Scientists have finally taken a close look at the first-ever soil samples brought back from the far side of the moon, and the results show that this lunar dirt behaves very differently from anything collected before.
China’s Chang’e-6 mission returned nearly two kilograms of soil from the South Pole–Aitken Basin on June 25, 2024.
This region is the largest, deepest, and oldest impact crater on the moon, making it one of the most scientifically important places to study.
Until now, all lunar samples—including those from the Apollo missions, the Soviet Luna program, and China’s earlier Chang’e-5 mission—came from the near side of the moon. These missions brought back large amounts of loose, powdery material.
But the Chang’e-6 team immediately noticed something unusual: the new samples looked “slightly more sticky and clumpy” than expected, according to Hu Hao, the mission’s chief designer.
To understand why, a research team led by Professor Qi Shengwen from the Chinese Academy of Sciences ran a series of experiments.
They wanted to measure the soil’s angle of repose, which describes how easily grains of sand or dust can flow and pile up.
Materials that are loose and dry, like beach sand, form gentle slopes, while stickier materials build much steeper piles. The scientists used fixed-funnel tests and a rotating drum to see how the Chang’e-6 soil behaved.
Their study, published in Nature Astronomy, revealed that the lunar far-side soil forms much steeper angles than soils from the near side. This means the material is far more cohesive—essentially stickier and less likely to flow freely.
The next step was figuring out why.
The researchers ruled out common explanations such as magnetic minerals or clay-like substances, since the samples contained almost none of those. Instead, they found that three forces were responsible for the high cohesion: friction between rough particle surfaces, van der Waals forces that pull tiny particles together, and electrostatic charges.
These attractive forces become stronger when particles are very small.
By analyzing particle sizes, the team found that the Chang’e-6 sample has a D60 value (the size at which 60% of particles are smaller) of just 48.4 micrometers—far finer than near-side soils. Surprisingly, these tiny grains were not smooth or rounded. High-resolution CT scans showed that they were highly irregular in shape, which increases friction and makes particles more likely to stick together.
Professor Qi suggests two reasons for this unusual texture. First, the soil contains more feldspar, a mineral that easily breaks into sharp fragments. Second, the moon’s far side experiences stronger space weathering, which constantly breaks down surface material.
This research offers the first detailed explanation of why far-side lunar soil behaves so cohesively.
It also provides valuable clues about the moon’s geological history and will help engineers design future exploration tools and landers for this untouched region of the lunar surface.
