The Moon has been bathed in solar wind for over four billion years, but not evenly. The near side — the face that always points toward Earth — receives a noticeably gentler bombardment than the far side, which faces outward into deep space. Now, thanks to China's Chang'e-6 mission, scientists finally understand why.
In a study published in Nature Geoscience, researchers from the Chinese Academy of Sciences analyzed noble gases trapped in lunar soil samples returned from the Moon's far side by the Chang'e-6 mission in 2024 — the first samples ever collected from the lunar far side. By comparing these with near-side samples from the earlier Chang'e-5 mission, they discovered a striking asymmetry in how solar wind particles are embedded in the soil.
The solar wind is a continuous stream of charged particles flowing from the Sun at speeds of 300 to 800 kilometers per second. Earth's global magnetic field, the magnetosphere, deflects most of this incoming radiation, protecting life on the surface. But the Moon orbits within Earth's magnetotail — the elongated region of the magnetosphere that stretches away from the Sun — for about five days each month. During this time, the near side of the Moon is partially shielded, while the far side remains fully exposed.
The research team analyzed five noble gases — helium, neon, argon, krypton, and xenon — whose chemical inertness makes them ideal tracers of physical processes. They found that krypton and xenon in the far-side samples were released only at very high temperatures, indicating they had been driven deep into soil grains by high-energy solar wind impacts. In near-side samples, these same gases showed a two-peak release pattern, revealing a mix of both high-energy and decelerated particles.
"Krypton and xenon are heavy noble gases that hardly diffuse once trapped in regolith grains," said Zhang Xuhang, first author of the study and a postdoctoral researcher at the Institute of Geology and Geophysics. "Their release temperature directly reflects the original implantation depth — indicating that solar wind particles penetrated far deeper into the far-side soil under full-speed conditions."
The discovery has implications beyond lunar science. It demonstrates that the Moon's regolith functions as a natural archive of solar system history, preserving records of solar wind properties, Earth's magnetic field evolution, and the changing space environment over billions of years. As future missions return samples from other locations on the Moon, scientists will be able to reconstruct a more complete timeline of how the Sun, Earth, and Moon have interacted through deep time.