While some details are still being worked out, it is agreed that the moon was created when a Mars-sized body collided with the early Earth. Part of the debris brought into orbit by the collision would then condense into the Moon.
One of the consequences of this is that the early moon has spent much of its history being bombarded by these debris, a process that should have allowed its surface to melt. This magma ocean would only slowly solidify when the shelling subsided, and the process of solidification would have left its mark on the composition of the moon. So far, it has been difficult to find clues. But now there are signals that the Chang'E-4 mission on the other side of the moon has finally discovered part of the Moon's mantle that contains signs of its magma ocean.
The End of an Ocean
At first sight The end of a magma ocean may seem simple: molten rock solidifies and leaves a solid body. But different minerals have their own melting points and densities, which allows the ocean to be layered. Ultimately, it is believed that the densest minerals solidify on the seafloor, while the crust would form lighter material that could solidify as it floats on the remaining magma. So we would expect to see certain minerals on the surface and another group of minerals deep in the mantle.
But to confirm that, we have to scan the mantle, which is not easy. Most of what we know about the mantle comes from seismic experiments conducted during the Apollo missions. None of the returned rocks appear to have been formed in the mantle, especially because there is no plate tectonics on the moon, so that no mantle material can be brought to the surface by this process.
An alternative method that could bring sheath material to the surface is a blow. This idea received a boost when the GRAIL mission mapped the lunar crust and discovered that it was thinner than previously thought. However, it is not clear how effectively an impact would lift the mantle. Simulations indicated that a large impact would leave a pool of molten rock where the same stratification process could occur. In other words, the same dense minerals may condense deeper beneath the crater than in a place where we could examine them.
If there is still a place where the mantle could have been excavated, the best candidate is the South Pole Aitken Basin, the largest impact crater of the Moon with a diameter of around 2,500 kilometers. The orbital uplift suggests that the pool has a higher iron content than other areas of the surface, which would be consistent with the cladding material. There is only one problem: The Aitken Basin is on the other side of the Moon and is out of the area where we can easily control a lander.
China felt that the scientific reason for exploring the Aitken Basin was strong enough. Change this by bringing the QueQiao relay satellite into orbit where it could enable exploration of the other side of the Moon. This was followed by the Chang'E-4 lander and the Yutu-2 rover, which had landed in the Von Kármán Crater, a smaller crater (1
Maybe a coat
It would be nice to think we could point a spectrometer at a rock and pop out a list of minerals present. The problem is that most rocks do not consist of a single mineral. As a result, you get a complicated spectrum that contains peaks of multiple materials, which makes their interpretation difficult. It is obvious that the spectrum does not look like that of the predecessor of the lander, Chang & # 39; E-3.
To find out what might be going on, the researchers built a model that allowed them to mix known minerals in the lunar crust, along with some that are expected to be deep in the mantle. The model could vary the proportion of these materials and then predict the spectrum it produces when reflecting sunlight.
The results of the model suggest that about half of the sample consisted of pyroxene. Importantly, most of this mineral has a low calcium content, resulting in a denser material and is expected to be present in the shell. In addition, the model revealed that nearly half of the sample was olivine, a dense material that is likely to be deep in the mantle. While this should not be considered a definitive proof of the presence of these minerals, the Chinese researchers mention that China's space program is preparing to return samples, so Yutu-2 can be considered as the basis.
It is noteworthy that these minerals occur more in rocks than in the soils of the region. If the entire impact basin contained material from one of the two impacts (the Aitken basin and the Von Kármán crater), it could be expected that part of this material would contribute to the regolith on the lunar surface. The researchers suspect, however, that the rocks were created elsewhere.
The Aitken Basin has several impacts, one of which – the Finsen Crater – produced jets of expelled material, two of which travel through the Von Kármán Crater. It turns out that the landing site of Change & # 39; E-4 (called Statio Tianhe) is exactly where one of these rays crosses the crater floor. The researchers therefore suspect that the Finsen impact is what has released casing material and allows Yutu-2 to find it there.
Nature 2019. DOI: 10.1038 / s41586-019-1189-0 (About DOIs).