Earth's magnetosphere a natural 'speed governor', Chinese study reveals

The farside of the Moon receives solar wind particles at nearly full speed, while its Earth-facing side is hit by the same wind after it has been slowed by nearly half, according to a study published in the journal Nature Geoscience on Wednesday.

The discovery, based on the first-of-its-kind noble gas analysis of lunar farside samples returned by China's Chang'e 6 mission in 2024, shows that Earth's magnetosphere acts as a natural "speed governor" — deflecting and decelerating solar wind before it reaches the nearside — and that the effect is permanently locked into the soil's noble gas record.

The solar wind — a continuous flow of charged particles from the Sun — is the primary carrier of volatile elements across the solar system. On Earth, a strong global magnetic field blocks most of this inflow, while ongoing geological activity continuously erases the planet's ancient magnetic signatures.

The Moon, by contrast, is geologically quiescent and lacks both a global magnetic field and a protective atmosphere. This leaves its surface fully exposed to solar wind and allows its regolith to act not only as a passive collector but also as a long-term "time capsule", preserving an archive of interactions among the Sun, Moon and Earth over billions of years.

Using soil samples retrieved from the South Pole-Aitken Basin, the oldest and largest impact basin on the lunar farside, Chinese researchers analyzed noble gases — helium, neon, argon, krypton and xenon — whose chemical inertness allows them to preserve a record of the physical processes that occurred before and after they were trapped in the regolith.

When comparing the farside samples with previous records from Chang'e 5, collected at nearly identical latitudes, the team found that krypton and xenon were released almost exclusively at high temperatures in a single-peak pattern, while the nearside samples exhibited a distinct bimodal pattern, with both low- and high-temperature peaks.

"Krypton and xenon are heavy noble gases that hardly diffuse once trapped in regolith grains," Zhang Xuhang, first author of the study and a postdoctoral researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, said.

"Their release temperature directly reflects the original implantation depth — indicating that solar wind particles penetrated far deeper into the farside soil under full-speed conditions, while the nearside received a substantial fraction of decelerated solar wind, forming a shallower, low-temperature component," Zhang said.

Simulations showed that when the Moon passes through Earth's magnetosheath — the turbulent outer layer of its magnetic shield — normal solar wind slows from roughly 400 kilometers per second to about 200 km per second. At the Chang'e 5 landing site, this slower wind accounts for about 25 percent of total solar wind exposure, while on the farside, solar wind is never decelerated.

Neon isotopes, atoms of the same element with different masses, told a more puzzling story. Chang'e 6 returned neon isotope ratios far below those measured in any nearside lunar sample — a signature indicating that the farside experienced an extreme level of isotopic fractionation, a process in which lighter isotopes are preferentially lost relative to heavier ones. The degree of fractionation exceeded what existing models can fully explain.

"The neon fractionation involves many overlapping factors, and the specific mechanism still requires further study," Zhang said.

He noted that the findings challenge the long-standing assumption that Earth's magnetic field simply blocks solar wind. Instead, it acts as a "speed governor", selectively decelerating particles that reach the lunar nearside.

"The Moon offers a different window — we can use today's observations to trace back the state of Earth's magnetic field in the deep past, a perspective that has not been explored before," he said.

He Huaiyu, corresponding author of the study and a professor at the institute, said that by analyzing heavy noble gases in older nearside samples, such as Chang'e 5 drill core samples, scientists could in principle reconstruct how the boundary between Earth's magnetosphere and the solar wind has shifted over time.