Specialists from China have determined the reason why lunar dust on the far side of the satellite is more prone to clumping than on the side facing Earth. Reports on the analysis of samples brought back by the Chang’e-6 mission were published in the journal Nature Astronomy. Immediately after retrieving the samples, scientists were intrigued by the unusual properties of this regolith—its pronounced tendency to aggregate and form clumps, which sharply differentiated it from the material on the near side of the Moon. To uncover the root of this feature, a series of laboratory tests were conducted, including tests using an inclined tray and a rotating cylinder, aimed at determining the angle of repose of the material—a key parameter characterizing the flowability of soils in mechanics. The results showed that the samples brought by Chang’e-6 had a significantly higher angle of repose compared to the rock from the visible side, making its behavior similar to certain types of cohesive soils on Earth. Careful compositional analysis ruled out magnetic fields or the presence of clay minerals as factors contributing to the enhanced cohesion (stickiness). Instead, a combined effect of three types of interparticle interactions was identified: friction, Van der Waals forces, and electrostatic forces. Notably, as particle size decreases, the influence of Van der Waals and electrostatic forces increases exponentially. The study established a threshold for particle size: when the D60 value—the diameter below which 60% of the sample mass lies according to granulometric composition—drops below approximately 100 micrometers, Van der Waals and electrostatic forces begin to significantly enhance soil cohesion. High-resolution tomography was used to analyze over 290,000 individual regolith particles from the Chang’e-6 samples. The minimum D60 value recorded was only 48.4 micrometers, indicating extremely fine dust. At the same time, these particles exhibit complex shapes and pronounced sharp edges. “This is quite atypical. Generally, the smaller the particles, the more spherical their shape tends to be. However, the Chang’e-6 soil, despite being smaller in size, displays a more complex morphology,” noted Professor Qi Shengwen from the Institute of Geology and Geophysics, Chinese Academy of Sciences. This atypicality may be due to a higher content of easily decomposable feldspars combined with potentially more intense space weathering effects on the lunar far side. The interaction between the finer and evidently rougher particles promotes their stronger clumping. “Our research for the first time provides a systematic explanation for the unique cohesiveness of the Chang’e-6 soil from the perspective of discrete element mechanics,” Qi emphasized. The findings will be valuable for planning future lunar expeditions, particularly for tasks such as building permanent bases and utilizing local resources, he added.
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