What do the samples brought back by the Chinese Chang’e 6 space mission tell us?

From Earth, we always see the same side of the Moon. Its hidden side conceals secrets, and it was a Chinese mission that went to discover them. The samples arrived on Earth in June 2024, and the analyzes are starting to bear fruit.

In June 2024, the Chinese Chang’e 6 mission marked a historic milestone by bringing back the first samples from the far side of the Moon, more precisely from the South Pole-Aitken basin.

The first analyzes of these samples revealed volcanic episodes 4.2 billion years old, and made it possible to refine the chronology of lunar events based on precise isotopic data.

These discoveries are not only important for better understanding the history of the Moon, but they could also have a major impact on our models of planet formation and our understanding of geological processes throughout the solar system.

Apollo and Luna transformed our understanding of the Moon

The returns of lunar samples carried out by the Apollo (USA) and Luna (Russia) missions between 1969 and 1976 radically transformed our understanding of the Moon.

Read more: Will we be able to grow plants on the Moon? Clues with the samples brought back by Apollo

Before these missions, scientists thought the Moon was either an asteroid captured by Earth’s gravity or formed at the same time as Earth. However, the chemical and isotopic characteristics of the lunar samples could not be explained by these scenarios.

A new theory then emerged to reconcile both the data from the samples and the physical dynamics of the Earth-Moon system: the Moon would have formed during a giant impact between Theia, a body the size of Mars (around 10 % of the mass of the current Earth) with the Earth, a few tens of millions of years after the formation of the Earth. This impact would have vaporized part of the Earth as well as all of Theia. Most of the ejected material would have fallen back to Earth, while a small fraction would have formed the Moon, which today represents only about 1% of Earth’s mass.

However, the Apollo and Luna samples come exclusively from the near side of the Moon, which poses a crucial question: are they representative of the entire satellite?

Attacking the hidden side of the Moon

Indeed, the far side of the Moon is quite different from what we see from Earth: it has a thick, heavily cratered primordial crust, with few or no “lunar seas” — these dark volcanic flows visible from Earth. It also displays lower concentrations of radioactive elements, such as thorium, than the visible side. Our knowledge of the hidden side is essentially based on orbital observations, the first having been carried out by the Luna 3 mission in 1959.

Although these differences between the two sides of our natural satellite are still poorly understood, they nevertheless suggest that analyzes based solely on samples from the visible side could be biased.

The Moon also serves as our astronomical “clock”

Sample returns from Apollo and Luna also allowed the first dating of lunar rocks. By comparing the age of these rocks to the density of associated craters, scientists were able to establish a correspondence between the density of the craters and the absolute age of the surfaces. Indeed, the older a planetary surface is, the more it has been exposed to meteorite impacts and therefore marked by numerous craters.

When a volcanic eruption occurs, it erases existing craters by covering the surface with lava, thereby resetting the “clock”. This time scale is now used to estimate the age of the surfaces of other bodies in the solar system for which we do not have samples such as Mercury or Venus. It thus constitutes a central tool for understanding the dynamics of planetary surfaces, which in turn reflects the internal dynamics of the planets.

However, this scale relies entirely on samples taken from the near side of the Moon, which could introduce bias. Indeed, it is possible that the meteorite flows were different between the visible and hidden sides of the Moon, calling into question the universality of this model.

First results from Chang’e 6 samples

To answer these questions and better understand the history of our satellite, the Chinese Chang’e 6 mission landed in June 2024 on its far side, in the South Pole-Aitken impact basin. This region, one of the oldest on the Moon, is notable for its high crater density and could even contain fragments of the lunar mantle, the layer beneath the primordial crust which has never been sampled until now .

Indeed, the South Pole-Aiken basin is the largest lunar basin. The gigantic impact at its origin would have “excavated” the surface, leaving an abnormally thin crust in this region. Some simulations even suggest that the impact could have reached the lunar mantle, making this area particularly interesting for scientific exploration.

Thanks to precise isotopic dating of numerous fragments of basalt (volcanic rocks resulting from the fusion of the lunar mantle under the crust) collected in the Chang’e 6 moon landing zone, the researchers identified two distinct volcanic episodes.

The oldest episode identified here is volcanic activity 4.2 billion years old. The basalt sample analyzed is rich in potassium, rare earths and phosphorus (a combination abbreviated “KREEP”), indicating that the volcanic activity that produced this basalt came from a region in the mantle rich in radioactive elements. This fragment constitutes the oldest sample of lunar basalt ever precisely dated.

In addition, other samples from Chang’e 6, basalts poorer in potassium, rare earths and phosphorus, show traces of more recent volcanism, around 2.8 billion years old. This discovery extends the known duration of volcanic activities on the far side, demonstrating that they spanned at least 1.4 billion years.

The most recent basalts of the South Pole-Aitken basin come from the fusion of a lunar mantle poor in KREEP and radioactive elements which constitute the main source of heat allowing the fusion of rocks and the production of lava.

However, as mentioned previously, the bottom of the South Pole-Aitken basin has an abnormally thin crust, and we thought until now that this favors the melting of the mantle and the rise of magma. However, it today turns out that this rise of lava is limited by the composition of the underlying mantle, depleted in radioactive elements, which limits partial fusion. This explains the absence of large volcanic plains in this region, despite apparently favorable conditions on the surface.

This confirms the origin of the low abundance of “seas” on the far side of the Moon compared to the visible side. However, the origin of this chemical dichotomy between the two sides remains subject to debate. A recent hypothesis would be linked to the very impact at the origin of the South Pole-Aitken basin, the power of which would have disrupted the distribution of material at depth, leading to an accumulation of material rich in KREEP under the visible side.

Chang’e 6 validates the astronomical “clock” established by Apollo and Luna

Another crucial aspect of the work is improving timeline models based on lunar crater counts. The isotopic ages of the youngest basalts reported by Chang’e 6 (2.8 billion years) agree with the ages determined by crater counting using the calibration established on the visible side.

Additionally, these ages provide a critical calibration point for refining these tools, not only for the Moon, but for other planetary bodies as well.

The first results of the Chang’e 6 mission thus confirm the hypothesis according to which the meteorite flow on the far side is similar to that of the visible side. This observation validates the use of the calibration established on the visible side for larger studies, reinforcing the reliability of chronological models applicable to other celestial bodies such as Mars or asteroids for example.