the rest if Earth was spinning once every 2.5 hours. The extra energy would have eventually translated into extra speed in Earth’s orbit around the sun and slightly enlarged its orbital path. This effect and the tidal pull of the new Earth-Moon system would have eventually slowed Earth’s rotation to once every 24 hours.

Further refinements of the head-on collision model opened up some exotic ideas. One of them came from Stewart’s then-graduate student, Simon Lock, who proposed that Earth could have looked like a giant molten bagel with its core bulging out where the bagel’s hole would have been shortly after the collision. The Moon could have formed first as a series of moonlets that eventually merge together in the torus of this odd-shaped structure known as a synestia. The Moon and modern Earth would have essentially formed from the same material in the aftermath of the collision.

According to Lock, the senestia model was more flexible than other models that would have required Theia to hit pretty precisely in order to them to work. “You would need a roughly Mars-mass body traveling at near the escape velocity and hitting the proto-Earth within a couple of degrees of the correct angle” in the canonical model, he says.

Synestias may actually happen with some regularity throughout the universe, says Lock, but the laws of physics dictate that they would be fairly short-lived. “There is the chance that we will never see one,” he says.

Other models suggest that Theia could have hit the proto-Earth more than once, first in a “sideswipe” and then possibly in a more head-on collision. Earth may also been hit by more than one body, which could have obscured the isotope signatures of each body. This could have launched enough material into Earth orbit to form some moonlets that eventually formed our Moon. Some isolated scientists like the University of Chicago’s Nicolas Dauphas have also proposed that Theia may have had isotope signatures that were more similar to Earth than the scientific community currently believes.

The exact model of how the Moon formed may not be settled until and unless more samples can be returned and studied. Scientists believe that the composition of the Moon is pretty well-known, but have proposed returning samples from Venus so that its isotope ratios can be compared to Earth’s and give them a better understanding of how similar Theia might have actually been.