Differences in lunar space weathering revealed by farside samples

Differences in lunar space weathering revealed by farside samples
by Simon Mansfield
Sydney, Australia (SPX) Apr 10, 2025

This study, led by Dr. XIAN Haiyang and Dr. ZHU Jianxi from the Guangzhou Institute of Geochemistry under the Chinese Academy of Sciences, presents groundbreaking findings from the Chang'e-6 lunar mission. On June 25, 2024, Chang'e-6 achieved the first successful return of samples from the Moon's farside. Following earlier contributions to the Chang'e-5 program, Dr. Xian was selected in August 2024 to analyze the initial batch of Chang'e-6 material. These samples provide the first direct evidence from the Moon's elusive farside and offer a new perspective on space weathering under unique environmental conditions.

Under Dr. Xian's supervision, graduate researcher Lin Jiarui performed detailed scanning electron microscopy (SEM) at the institute's Electron Microscopy Center. To preserve surface integrity, she evenly distributed the powdered material on conductive adhesive, applied a 10-nanometer carbon layer, and utilized low-voltage imaging at 3 kV. Among nearly 1,000 grains studied, Lin observed that the Chang'e-6 specimens bore fewer signs of surface melting or splashing, features often linked to micrometeorite impacts. She further selected seven mineral types using energy-dispersive spectroscopy (EDS) to represent the Moon's main mineralogical components.

Using focused ion beam (FIB) techniques, the team prepared feldspar particle P2-001 for transmission electron microscopy (TEM). Unlike Apollo samples, which commonly exhibit nanophase metallic iron (npFe degree ) formed by micrometeorite vapor deposition, this specimen lacked such features. EDS mapping across seven TEM sections showed that regions affected by space weathering mirrored the composition of their underlying minerals. This suggests that damage on the lunar farside is primarily the result of solar wind interactions with native minerals, rather than vapor deposition from impacts.

Further analysis involved measuring amorphized layer thicknesses, npFe degree grain sizes, and quantifying solar wind tracks in pyroxene and olivine. The results revealed that these particles experienced minimal exposure to solar wind, comparable to the lowest levels observed in Apollo 11 samples, and less than those from Chang'e-5. Yet paradoxically, npFe degree grains were larger in the Chang'e-6 specimens. "This might suggest that solar wind radiation in this region leads to more pronounced segregation and aggregation of iron," Lin explained. These findings point to a dominant role of solar wind in space weathering on the lunar farside, contrasting with earlier results from nearside samples.

This discrepancy may be explained by differing environmental exposures. Each lunar month, the nearside passes through Earth's magnetotail, receiving partial shielding from solar wind, while the farside remains fully exposed. Additionally, the velocity of incoming meteoroids varies with lunar phase, affecting the relative energy of impacts. As a result, both micrometeoroid bombardment and solar wind radiation must be considered in balance when analyzing space weathering.

The Chang'e-6 findings indicate that on the lunar farside, solar wind effects outweigh those of micrometeorite impacts. This sample-based evidence supports the hypothesis that the space environment itself differs significantly between the two hemispheres of the Moon. While the farside's unusual topography has been documented since 1959, this study provides the first direct physical confirmation of a corresponding environmental dichotomy.

These insights not only enhance our understanding of lunar surface evolution but also have implications for other airless planetary bodies, shedding light on the complex interplay of space weathering agents beyond Earth.

Research Report:Differences in space weathering between the near and far side of the Moon: Evidence from Chang'e-6 samples