A recent analysis of lunar samples reveals profound differences between its two sides, suggesting that the Moon's interior is not as uniform as previously thought.
The Chinese Chang'e 6 mission made history in June 2024 by bringing back the first samples ever collected from the far side of the Moon. These fragments specifically come from the South Pole-Aitken basin, a massive impact structure that ranks among the largest craters in the solar system. The meticulous analysis of these rocks demonstrated that they formed at temperatures approximately 100 degrees Celsius lower than samples collected from the near side during NASA's Apollo missions.
The far side of the Moon, covered in craters, photographed by the Orion spacecraft.
Credit: NASA
Researchers combined several approaches to achieve these results. They used computer simulations coupled with satellite data to reconstruct the thermal history of the rocks. By studying how these materials crystallized, the team was able to estimate the temperature of the parent rock, meaning the original material that melted to form magma before solidifying into lunar samples. This method also allowed them to date these samples to approximately 2.8 billion years ago.
The thermal difference would be explained by an uneven distribution of heat-producing elements in the lunar mantle. The near side would be richer in elements like uranium, thorium, and potassium, which generate heat through radioactive decay. These elements, collectively called KREEP, would have migrated to the near side early in lunar history, keeping this region warmer and more volcanically active than its counterpart.
Several hypotheses attempt to explain this original asymmetry. Some mention a colossal asteroid impact that would have redistributed the lunar interior, while others suggest that the Moon might have once had a small companion that merged unevenly. Earth's gravitational pull could also have played a role in this particular distribution of elements, permanently influencing the thermal evolution of our satellite.
These discoveries, published in the journal
Nature Geoscience, represent a significant advance in our understanding of the Moon's internal structure. Although the study does not reveal the current temperatures inside the Moon, it suggests that this thermal division may have persisted for billions of years, deeply shaping the lunar landscapes we observe today from Earth.
Artist's view of the Chang'e 6 mission
© CNSA / CLEP The mystery of the two lunar faces
The Moon exhibits a remarkable dichotomy between its near side and far side. While the hemisphere facing Earth is characterized by vast dark volcanic plains called lunar maria, the opposite side appears much more rugged, with a thicker crust and a much higher density of impact craters.
This morphological difference is explained by distinct geological processes that affected both sides throughout lunar history. The near side experienced more intense and prolonged volcanic activity, partially covering ancient craters under lava flows. In contrast, the far side has preserved a more primitive landscape, marked by meteorite impacts accumulated over billions of years.
The recent thermal discoveries now complete this picture by revealing that these differences are not limited to the surface. The observed asymmetry in mantle temperature distribution shows that internal processes also evolved differently in the two hemispheres, creating a true structural duality.
The Chang'e 6 mission and its scientific importance
The Chang'e 6 mission represents a major step in modern lunar exploration. Developed by the Chinese space agency, this mission's primary objective was to collect and return samples from the far side of the Moon to Earth - a first in the history of space exploration.
The chosen landing site, the South Pole-Aitken basin, is particularly interesting to scientists. This massive impact structure, one of the largest in the solar system, excavated materials from deep layers of the lunar crust, thus providing unique access to the ancient geological history of our satellite.
The success of this mission opens new perspectives for lunar research. By enabling direct analysis of materials from regions inaccessible to previous missions, Chang'e 6 provides data to understand the formation and evolution of the Moon as a whole, beyond the limitations imposed by exclusively studying the near side.