China’s Chang’e-6 mission has unveiled significant findings about lunar soil collected from the far side of the Moon. On June 25, 2024, the mission successfully returned 1,935.3 grams of lunar soil from the South Pole–Aitken Basin, the largest and oldest impact structure on the Moon. This groundbreaking research highlights the cohesive properties of the samples, which differ markedly from those gathered in previous missions.
Previous lunar sample-return efforts, including the Apollo, Luna, and Chang’e-5 missions, have provided approximately 383 kilograms of material from the Moon’s near side. While these samples advanced the understanding of lunar geology, the lack of far-side samples limited insights into its unique composition and geological history. The Chang’e-6 mission marks a pivotal moment by filling this gap in scientific knowledge.
Key Findings on Lunar Soil Cohesion
The analysis of the Chang’e-6 samples, led by Prof. Qi Shengwen from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS), revealed that the lunar soil exhibits a “slightly more viscous and somewhat clumpier” texture compared to the materials returned by Chang’e-5. To quantify this behavior, the research team conducted experiments using fixed-funnel and rotating-drum methods to measure the angle of repose, a crucial indicator of how granular materials flow.
The study, published in Nature Astronomy, found that the angle of repose for the Chang’e-6 soil is significantly higher than that of near-side samples. This suggests that the far-side lunar soil displays flow characteristics typical of cohesive materials. Further analysis indicated that the samples contained only trace amounts of magnetic minerals and no clay minerals, ruling out magnetic and cementation effects as factors in the observed cohesiveness.
Interparticle Forces and Soil Composition
The research identified three interparticle forces contributing to the cohesiveness of the Chang’e-6 soil: friction, van der Waals forces, and electrostatic forces. Friction is influenced by the roughness of the particle surfaces, while van der Waals and electrostatic forces become more significant as particle size decreases. Using the D 60 metric, which indicates the particle diameter at which 60% of the sample is finer, the team established a critical size threshold of approximately 100 micrometers. Below this threshold, fine non-clay mineral particles begin to show cohesive behavior.
High-resolution CT imaging revealed that the Chang’e-6 samples have a D 60 of only 48.4 micrometers, indicating a finer and more irregular shape compared to near-side soils. The samples demonstrated lower particle sphericity, which is unusual for fine-grained materials. Prof. Qi commented on this finding, stating, “Finer particles are typically more spherical. Despite being fine-grained, Chang’e-6 soil displays more complex particle morphologies.”
This complexity may result from two factors: the higher feldspar content of approximately 32.6% in the samples, which is prone to fragmentation, and more intense space weathering on the far side of the Moon. These characteristics enhance interparticle forces, leading to the observed high cohesion.
The findings from the Chang’e-6 mission provide a systematic explanation of the cohesive behavior of lunar soil, offering new perspectives on the physical properties of far-side regolith. This research not only enriches our understanding of lunar geology but also sets the stage for future explorations and studies of the Moon’s far side.
For more information, refer to the study by Shengwen Qi et al, “Strongly cohesive lunar soil identified at the Chang’e-6 landing site,” published in Nature Astronomy (2025). DOI: 10.1038/s41550-025-02715-3.
