Chang’e-6 Returns Unprecedented Lunar Samples, Reveals Cohesion

China’s Chang’e-6 mission has successfully returned lunar soil samples from the South Pole–Aitken Basin, shedding light on the unique properties of lunar regolith. The mission, which concluded with the retrieval of 1,935.3 grams of material on June 25, 2024, marks a significant advancement in lunar exploration, particularly as it is the first to bring back samples from the far side of the Moon.

Prior missions, including Apollo, Luna, and Chang’e-5, have provided approximately 383 kilograms of lunar materials, primarily from the Moon’s near side. These past efforts have greatly enhanced our understanding of lunar geology. However, the lack of far-side samples until now has limited insights into the Moon’s diverse composition and geological history.

The Chang’e-6 mission team, led by chief designer Hu Hao, noted that the returned samples exhibited a “slightly more viscous and somewhat clumpier” texture compared to the finer, looser material from Chang’e-5. To quantify these differences, a research team headed by Prof. Qi Shengwen from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) conducted experiments to measure the angle of repose, a crucial parameter that reflects the flowability of granular materials. Their findings were published in the journal Nature Astronomy.

Distinct Properties of Far-Side Lunar Soil

The research revealed that the soil from Chang’e-6 has a significantly higher angle of repose than samples collected from the Moon’s near side. This indicates a flow behavior typical of cohesive soils. Investigations confirmed that magnetic and cementation effects were not responsible for this cohesion, as the samples contained only trace amounts of magnetic minerals and no clay minerals.

Instead, the cohesive behavior of the lunar soil is attributed to three primary interparticle forces: friction, van der Waals forces, and electrostatic forces. The study established a critical size threshold of approximately 100 micrometers. Below this size, fine non-clay mineral particles begin to show cohesive properties. The Chang’e-6 samples were found to have a D 60—denoting the particle diameter at which 60% of the sample is finer—of only 48.4 micrometers, making them substantially finer and more irregular in shape than near-side soils, with lower particle sphericity.

Prof. Qi remarked on the unusual nature of these findings, stating, “Finer particles are typically more spherical. Despite being fine-grained, Chang’e-6 soil displays more complex particle morphologies.” This complexity may be due to the samples’ higher feldspar content, estimated at 32.6%, as well as the more intense space weathering effects experienced on the Moon’s far side.

The intricate texture and morphology of these samples reinforce interparticle forces, contributing to the high cohesion observed. This study provides a 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.

As lunar exploration continues to evolve, the Chang’e-6 mission serves as a pivotal step toward understanding the Moon’s geology and the implications it holds for future exploration endeavors. More detailed findings are documented in the research conducted by Shengwen Qi and colleagues in Nature Astronomy, providing a foundation for future studies in lunar science.