An analysis of material collected by China's Chang'e 6 mission now suggests that Earth's magnetosphere is responsible for much of this contrast. The findings were published in Nature Geoscience.

Moon Dust Preserves a Solar Wind Record

The solar wind is a constant flow of fast-moving charged particles released by the Sun. Because the Moon lacks a thick atmosphere and a global magnetic field, these particles strike its surface directly.

Over time, the lunar regolith has stored evidence of this exposure. It acts as a natural archive of volatile materials delivered by the solar wind, including the noble gases (He, Ne, Ar, Kr, Xe). Because these elements rarely react chemically with other materials, scientists can use them as reliable markers of how solar wind particles entered and accumulated in lunar soil.

Until recently, researchers could study samples only from the Moon's near side. Without material from the far side, they could not directly test whether solar wind implantation differed systematically between the two hemispheres.

That changed when China's Chang'e 6 mission brought back 1.935 grams of regolith from the South Pole Aitken basin on the lunar far side. The samples provided the first direct opportunity to compare how solar wind particles were implanted into soil from both sides of the Moon.

Chang'e 6 Samples Reveal an Isotopic Difference

A team led by the Institute of Geology and Geophysics (IGG) of the Chinese Academy of Sciences (CAS) examined the concentrations and isotopic compositions of helium, neon, argon, krypton, and xenon in the Chang'e 6 material.

The study was conducted by Xuhang Zhang, a postdoctoral researcher at IGG working under the supervision of Professor HE Huaiyu. The project also included researchers from the University of Science and Technology of China and members of the Chang'e 7 volatile payload team.

One of the clearest differences appeared in the neon isotopes. The Chang'e 6 regolith had an average ²⁰Ne/²²Ne ratio of 11.34 ± 0.22. That value is considerably lower than measurements from all previously studied near-side samples, but it closely matches the theoretical composition expected after strong solar wind fractionation.

This pattern indicates that the far side experienced more intense isotopic fractionation, causing the heavier neon isotope to become relatively more abundant.

Solar Wind Reached Deeper on the Far Side

Krypton and xenon provided additional evidence that the two hemispheres were exposed to particles with different energies.

During stepwise heating experiments, xenon delivered by the solar wind was released from the Chang'e 6 material mainly at high temperatures, creating a single high-temperature peak. Chang'e 5 samples from the near side showed a different pattern, with substantial xenon released at both low and high temperatures.

The contrast suggests that solar wind particles penetrated much farther into the far side regolith. Deeper implantation generally requires particles with greater energy, indicating that the Moon's far side was exposed to a faster and more energetic solar wind.

How Earth Slows the Solar Wind

The researchers explain the difference through the "speed-governing" effect of Earth's magnetosphere.

As the Moon travels around Earth, it sometimes moves through the magnetosheath, a buffer region surrounding the magnetosphere. Within this zone, the solar wind slows from its usual speed of about 400 km/s to roughly 200 km/s.

The reduced speed mainly affects the Moon's near side, which faces Earth. Lower-energy particles do not travel as deeply into the surface, so their implantation remains closer to the top of the regolith.

The far side, which always faces away from Earth, is not affected in the same way. It remains exposed to the undisturbed solar wind, allowing faster particles to penetrate farther into the lunar soil.

The researchers estimate that approximately 25% of the total solar wind exposure recorded at the Chang'e 5 landing site involved this slower flow. By comparison, the Chang'e 6 site on the far side showed no evidence of receiving the same protective effect.

Lunar Soil Could Record Earth's Magnetic Past

These far side samples provide the first direct physical evidence that Earth's magnetosphere controls the speed of solar wind particles reaching different parts of the Moon. The influence is permanently recorded in both the depth at which particles entered the regolith and the isotopic signatures of noble gases trapped there.

The researchers also propose that heavy noble gases in lunar soil could act as "fossil records" of earlier interactions between the solar wind and Earth's magnetosphere. When studied alongside paleomagnetic evidence, these gases may offer a new way to trace how Earth's magnetosphere changed over long periods of time.

The results reveal that the relationship among the Sun, Earth, and Moon is more complicated than scientists previously understood. They also suggest that the Moon has preserved hidden evidence of these ancient interactions, giving researchers a new way to investigate the long-term history of Earth's magnetic environment.