While the Moon formed from impact debris early in the history of the Solar System, a continuous stream of orbital forcings is believed to have formed an ocean of magma, leaving the body liquid. 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 glimpse into the far side of the Moon.
It turns out to be very different from the side we know, with very few dark areas, called ponds, dominating 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 drop 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 breakdown
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 formed after the period of rocky oceans, the contours of which froze after the impact. But it is also very old, and may have formed before many of the volcanic features we can see on the near side. Oddly enough, the greatest concentration of volcanic puddles is north of the near side, roughly on the far side of the Moon at the same collision. Could it be related?
It is clear that an impact of this magnitude could generate a lot of heat inside the Moon and potentially have affected or even restarted the convection of the material there. But it’s less clear that this could be producing volcanic activity so far from the impact site.
To better understand the situation, a team of Chinese researchers has built a model of the interior of the moon. This model combines software capable of simulating the impact with models of the Moon’s interior that can take into account warming and additional material from the gravitational and gravitational impact of neighboring Earth.
big churning
As expected, the model shows that heat derived from the collision restarts convection within the Moon. But it does not resume uniformly. This is because the body that created the crater is also injecting a lot of material into the moon, and this material is gradually expanding from the impact site in all directions. For most of the Moon’s interior, this disrupts regulated convection.
This regulated load 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 hot, deep material only approaches the surface on the opposite side of the impact pit. On the Moon, this substance also contains higher concentrations of radioactive isotopes, which will keep it warm for much longer, fueling the long period of volcanoes that created the Persians.
Not all effects will produce this type of effect. If the angle of impact is too small, the spread of the material is not wide enough to create a large 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 work well, so researchers will likely want to re-examine this work with 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 better idea 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 the 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 geology science، 2022. DOI: 10.1038 / s41561-021-00872-4 (About DOIs).