Chang’e-6 Returns Unique Lunar Samples, Reveals Cohesive Soil Properties

The recent analysis of lunar samples returned by China’s Chang’e-6 mission has unveiled distinct properties of soil from the moon’s far side. On June 25, 2024, the mission successfully retrieved 1,935.3 grams of lunar soil from the South Pole–Aitken Basin, the largest and oldest impact structure on the moon. This marks a significant advancement in lunar research, as previous missions primarily focused on the near side, leaving a gap in understanding the far side’s unique composition and geological history.

Historically, lunar sample-return missions, including Apollo, Luna, and Chang’e-5, have contributed approximately 383 kilograms of lunar material, enhancing knowledge of lunar geological evolution. However, the lack of far-side samples has limited detailed studies. The Chang’e-6 samples are particularly notable for their “slightly more viscous and somewhat clumpier” texture, according to Hu Hao, the chief designer of the mission.

Research Findings on Lunar Soil Cohesion

A research team led by Prof. Qi Shengwen from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) conducted experiments to quantify the observed properties of the Chang’e-6 samples. Using fixed-funnel and rotating-drum techniques, the team measured the angle of repose, an essential parameter indicating the flowability of granular materials. Their findings, published in Nature Astronomy, revealed that the soil from Chang’e-6 has a significantly higher angle of repose compared to samples from the near side, indicating cohesive behavior typically associated with certain soil types.

Further analysis eliminated the influence of magnetic and cementation effects, as the lunar soil contained only trace amounts of magnetic minerals and no clay minerals. Instead, the researchers attributed the high angle of repose to three interparticle forces: friction, van der Waals forces, and electrostatic forces. This cohesion is particularly pronounced in particles smaller than 100 micrometers, as the study identified that fine non-clay mineral particles begin to exhibit cohesive behavior below this critical size threshold.

Implications of Unique Soil Characteristics

High-resolution CT imaging revealed that the Chang’e-6 samples possess a D 60 metric of only 48.4 micrometers, indicating a finer and more irregular particle shape compared to near-side soils. Prof. Qi noted that, “Finer particles are typically more spherical. Despite being fine-grained, Chang’e-6 soil displays more complex particle morphologies.” This irregularity may be linked to two factors: the higher feldspar content of approximately 32.6% and more intense space weathering experienced on the far side.

These findings offer a systematic explanation of the cohesive behavior of lunar soil from a granular mechanics perspective. They contribute valuable insights into the physical properties of far-side regolith, enhancing the understanding of its geological evolution and potential implications for future lunar exploration efforts.

This research not only strengthens the link between orbital remote sensing and ground-truth measurements but also opens new avenues for investigating the far side of the moon. The Chang’e-6 mission, through its unique samples, has provided a pivotal resource for advancing lunar science and understanding the complexities of our lunar neighbor.