Lunar samples serve as a critical link between orbital remote sensing and ground-truth measurements. Previous sample-return missions—Apollo, Luna, and Chang'e-5—have collectively brought back approximately 383 kilograms of lunar soil and rock from the Moon's near side, advancing our understanding of lunar geological evolution and regolith properties. However, the absence of samples from the far side has limited investigations into its unique composition and geologic history.

On June 25, 2024, China's Chang'e-6 mission successfully returned 1,935.3 grams of lunar soil from the South Pole–Aitken Basin on the lunar far side—the Moon's largest, deepest, and oldest impact structure. According to HU Hao, chief designer of the Chang'e-6 mission, the returned samples appeared "slightly more viscous and somewhat clumpier" than the relatively fine, loose material collected by Chang'e-5.

To quantify this observation, a research team led by Prof. QI Shengwen from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) conducted fixed-funnel and rotating-drum experiments to measure the angle of repose, a key parameter reflecting the flowability of granular materials. The results showed that Chang'e-6 soil has a substantially higher angle of repose than near-side samples, exhibiting flow behavior characteristic of cohesive soils.

Subsequent analysis ruled out magnetic and cementation effects, as the samples contain only trace amounts of magnetic minerals and no clay minerals. Instead, the elevated angle of repose is attributed to three interparticle forces: friction, van der Waals forces, and electrostatic forces. While friction is proportional to particle surface roughness, the contribution of van der Waals and electrostatic forces increases as particle size decreases. Using the D₆₀ metric—the particle diameter at which 60% of the sample is finer—the researchers identified a critical size threshold of approximately 100 micrometers. Below this threshold, fine non-clay mineral particles begin to exhibit cohesive behavior.

High-resolution CT imaging revealed that Chang'e-6 samples have a D₆₀ of only 48.4 micrometers—substantially finer and more irregular in shape than near-side soils, with much lower particle sphericity.

"This is unusual," noted Prof. QI. "Finer particles are typically more spherical. Despite being fine-grained, Chang'e-6 soil displays more complex particle morphologies."

This phenomenon may stem from two factors: the samples' higher feldspar content (~32.6%), a mineral susceptible to fragmentation, and more intense space weathering on the far side. These textural and morphological characteristics strengthen interparticle forces, resulting in the observed high cohesion.

This study, published in Nature Astronomy on November 24, provides the first systematic explanation of the cohesive behavior of lunar soil from a granular mechanics perspective, offering new insights into the physical properties of far-side regolith.