Chinese Research Reveals Slowing Solar Wind on the Near Side of the Moon

Illustration of the solar wind hitting the Moon. Chinese research says Earth's magnetosphere can slow down solar wind particles before they reach the Moon's side facing the Earth. Illustration by China Daily.

JAKARTA - Samples of lunar soil brought back by China's Chang'e 6 mission have opened up new findings. The far side of the Moon receives solar wind at almost full speed, while the near side of the Moon stores traces of particles slowed down by the Earth's magnetosphere.

China Daily, quoted Saturday, July 25, reported that the findings were published in the journal Nature Geoscience. The study used noble gas analysis of samples of the far side of the Moon brought home by Chang'e 6 in 2024.

The results show that Earth's magnetosphere is not just a shield. The magnetic protective layer also acts like a natural speed regulator that deflects and slows down the solar wind before it reaches the near side of the Moon.

The magnetosphere is a protective region around Earth that is formed by the Earth's magnetic field. This layer can deflect and slow down charged particles from the Sun called the solar wind.

Solar wind is a flow of charged particles that constantly comes from the Sun. Solar wind also carries volatile elements to various parts of the solar system.

Volatile elements are elements or compounds that are easily vaporized. On Earth, most of the solar wind flow is blocked by the global magnetic field. However, the Earth's ancient magnetic traces are largely erased by ongoing geological activity.

The Moon is different. Geologically, the Moon is much quieter. The Moon also has no global magnetic field and a protective atmosphere like Earth. As a result, its surface is open to exposure to the solar wind.

This condition makes the regolith of the Moon an important natural archive. Regolith is a layer of soil, dust, and rock fragments on the Moon's surface. This layer can store a record of the interaction between the Sun, Moon, and Earth for billions of years.

In the study, Chinese researchers used soil samples from the South Pole-Aitken Basin, the oldest and largest impact basin on the far side of the Moon.

They analyzed five noble gases, namely helium, neon, argon, krypton, and xenon. Noble gases are a group of gases that are very difficult to react with other elements. Helium is commonly known as a balloon filling gas, neon is known from neon lights, while argon, krypton, and xenon are also used in lights, lasers, and certain technologies.

Because they are difficult to react, these gases can store traces of physical processes that occur on the Moon's soil. The traces are recorded in regolith, which is a layer of dust, soil, and rock fragments that cover the Moon's surface.

China Daily also said that the team then compared the sample of the far side of the Moon from Chang'e 6 with previous records from Chang'e 5. The Chang'e 5 sample was taken from the near side of the Moon at almost the same latitude.

As a result, the krypton and xenon from the far side sample of the Moon are almost entirely released at high temperatures with a single peak pattern. In contrast, the near side sample of the Moon shows a two-peak pattern, namely at low and high temperatures.

Zhang Xuhang, the first author of the study and a postdoctoral researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences, said krypton and xenon are heavy noble gases that are almost non-diffusive after being trapped in regolith grains.

"The temperature of the release directly reflects the depth of the initial planting," said Zhang.

According to Zhang, the pattern shows that solar wind particles penetrate much deeper into the far side of the Moon because they come in full speed.

In contrast, the near side of the Moon receives a fraction of the slowed-down solar wind. The particles form a more shallow component that is released at low temperatures.

Simulations show that when the Moon passes through the Earth's magnetosheath, the normal solar wind slows from about 400 kilometers per second to about 200 kilometers per second.

The magnetosheath is the turbulent outer layer of the Earth's magnetic shield.

At the Chang'e 5 landing site, this slower solar wind covers about 25 percent of the total solar wind exposure. On the far side of the Moon, the solar wind never experiences such a slowdown.

Zhang said these findings challenge the long-held assumption that Earth's magnetic field only blocks the solar wind. According to him, the magnetosphere also slows down certain particles that reach the near side of the Moon.

"The moon offers a different window. We can use current observations to retrace the state of the Earth's magnetic field in the very distant past," said Zhang.

He Huaiyu, corresponding study author and professor at the institute, said analysis of heavy noble gases in samples of the near side of the Moon that are older could open up new opportunities.

According to He, samples such as the Chang'e 5 drill core could in principle help scientists reconstruct the boundary changes between Earth's magnetosphere and solar wind over time.