Huge differences found between the Moon’s near and farside

China’s Chang’e-6 mission made headlines on June 25, 2024 for successfully collecting the first direct samples from the Moon’s farside and returning them to Earth in August. 

Following his notable work analyzing Chang’e-5 specimens, Dr. Xian Haiyang of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), was awarded the earliest batch of Chang’e-6 material. 

Together with his colleague, Dr. Zhu Jianxi, he aims to deepen our understanding of how space weathering shapes the lunar farside – an unexplored region that may experience environmental processes distinct from those on the near side of the Moon.

A glimpse into farside soil

Under Dr. Xian’s guidance, graduate researcher Lin Jiarui at the Electron Microscopy Center of the Guangzhou Institute of Geochemistry began her work by examining the newly arrived samples from the Moon with a scanning electron microscope (SEM). 

She first distributed the fine-grained lunar powder evenly onto conductive adhesive, then carefully applied a 10 nm carbon film to stabilize each grain’s surface before imaging. 

Operating at a voltage of just 3 kV, Lin meticulously documented nearly a thousand particles and noted how the Chang’e-6 specimens showed noticeably fewer melt droplets or splashes than samples from Apollo or Chang’e-5 missions.

To systematically probe space weathering signatures, she selected seven mineral grains – identified using energy-dispersive spectroscopy (EDS) – that represented the primary lunar minerals. 

Observing each of these in detail, Lin sought to understand the cumulative effects of micrometeoroid impacts, solar wind radiation, and other processes that gradually transform the topmost layers of lunar soil.

Absence of surface iron nanoparticles

In a follow-up investigation using transmission electron microscopy (TEM), Lin and colleagues applied focused ion beam (FIB) techniques to section one feldspar grain, designated P2‑001. 

Unlike Apollo samples, whose surfaces often carry a vapor-deposited film studded with nanophase metallic iron (npFe⁰), this Chang’e-6 feldspar showed no such feature. Instead, the newly recovered piece offered a different picture of how surface layers had evolved under lunar conditions.

Further EDS mapping on seven more FIB-prepared sections revealed an important clue. Any indications of space weathering, such as amorphous layers, vesicles, or small npFe⁰ grains, reflected the substrate’s intrinsic composition rather than the signature of external material condensing onto the rock. 

This suggests that, at least for these minerals, the solar wind may have been the dominant driver, causing localized damage without adding additional vapor components from micrometeoroid impacts.

Different exposure, different iron grains

Lin then quantified the thickness of the amorphized zones, measured the diameters of npFe⁰ grains, and counted solar wind tracks in minerals like pyroxene and olivine. The goal was to estimate how long these particles were exposed to solar radiation on the Moon. 

The results indicated that the Chang’e-6 grains had one of the shortest solar wind exposure times observed among lunar samples – on par with the least-exposed Apollo 11 grains and somewhat briefer than Chang’e-5’s. 

Yet, intriguingly, the iron nanoparticles found in these samples were larger. “This might suggest that solar wind radiation in this region leads to more pronounced segregation and aggregation of iron,” noted Lin.

The findings add fresh context to an ongoing puzzle: the lunar farside appears to be space-weathered in ways that deviate from patterns documented by earlier Apollo or Chang’e-5 missions. Rather than micrometeoroid-induced vapor deposits shaping these surfaces, the solar wind seems to play a more central role.

The Moon’s farside is unique

Scientists have long noted differences between the lunar near side and far side. While the near side periodically passes through Earth’s magnetotail, which partially shields it from solar wind flux, the farside remains openly exposed.

And although micrometeoroid impacts certainly occur on both sides, varying orbital velocities can alter how those impacts affect the regolith. For instance, during a full moon, the Moon and meteoroids travel in roughly the same orbital direction, increasing the relative collision speeds.

Micrometeoroid impacts and solar wind sputtering are recognized as the two major processes that gradually alter the chemical and physical surface properties of airless bodies like the Moon. 

Yet the Chang’e-6 samples highlight that on the farside, solar wind may outstrip micrometeorite activity in shaping the regolith’s mineralogy and microscopic structure.

Constant transformation of the Moon

Ever since the first images of the lunar farside in 1959, scientists have highlighted its stark contrast with the near side – a phenomenon dubbed the Moon’s “dichotomy.” But whether that difference extends to their space weathering processes has remained largely speculative. 

Now, direct analysis of Chang’e-6’s farside soil offers concrete evidence that the unique environmental conditions on the far side lead to distinct signatures, emphasizing solar wind’s importance relative to micrometeoroid impacts.

This finding broadens our understanding of how the space environment molds the lunar surface. With every new mission, researchers refine their knowledge of airless-body evolution – an insight valuable not just for comprehending Earth’s closest neighbor but for interpreting the histories of other rocky objects throughout the solar system. 

The Chang’e-6 results suggest that future missions could uncover additional nuances in how cosmic rays, magnetic fields, and orbital geometry each shape the surfaces of bodies with little or no atmosphere.

For now, the novel data reveal that the lunar farside experiences a heightened influence from solar wind, offering a new lens through which to view our ancient satellite’s constant transformation under cosmic forces.

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