The Moon has been exposed to the solar wind for billions of years, but its two hemispheres are struck by the solar wind at different speeds and energies. Now, research based on samples from China’s Chang’e-6 mission shows that Earth’s magnetosphere shaped this difference.
Two Lunar Hemispheres Bombarded by the Solar Wind
The solar wind — a constant stream of high-speed charged particles from the Sun — directly bombards the lunar surface. Lunar regolith has preserved a record of this bombardment, acting as a natural archive of solar-wind-derived volatile substances, including noble gases such as He, Ne, Ar, Kr, and Xe. These chemically inert elements are reliable markers of solar-wind implantation and provide valuable clues about the process.
Before this study, the lack of samples from the far side of the Moon made it impossible to conduct direct experiments on systematic differences in solar-wind implantation between the two hemispheres. However, China’s Chang’e-6 mission returned 1.935 grams, or 0.07 ounces, of regolith from the South Pole–Aitken Basin on the far side of the Moon, providing the first opportunity to directly compare solar-wind implantation processes on the near and far sides.
Using these lunar samples, a research team led by the Institute of Geology and Geophysics of the Chinese Academy of Sciences conducted an isotopic study of noble gases in the Chang’e-6 regolith and determined the concentrations and isotopic composition of the inert gases.
Differences in Noble-Gas Isotopic Composition on the Two Sides of the Moon
In their analysis, the researchers first noticed that the isotopic composition of neon in the Chang’e-6 regolith was highly unusual. The average ²⁰Ne/²²Ne ratio was 11.34 ± 0.22, significantly lower than in all previously analyzed samples from the Moon’s near side, but close to the theoretical isotopic composition expected after strong fractionation of the solar wind. This means that the far side of the Moon experienced stronger isotopic fractionation, leading to relative enrichment in the heavier isotope.
For krypton and xenon, their release behavior also differed from near-side samples. In stepwise heating experiments, solar-wind-derived xenon in the Chang’e-6 regolith was released mainly at high temperatures, forming a single high-temperature release peak. By contrast, Chang’e-5 samples showed a distinct two-peak release pattern, with noticeable xenon release at both low and high temperatures. This indicates that solar-wind ions penetrated much deeper into the far-side regolith than into near-side regolith — meaning the far side was exposed to higher-energy particles.
But why do the two hemispheres of the Moon receive solar wind with different energies?
Earth’s Magnetosphere Slows the Solar Wind on the Near Side
The research team explains this difference through the “speed-regulating” effect of Earth’s magnetosphere. As the Moon orbits Earth, it periodically passes through the magnetosheath — a buffer zone around the magnetosphere — where the solar wind slows from a typical speed of 400 km/s to about 200 km/s.
This slower solar wind mostly reaches the side of the Moon facing Earth, resulting in shallower implantation of particles into the regolith on that side. By contrast, the far side, which constantly faces away from Earth, remains directly exposed to undisturbed solar wind, allowing ions to penetrate more deeply into the regolith.
The researchers suggest that about 25% of the total solar-wind exposure at the Chang’e-5 landing site was influenced by this slowed solar wind, while the Chang’e-6 landing site did not experience such a shielding effect.
Noble Gases Preserve Records of Magnetosphere–Solar Wind Interactions
By providing the first direct empirical evidence from far-side lunar samples, this study confirms the speed-regulating effect of Earth’s magnetosphere on solar-wind implantation into the lunar surface — an effect permanently preserved both in the depth distributions of implanted particles and in the isotopic signatures of noble gases in the regolith.
In addition, the researchers noted that heavy noble gases in lunar soils may act as “fossil records” of past interactions between Earth’s magnetosphere and the solar wind, offering a new approach to reconstructing the long-term evolution of Earth’s magnetosphere in combination with paleomagnetic records.
The results also show that interactions within the Sun–Earth–Moon system are more complex than previously thought. According to the researchers, these findings open a new window into ancient dynamics, showing that Earth’s nearest celestial neighbor preserves previously unknown records of these interactions.
