The moon has been exposed to the solar wind for billions of years. But new evidence shows that neither hemisphere experiences bombardment in the same way. Particles impacting the near and far sides arrive at different velocities and carry different amounts of energy.
Analysis of material collected by China’s Chang’e 6 mission suggests that Earth’s magnetosphere is responsible for much of this contrast. The survey results are natural earth science.
Moon dust preserves records of the solar wind
The solar wind is a constant stream of fast-moving charged particles emitted by the Sun. Because the Moon does not have a thick atmosphere and no global magnetic field, these particles directly impact the Moon’s surface.
The lunar regolith has stored evidence of this exposure over time. It acts as a natural archive for volatile substances such as noble gases (He, Ne, Ar, Kr, Xe) carried by the solar wind. Because these elements have little chemical reaction with other substances, scientists can use them as reliable markers of how solar wind particles entered and accumulated in the lunar soil.
Until recently, researchers were only able to study samples from the moon’s near side. Without material from the far side, it was not possible to directly test whether solar wind injection differs systematically between the two hemispheres.
The situation changed when China’s Chang’e 6 mission brought back 1.935 grams of regolith from the Antarctic Aitken Basin on the moon’s far side. This sample provided the first direct opportunity to compare how solar wind particles were injected into the soil from both sides of the moon.
Isotopic differences revealed in Chang’e 6 samples
A team led by the Institute of Geology and Geophysics (IGG) of the Chinese Academy of Sciences (CAS) investigated the concentrations and isotopic composition of helium, neon, argon, krypton, and xenon in the Chang’e 6 material.
This research was conducted by IGG postdoctoral researcher Xuhang Zhang under the supervision of Professor HE Huaiyu. The project also involved researchers from the University of Science and Technology of China and members of the Chang’e 7 volatile payload team.
One of the most obvious differences appeared in neon isotopes. The average 20Ne/22Ne ratio of the Chang’e 6 regolith was 11.34 ± 0.22. This value is considerably lower than measurements from all nearside samples studied so far, but is broadly consistent with the theoretical composition expected after fractionation of the strong solar wind.
This pattern indicates that more intense isotopic fractionation occurred on the opposite side, resulting in a relative abundance of heavier neon isotopes.
Solar wind reaches far away
Krypton and xenon provided additional evidence that the two hemispheres were exposed to particles with different energies.
During the gradual heating experiment, the xenon carried by the solar wind was mainly released from the Chang’e 6 material at high temperature, forming a single high temperature peak. Chang’e 5 samples from the near side showed a different pattern, with significant xenon released at both low and high temperatures.
This contrast suggests that solar wind particles have penetrated deeper into the backside regolith. In general, deeper injections require particles with more energy, indicating that the far side of the moon was exposed to faster, more energetic solar winds.
How the Earth slows down the solar wind
The researchers explain the difference through a “velocity-controlling” effect of Earth’s magnetosphere.
As the moon moves around the Earth, it may move through a magnetic sheath, a buffer region that surrounds the magnetosphere. Within this zone, the speed of the solar wind slows from its normal speed of about 400 km/s to about 200 km/s.
The reduction in speed primarily affects the near side of the Moon that faces Earth. Low-energy particles do not travel deep into the surface, so their injection remains near the top of the regolith.
The back side, which always faces away from the Earth, is equally unaffected. It remains exposed to the solar wind unhindered, allowing faster particles to penetrate even deeper into the lunar soil.
Researchers estimate that about 25% of the total solar wind exposure recorded at the Chang’e 5 landing site is related to this slow flow. By comparison, the Chang’e 6 site on the other side showed no evidence of similar protection.
Moon’s soil may record Earth’s magnetic past
These farside samples provide the first direct physical evidence that Earth’s magnetosphere controls the speed of solar wind particles reaching different parts of the moon. This effect is permanently recorded both in the depth at which the particles penetrate the regolith and in the isotopic signature of the noble gases trapped there.
The researchers also propose that heavy noble gases in the moon’s soil could serve as a “fossil record” of early interactions between the solar wind and Earth’s magnetosphere. Studying these gases in conjunction with paleomagnetic evidence could provide new ways to track how Earth’s magnetosphere has changed over time.
The results reveal that the relationship between the Sun, Earth, and Moon is more complex than scientists previously understood. They also suggest that the Moon preserves hidden evidence of these ancient interactions, providing researchers with new ways to investigate the long-term history of Earth’s magnetic environment.
