惑星科学：太陽系最大の衝突から得られた知見

The latest analyses of 2.8-billion-year-old lunar samples collected by China’s Chang’e-6 mission add new insights about the Moon’s formation and the effect of giant impacts. The findings, published in Nature this week, indicate that the deep lunar interior that produced the rocks sampled by the Chang'e-6 mission already had a large amount of melt extracted from it.

Observations of the Moon have revealed differences in crustal thickness, volcanic activity, and the chemistry of rocks between the nearside and farside of the Moon. However, the origins of these differences have been debated. In 2024, China’s Chang’e-6 returned basalt samples (rock formed from cooling lava) from the South Pole–Aitken basin on the lunar farside, allowing scientists to explore differences between the two hemispheres.

Wei Yang and colleagues analysed the isotopic composition of samples of volcanic rock from the Chang’e-6 mission, taken from the South Pole–Aitken basin on the farside of the Moon, and compared them to samples obtained during the Apollo missions from the nearside. The authors found that rocks from the South Pole–Aitken basin shared some characteristics with the nearside mantle, but appear to have sampled a part of the lunar interior that was already depleted of melt. These properties suggest that the basalts may have formed from deep (mantle) rocks depleted in melt from the early magma ocean stage of the Moon and/or further melted during the South Pole–Aitken impact event. The authors conclude that the samples recovered by the Chang’e-6 mission provide a means to study early lunar mantle formation, and that further studies may offer more insights into the processes that underlie the differences between the nearside and farside.