While some of the details are still being developed, it is generally accepted that the moon is formed when a Mars-sized body collides with the early Earth. Some of the remains in the orbit of the collision will continue to condense in the moon.
One consequence of this is that the early moon has spent much of its history bombarded by this remnant, a process that must have left its surface melted. This magmatic ocean will only harden slowly until the bombardment is dropped and the curing process should leave a trace on the moon's composition. It's hard to find indications so far. But now there are indications that the Chang-E-4 mission to the far side of the moon has finally noticed some of the moon's mantle that contains signs of its Magma Ocean.
The end of the ocean
At first sight, the end of the ocean of magma may seem simple: the molten rock solidifies, leaving behind a solid body. But different minerals have their own melting points and densities that can cause ocean breakage. Ultimately, it is believed that the densest minerals will solidify at the base of the ocean, while the bark will form from a lighter material that can harden while floating on the remaining magma. So we expect to see some minerals on the surface and a different group of minerals deep in the mantle.
Confirmation of this, however, requires us to take the mantle, something that is not quite simple. Most of what we know about mantle comes from seismic experiments created during Apollo's missions. None of the returned rocks, however, seems to originate from the mantle, largely because the plate tectonics does not exist on the Moon, so this process can not bring surface material to the mantle.
An alternative process that can bring the mantle to the surface is an impact. This idea intensified when Mission GRAIL pinpointed the moon and found it thinner than previously thought. But it is not clear how effective it is to dig the mantle. Simulations show that a big blow will leave behind a pool of molten rock in which the same stratification process can occur. In other words, the same dense minerals can go deep beneath the crater, not a place to study them.
However, if there is a place where the mantle could be dug, the best candidate is the South Pole of the Aitken Basin, the largest crater of the Moon, with a diameter of about 2500 kilometers. And the orbital visualization suggests that the pool is higher in the iron content than other surface areas, which would be consistent with that containing the mantle material. There is only one problem: Aitken's pool is on the other side of the moon, leaving it outside the area where we can easily control a collector.
Feeling that the scientific case for exploring the Aitken basin is strong enough, China decided to change this by putting the QueQiao satellite in orbit where it could allow exploration on the far side of the Moon. This was followed by the port of Chang-E-4 and the Yutu-2 rifle, which were unloaded in the Von Kármán crater, a smaller (185 km in diameter) crater on the Aitken pool floor. Yutu-2 wore the nearby infrared spectrometer, which allowed him to determine the composition of the rocks in the area.
Maybe some mantle
It would be nice to think that we could direct a spectrometer to a rock and pop up a list of available minerals. The problem is that most rocks are not made up of a mineral. As a result, you get a complex spectrum that includes peaks of multiple materials, making them difficult to interpret. Obviously, the spectrum does not look anything like those collected by Earth's predecessor Chan E-3.
To understand what could happen, the researchers built a model that allowed them to mix minerals known to be present in the lunar crust along with some that are expected to be deep in the mantle. The model can change the fraction of available materials and then predict the spectrum it produces in reflecting sunlight.
The results obtained from the model suggest that approximately half of the sample is composed of pyroxene. It is crucial that most of this mineral is low in calcium, resulting in a thicker material and is expected to be present in the mantle. In addition, the model shows that almost half of the sample consists of an olivine, dense material that is also expected to be found deep in the mantle. Although this should not be seen as a definitive proof of the presence of these minerals, Chinese researchers have mentioned that China's space program is preparing to return samples, so Yutu-2 can be seen as the basis for this.
It is remarkable that these minerals appear in rocks rather than in the soils of the area. If the entire impact pool included material from the two effects (those that formed the Aitken pool and the Karman crater), then you can expect some of this material to contribute to the regrowth on the surface of the Moon. But the researchers suggest that the rocks probably originated elsewhere.
The Aitken swimming pool contains numerous impacts, and one of them – a Fine Crater – produced rays of discarded material, two of which cross the Von Kármán crater. It turns out that Change E-4 landing site (called Statio Tianhe) is right where one of these rays passes through the crater floor. Thus, researchers suggest that Finsen's impact is what blasting mantle loosens material, allowing Yutu-2 to find it there.
scienceDOI: 10.1038 / s41586-019-1189-0 (for DOIs).