For billions of years, the moon has been constantly bombarded by the solar wind—a stream of charged particles flowing outward from the sun.
Scientists have long known that this invisible rain of particles leaves its mark on the moon’s dusty surface.
Now, for the first time, researchers have discovered that Earth’s magnetic field changes how the solar wind reaches different sides of the moon.
The discovery comes from the first-ever soil samples collected from the moon’s far side by China’s Chang’e-6 mission.
The findings, published in Nature Geoscience, reveal that Earth’s magnetic environment slows the solar wind before it reaches the moon’s near side, creating a lasting difference between the two lunar hemispheres.
The moon’s surface is covered by a layer of loose dust and broken rock called regolith. Because the moon has almost no atmosphere to protect it, solar wind particles strike the surface directly.
Tiny amounts of gases carried by the solar wind, including helium, neon, argon, krypton, and xenon, become trapped in the regolith. Since these noble gases are chemically stable, they preserve a reliable record of the moon’s long history of solar wind exposure.
Until now, scientists had only studied samples from the moon’s near side. That changed when the Chang’e-6 mission returned nearly two grams of regolith from the South Pole-Aitken Basin on the far side, allowing researchers to compare the two regions directly for the first time.
The research team analyzed the noble gases trapped inside the far-side samples and found clear differences from those collected by the earlier Chang’e-5 mission on the near side. The isotopes of neon showed much stronger fractionation, meaning heavier forms of the gas had become more concentrated over time.
The behavior of xenon also differed. When the samples were heated in the laboratory, xenon trapped in the far-side soil was released mainly at higher temperatures, suggesting it had been implanted deeper beneath the surface.
These findings indicate that solar wind particles reached the far side with greater energy, allowing them to penetrate farther into the lunar soil than particles striking the near side.
The researchers believe Earth’s magnetosphere explains this difference. As the moon travels around Earth, it regularly passes through the magnetosheath, a region surrounding Earth’s magnetic field where the solar wind slows dramatically—from around 400 kilometers per second to roughly 200 kilometers per second.
This slower solar wind mainly reaches the moon’s Earth-facing side, where the lower-energy particles do not burrow as deeply into the surface. The far side, which always faces away from Earth, remains exposed to the faster, undisturbed solar wind.
The researchers estimate that about one-quarter of the solar wind reaching the Chang’e-5 landing site on the near side had been slowed by Earth’s magnetic shield, while the Chang’e-6 landing site on the far side received no such protection.
Beyond revealing a surprising difference between the moon’s two faces, the study suggests that noble gases trapped in lunar soil may preserve an ancient record of how Earth’s magnetic field has changed over billions of years.
By combining these lunar records with evidence from rocks on Earth, scientists may one day reconstruct the long-term history of Earth’s magnetic shield and better understand the complex relationship between the sun, Earth, and the moon.
