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Chang'e-6 samples unlock secrets of the moon's farside

The moon's near and far sides exhibit striking asymmetry—from topography and crustal thickness to volcanic activity—yet the origins of these differences have long puzzled scientists.

China's Chang'e-6 mission, launched on May 3, 2024, changed this by returning 1,935.3 grams of material from the lunar farside's South Pole–Aitken Basin (SPA), the moon's largest, deepest, and oldest known impact structure, measuring 2,500 kilometers in diameter. The samples arrived on Earth on June 25, 2024.

Previous studies indicated that the SPA was formed by a colossal impact approximately 4.25 billion years ago, releasing energy greater than that of a trillion atomic bombs. But the effect of this impact on lunar geology and thermal evolution was one of planetary science's greatest unsolved questions until recently.

In the past year, research teams led by CAS institutions including the Institute of Geology and Geophysics (IGG) and the National Astronomical Observatories (NAOC), along with Nanjing University and others, have made four landmark discoveries based on the SPA samples. Their were published in four cover articles in the journal Nature.

According to Prof. Wu Fuyuan, a member of the Chinese Academy of Sciences and a researcher at IGG, the profound geological consequences of the impact that formed the SPA are, for the first time, revealed collectively in these four Nature papers.

The cover stories focus on the following areas:

• Prolonged volcanic activity: Analysis identified two distinct volcanic phases on the lunar farside—4.2 billion and 2.8 billion years ago—indicating that volcanic activity persisted for at least 1.4 billion years, far longer than previously thought.
• Fluctuating magnetic field: Measurements of paleomagnetic intensities in basalt clasts revealed a rebound in the moon's magnetic field 2.8 billion years ago, suggesting that the lunar dynamo, which generates magnetic fields, fluctuated episodically rather than fading steadily.
• Asymmetric water distribution: The farside mantle was found to have significantly lower water content than the nearside mantle, indicating that volatile elements are unevenly distributed within the lunar interior—adding another aspect to the moon's asymmetry.
• Mantle depletion signatures: Geochemical analysis of basalt points to an "ultra-depleted" mantle source, likely resulting from either a primordial depleted mantle or massive melt extraction triggered by large impacts. This highlights the role of major impacts in shaping the moon's deep interior.

The first analysis of the samples was published by NAOC and its collaborators, detailing the samples' physical, mineralogical, and geochemical properties.

The Guangzhou Institute of Geochemistry at CAS subsequently confirmed 2.8-billion-year-old farside volcanic activity, linking it to a highly depleted mantle. IGG, in turn, dated the SPA to 4.25 billion years ago, providing a critical reference point for studying early solar system impacts.

These findings not only illuminate the evolution of the moon's farside but also underscore the transformative impact of the Chang'e-6 mission, paving the way for deeper insights into planetary formation and evolution.