China’s Chang’e-6 returned lunar samples reveal differences in space environment in the Moon’s near and far side

This study is led by Dr. XIAN Haiyang and Dr. ZHU Jianxi of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. On June 25, 2024, the Chang’e-6 mission successfully achieved the world’s first sampling of the lunar farside and returned safely to Earth. In August, based on his outstanding performance in the Chang’e-5 sample research, Dr. Xian received the first batch of Chang’e-6 samples. This marks the first time in human history that samples have been directly obtained from the lunar farside, and the research team hopes to use these samples to gain insights into the space weathering characteristics on the lunar farside. The space weathering features recorded in lunar samples can sensitively capture information about the surrounding space environment, helping us better understand the conditions on the lunar farside.

Under Dr. Xian’s guidance, his student Lin Jiarui at the Electron Microscopy Center of the Guangzhou Institute of Geochemistry analyzed these precious samples one by one using a scanning electron microscope (SEM). To preserve as much of the surface information as possible, she chose to evenly distribute the fine-grained powder on conductive adhesive and then deposited a 10 nm carbon film, observing the samples at a low voltage of 3 kV. After examining nearly a thousand particles, she noted that the Chang’e-6 samples exhibited noticeably less melt drops and melt splashes on their surfaces. To further systematically study the space weathering characteristics, she selected seven mineral particles with distinct compositions using energy-dispersive spectroscopy (EDS), which together represent the main lunar mineral types.

In subsequent transmission electron microscopy (TEM) investigations, Lin and other team members prepared a feldspar particle (designated P2‑001) using focused ion beam (FIB) techniques and discovered that its surface lacked the nanophase metallic iron (npFe⁰) particles commonly found in Apollo samples. Typically, the surface of feldspar in Apollo samples exhibits a vapor‑deposited layer generated by micrometeorite impacts that contains npFe⁰. They further conducted EDS mapping on the other seven FIB sections under TEM, and the results showed no significant compositional differences between the edges and interiors of these minerals. All regions displaying observable space weathering features—including the amorphized layers, vesicles, and npFe⁰ grains—were consistent with the substrate mineral composition, indicating that these features can be attributed to the damage caused by solar wind radiation on the substrate minerals.

Lin further measured the thickness of the amorphized layers and the grain sizes of npFe⁰, and counted the solar wind tracks in pyroxene and olivine to estimate the solar wind exposure time of the particles. The study found that the solar wind exposure time of the Chang’e-6 samples was close to the minimum observed in the Apollo 11 samples, lower than that of the other Apollo samples, and slightly shorter than that of the Chang’e-5 samples. However, surprisingly, the npFe⁰ grain sizes in the Chang’e-6 samples were larger. “This might suggest that solar wind radiation in this region leads to more pronounced segregation and aggregation of iron,” she noted. These exciting new results add to the growing evidence that space weathering on the lunar farside may differ from that on the nearside, and, contrary to previous findings from Apollo and Chang’e-5 samples, solar wind radiation plays a more dominant role in the space weathering process on the lunar farside.

There are differences in the solar wind’s influence on different regions of the Moon. During each synodic month, the near side of the Moon enters Earth’s magnetotail, where the protection afforded by Earth’s magnetic field reduces its exposure to the solar wind; in contrast, the farside is continuously exposed to direct solar wind radiation. Moreover, due to orbital dynamics, different locations on the Moon experience varying impact velocities from cometary and asteroidal meteoroids. The relative velocity between the Moon’s surface and impacting meteoroids changes with the lunar phase: during a full moon, when the Moon and meteoroids move in the same orbital direction, the relative velocity increases; the opposite occurs during a new moon.

Micrometeoroid impacts and solar wind radiation are the two primary processes driving space weathering, but the effective sputtering rate from the solar wind and vapor deposit from micrometeorite impacts offset each other. Therefore, when discussing space weathering mechanisms, it is essential to consider the relative contributions of both factors under different space environments. The findings from the Chang’e-6 samples indicate that on the lunar farside, the effect of the solar wind exceeds that of micrometeorite impacts, further demonstrating that the space weathering process is regulated by variations in the space environment.

Since the first images of the lunar farside were captured in 1959, it has been evident that its topography is markedly different from that of the nearside—a phenomenon known as the Moon’s “dichotomy.” However, whether the lunar space environment also exhibits a similar “dichotomy” had until now only been inferred from remote sensing spectra. The latest analysis of the Chang’e-6 samples provides direct, sample‑based evidence for this hypothesis, highlighting the critical role of space environmental variables in the space weathering process. This discovery not only deepens our understanding of how solar wind radiation and micrometeorite impacts shape the lunar surface, but also offers important insights for studying the space weathering evolution of other airless bodies.

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See the article:

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

https://doi.org/10.1093/nsr/nwaf087

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