Did a large impact remix the Moon’s interior?

Zoom / The blue area is the basin formed by the greatest impact on the moon. Additional craters were formed by subsequent impacts.

As the Moon coalesced from impact debris early in the history of the Solar System, a continuous stream of orbital forcings is thought to have formed an ocean of magma, leaving the body fluid. This was supposed to allow its ingredients to mix evenly, creating an almost uniform body. But as space exploration began, we were finally able to get our first good look at the far side of the moon.

It turns out that it looks very different from the side we were familiar with, with very little in the way of the dark areas, called Persians, that dominate the land-facing side. These differences are also reflected in the chemical composition of the rocks on different sides. If the entire moon was a well-mixed bubble of magma, how did it end up with such a huge difference between two faces? A new study links this difference to the largest crater on the moon’s surface.

big accident

The South Pole-Aitken Basin is one of the largest impact craters in the Solar System, but then again, we didn’t realize it was there until after we put a craft into orbit around the Moon. All we can see from the ground are some hills that are part of the crater’s outer wall. Most of the 2,500-kilometre crater extends to the far side of the Moon.

It is clear that the crater was formed after the period of the magmatic ocean, based on the fact that its contours froze after the impact. But it’s also very old, and could have formed before many of the volcanic features we can see on the near side. Interestingly, the largest concentration of volcanic Persians is in the north of the near side – roughly on the far side of the moon from the impact itself. Could it be related?

It is clear that an impact of this magnitude could generate a lot of heat inside the Moon and potentially affect or even restart the convection of the material there. But it is less clear that this could produce volcanic activity far from the site of impact.

To better understand the situation, a team of Chinese researchers built a model of the moon’s interior. This model combines software that can simulate the impact with models of the moon’s interior that can take into account the heating, additional material for impact, and the gravitational effect of nearby Earth.

big labour

As expected, the model shows that the heat from the impact does indeed re-convection the interior of the Moon. But it does not reboot evenly. This is because the body that created the crater also injected a lot of material into the moon’s interior, and this material gradually spreads out from the impact site in all directions. For much of the moon’s interior, this disrupts regulated convection.

This regulated convection is what allows warmer, deeper material to make its way to the surface and attract cooler material from the surface inward. The end result is that the warm, deep material makes its way closer to the surface on the opposite side of the impact crater. On the Moon, this substance also contains higher concentrations of radioactive isotopes, which will keep it warm longer, prolonging the period of the volcanoes that created the Persians.

Not every effect will produce this kind of effect. If the angle of impact is too shallow, the spread of the material is not wide enough to create significant asymmetry. The details of the asymmetry are sensitive to the size of the collider and to the viscosity of the material being injected into the moon’s interior.

Obviously, this kind of complex mechanism requires a lot of things to go right, so the researchers might want to re-examine this work using independent convective models. The study authors suggest that looking at the rocks near the Chang’e-5 landing site on the northern part of the near side might give us a greater sense of the composition of the material that erupted there.

But as the authors note, there are many competing models to explain the asymmetry, so we’ll have to wait for scientists to compare models to see if there are any obvious differences in what they produce. And then we have to see if we can expect to get any relevant evidence from the Moon.

natural earth sciences, 2022. DOI: 10.1038/s41561-021-00872-4 (about DOIs).

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