Sat. Feb 4th, 2023
Model shows that multiple impacts could have caused our moon

A new article in the magazine Nature challenges the leading explanation for the moon’s formation. The prevailing idea is that the moon formed after a planetary body about 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 dismissed idea that a series of smaller impacts with Earth may have collectively built the moon.

Moon history

The giant impact hypothesis was first proposed in the 1970s. When computers got powerful enough, we found that it worked in simulations. A fleeting blow from a Mars-sized planetesimal leads to a disc of material surrounding the young Earth that, over time, coalesces into the moon. And planetesimals were readily available in the early solar system, flying in strange orbits that made planetary collisions highly likely.

In terms of 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 ran into problems with this model.

For starters, the simulations predicted that the moon would be formed from about four parts of planetesimal material and one part of Earth material. If that’s the case, the Moon should have a substantially different composition than Earth’s. But recent studies have shown that the two bodies are a close match, at least in terms of the isotopes of oxygen, titanium and tungsten.

Artist's depiction of the giant impact scenario, in which a planetesimal collides with the young Earth.
Enlarge / Artist’s depiction of the giant impact scenario, in which a planetesimal collides with the young Earth.

That doesn’t necessarily mean the giant impact hypothesis is wrong. It could just be that the isotopes in the original planetesimal were already comparable to those in the Earth. If that’s the case, it wouldn’t matter which body most of the moon’s material came from; we would get the moon we observe anyway. But that explanation is uncomfortable. For starters, the isotopes found throughout the solar system are not identical to Earth’s. So while such an Earth-like planetesimal could form, it is much more likely that the composition of the planetesimal differs from that of Earth.

The other possibility is that the planetesimal was different from Earth, but most of the moon’s material came from Earth instead of the planetesimal. The problem with this theory is that only an extremely powerful collision would send so much of Earth’s mass into space. But the resulting moon would have a different amount of angular momentum, one that probably no longer matches the real moon’s angular momentum.

It is still possible to get a collision that yields the observed composition and angular momentum, but that would have to be a very specific kind of collision. Scientists tend to distrust solutions that need to be fine-tuned to the data.

Multiple effects

In the 1980s, an alternative hypothesis was proposed in which the Moon formed from multiple impacts rather than a single one. The impactors would still be planetesimals, albeit smaller. Each impact would send a new disk into Earth’s orbit. These disks would merge individually into small moons, or “moonlets.” Gravity would pull these moons together and merge them into the moon we know. The merger would be possible because moonlets slow down as they move away from Earth due to tidal forces from the planet.

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

The researchers performed a series of 864 simulations in which Earth was hit by multiple medium-to-large-sized planetesimals (0.01 to 0.1 Earth masses). They tried different initial conditions for the simulations: different speeds, object masses, angles of attack.

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

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

That means a significant portion of any moonlet’s composition still comes from a planetesimal. But that’s probably not a problem. The planetesimals probably have different compositions from each other, so if you mix enough of them they just dilute each other. Because of this, the chemistry of the earth remains the dominant ingredient.

Challenges

A pitfall to this idea is the number of effects it requires. The researchers estimate that about 20 impacts would get the job done, but this assumes that all of the moonlets merged perfectly into the moon and that very little of their material was lost to the solar system as a whole. The reality, unfortunately, is often messier than that, meaning it would take a lot more impact to make up for the losses.

If so, the multiple impact model is starting to look less and less likely – possibly even less likely than some versions of the giant impact model. Of course, that depends on how many planetesimals flew around the early solar system. The more there were, the more often one of them was likely to hit the earth. Future work could make better estimates of the early planetesimal population; if the early solar system was full enough, the multiple impact model could be plausible. However, if that were the case, then you run into the question of why Venus also has no moon.

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

Finally, if the multi-impact scenario is true, it may have observable consequences. For example, since each impact would have been significantly less powerful than the single impact of the giant impact scenario, the collisions would have left the original Earth intact. If so, it could mean reservoirs of pristine material are hiding somewhere on Earth. And indeed, evidence has surfaced in recent studies of pristine reservoirs.

Natural Geosciences2017. DOI: doi:10.1038/NGEO2866 (About DOIs)

By akfire1

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