A new paper in the journal Nature challenges the leading explanation for the Moon’s formation. The predominant idea is that the Moon was created after a planetary body roughly the size of Mars collided with the early Earth. The debris it sent up later coalesced into the Moon. But researchers are now revisiting the largely discarded idea that a series of smaller impacts with the Earth may have collectively built the Moon.

Moon history

The giant impact hypothesis was first proposed in the 1970s. When computers became powerful enough, we found that it worked in simulations. A glancing blow from a Mars-sized planetesimal leads to a disc of material around the young Earth that, over time, coalesces into the Moon. And planetesimals were readily available in the early Solar System, flying around on weird orbits which made collisions with planets very probable.

In terms of its mass, angular momentum, and iron content, the Moon formed in these simulations was very similar to the real one we observe. But over the years, researchers kept running into difficulties with this model.

For one thing, the simulations predicted that the Moon would be formed from about four parts material from the planetesimal and one-part material from Earth. That being the case, the Moon should have a substantially different composition from that of the Earth. But recent studies have shown that the two bodies are a close match for each other, at least in terms of isotopes of oxygen, titanium, and tungsten.

That doesn’t necessarily mean the giant impact hypothesis is wrong. It could just be that the isotopes in the original planetesimal were already similar to those in the Earth. If that’s the case, it wouldn’t matter which body the bulk of the Moon’s material came from; we’d get the Moon we observe either way. But that explanation is uncomfortable. For one thing, the isotopes found throughout the Solar System aren’t identical to the Earth’s. So while such an Earth-like planetesimal could form, it’s far more likely the planetesimal’s composition would differ from the Earth’s.

The other possibility is that the planetesimal was different from the Earth, but most of the Moon’s material came from the Earth, rather than the planetesimal. The trouble with this theory is that only an extremely powerful collision would send that much of the Earth’s bulk into space. But the resulting Moon would have a different amount of angular momentum, one that probably wouldn’t match the angular momentum of the real Moon anymore.

It’s still possible to get a collision that produces the observed composition and angular momentum, but that would need to be a very specific kind of collision. Scientists tend to mistrust solutions that need to be finely tailored to match the data.

Multiple impacts

In the 1980s, an alternative hypothesis was proposed in which the Moon formed out of multiple impacts rather than a single one. The impactors would still be planetesimals, albeit smaller ones. Each impact would send a new disc into Earth’s orbit. These discs would individually coalesce into tiny moons, or “moonlets.” Gravity would draw these moonlets toward each other and merge them into the Moon we know. The merger would be possible because moonlets slow down as they move away from the Earth due to tidal forces from the planet.

It wasn’t clear whether these impacts could create large enough moonlets for this hypothesis to work, and the angular momentum might also be off. This model subsequently fell by the wayside, due to the giant impact hypothesis’ simplicity and elegance. But the problems with the giant impact hypothesis have induced a team of researchers to revisit the multiple-impact scenario.

The researchers ran a series of 864 simulations involving the Earth being hit by multiple planetesimals of medium to large size (0.01 to 0.1 Earth masses). They tried a variety of initial conditions for the simulations—different speeds, object masses, angles of impact.

Moonlets of varying masses formed from collisions in 750 of the 864 simulations. Furthermore, in cases where the planetesimal has a head-on collision with the simulated Earth, a large proportion of the resulting moonlet’s mass came from the Earth’s material.

“We believe the Earth had many previous moons,” said Hagai Perets of the Technion-Israel Institute of Technology, one of the paper’s authors. “A previously formed moon could therefore already exist when another moon-forming giant impact occurs.”

That means a substantial proportion of each moonlet’s composition still originates from a planetesimal. But that’s probably not an issue. The planetesimals probably have different compositions from each other, so when you mix enough of them together, they just dilute each other. This leaves the Earth’s chemistry as the dominant ingredient.

Challenges

One pitfall with this idea is the number of impacts it requires. The researchers estimate that about 20 impacts would get the job done, but this assumes that all the moonlets merged perfectly into the Moon, and very little of their material was lost to the Solar System at large. Reality, unfortunately, is often messier than that, which means that a lot more impacts would be needed to make up for the losses.

If that’s the case, the multiple-impact model starts to look less and less probable—potentially even less probable than some versions of the giant impact model. Of course, that depends on how many planetesimals were flying around the early Solar System. The more there were, the more often one of them would likely smack into the Earth. Future work could make better estimates of the early planetesimal population; if the early Solar System was crowded enough, the multiple impact model could be plausible. If that were the case, however, then you run into the question of why Venus doesn’t have a moon as well.

Future work could also estimate the efficiency of moonlet mergers and how much material is lost to space during the process. The present work made no attempt to do so.

Finally, if the multiple-impact scenario is true, it might have observable consequences. For example, since each impact would be substantially less powerful than the single impact of the giant impact scenario, the collisions would have left the original Earth more intact. If so, that might mean that reservoirs of pristine material are hiding somewhere on Earth. And indeed, evidence has turned up in recent studies of pristine reservoirs.

Nature Geoscience, 2017. DOI: doi:10.1038/NGEO2866 (About DOIs)