The Moon, our faithful celestial companion, still conceals many secrets, particularly on its hidden side which remains eternally invisible from Earth. This little-known side, although mapped by probes, remains largely unexplored. The Chinese Chang’e 6 mission, carried out in 2024, marked a major breakthrough by bringing back to Earth the first rock samples from this mysterious region. These fragments of solidified lava tell us a fascinating story: volcanoes were erupting on the far side 2.8 billion years ago. Furthermore, this account also reveals striking differences between the two sides of the Moon, enriching our understanding of its evolution.
A common, but different, volcanic history
Scientists have long agreed that the Moon was volcanic in its past. So far, the evidence has focused mainly on the near side, where vast lava flows, called marine basalts, cover about 30% of the surface. These dark plains are easily visible from Earth with the naked eye and were long mistaken for “lunar seas” before the first missions proved their basalt nature.
With the Chinese Chang’e 6 mission, samples from the far side could be analyzed for the first time. These rocks, from the region of South Pole-Aitken basinrevealed a surprising dating: they are 2.8 billion yearswhich proves that volcanoes were also active on this hidden side at that time. This result agrees with the ages recorded on the visible side, confirming that the two sides of the Moon shared common volcanic activity.
However, the similarities end there. While the visible side shows abundant traces of recent volcanism (until 2 billion years ago and maybe even 120 million years ago), the hidden side shows few signs of such prolonged activity. Only 2% of its surface is covered with marine basalts, which contrasts sharply with the richness of lava on the visible side. This disparity intrigues scientists and the results from Chang’e 6 provide important clues to solve this mystery.
A geology influenced by missing elements
The study of the samples revealed a major difference in geochemical composition basalts on both sides. The lava on the far side comes from a lunar mantle surprisingly poor in certain elements such as potassium, rare earths and phosphorus, often abbreviated by the acronym KREEP. On the visible side, these elements are present in abundance and play a crucial role in maintaining prolonged volcanic activity.
For example, potassium and uranium are radioactive. Their disintegration releases heat, which helps melt the mantle and fuel volcanism. Their relative absence on the far side could explain why this region saw its volcanism die out much earlier..
This uneven distribution of elements could find its origin in the colossal impact which created the South Pole-Aitken basin, one of the largest impact craters in the solar system. This cataclysmic event would not only have redistributed KREEP-rich materials towards the visible side, but would also have caused massive melting of the underlying mantle, depleting this region of elements necessary for prolonged volcanic activity.
What these discoveries teach us
Thus, the Chang’e 6 results not only confirm the existence of ancient volcanism on the far side of the Moon; they highlight the profound differences between the two hemispheres of our natural satellite that have been shaped by major geological events.
These findings have important implications for our understanding of lunar history and planet formation in general. They illustrate how catastrophic events, such as meteorite impacts, can shape not only the surface, but also the internal structure of a celestial body.
Finally, this work underlines the importance of missions to explore the hidden side of the Moon. This region, although more difficult to access, holds essential keys to understanding the history of the solar system. With future missions that could include lunar bases, the Dark Side may well move from shadow to light, revealing even more of its buried secrets.
