Has the secret of the moon's "color palette" been solved? The answer lies in the lunar soil brought back by Chang'an 5!

Produced by: Science Popularization China

Author: Keke (popular science creator)

Producer: China Science Expo

When we look up at the moon in the night sky, its silvery white light seems to be eternal. However, in the eyes of scientists, the surface of the moon is undergoing a slow and continuous change. This change not only affects the appearance of the moon, but also contains rich scientific information.

Recently, Chinese scientists have revealed the secrets behind this mysterious change by conducting in-depth research on lunar soil samples brought back by Chang'e 5. This research, jointly completed by the team of Academician Wang Weihua from the Institute of Physics of the Chinese Academy of Sciences, the team of Academician Yang Mengfei from the China Academy of Space Technology, and the team of Academician Zou Zhigang from Nanjing University, not only deepens our understanding of the evolution of the lunar surface, but also provides important inspiration for future exploration of other celestial bodies.

Let us uncover the mystery of the changing "color" of the moon.

Lunar glass: a space-weathered time capsule

In the lunar soil samples brought back by Chang'e 5, scientists discovered a special substance - lunar glass beads. These tiny glass beads are not man-made, but formed naturally. They are like time capsules that record the process of space weathering and contain a wealth of information.

Weathering of the lunar surface by (micro)meteorite impacts and solar wind radiation

(Image source: Reference 1)

The formation process of these glass beads is very unique. When high-speed meteorites or micrometeorites hit the lunar surface, the huge energy will instantly melt the rocks near the impact point. These molten rock materials are thrown into the air under the impact shock wave, and quickly cool during the flight, forming glass particles of various shapes. Among them, some molten droplets rotate at high speed during the flight, and finally solidify into spherical or ellipsoidal glass beads.

The research team found a large number of nano-sized metallic iron particles in these glass beads. These tiny iron particles are the key factor causing the changes in the optical properties of the lunar surface. The presence of nano-iron particles will change the reflection characteristics of the lunar surface to light of different wavelengths, thus affecting the "color" of the moon we observe.

Scientists have found that these nano-iron particles are not evenly distributed in the glass beads. There are significant differences in their size, distribution and formation mechanism, which provides important clues for understanding the evolution of the lunar surface.

The researchers used advanced in-situ electron microscopy to achieve dynamic observation of the formation process of nano-iron particles. This technique allowed scientists to "see" how the nanoparticles grew and migrated in the glass beads, providing direct experimental evidence for understanding their formation mechanism.

There are two different sizes of nano-iron particles in the glass beads, one is larger and the other is smaller. Among them, the larger nano-iron particles (with a size of tens of nanometers) are mainly distributed at both ends of the glass beads. The formation of these large particles is closely related to meteorite impact.

When high-speed meteorites hit the lunar surface, the high temperature and high pressure conditions generated melted iron-containing minerals and converted them into elemental iron. This process may involve two mechanisms: one is the disproportionate reaction triggered by the impact, that is, iron-containing oxides decompose into elemental iron and oxygen under extreme conditions; the other is the thermal decomposition process, that is, iron-containing sulfides decompose at high temperatures to produce elemental iron.

The newly generated elemental iron rapidly nucleates and grows in the molten glass beads, forming larger nano-iron particles. Interestingly, scientists also observed the special distribution of these iron particles in the glass beads. During the rotation of the glass beads, the centrifugal force causes these iron particles to migrate to the two ends, and finally they are "frozen" at the two ends when the glass beads solidify.

Even more surprising is that the convergence caused by this rotation sometimes produces ultra-large metallic iron particles with sizes of hundreds of nanometers or even micrometers. This discovery provides us with a new perspective on the microstructure of the lunar surface.

In addition, the research team also found that in addition to the common round glass beads, there are also some ellipsoidal or dumbbell-shaped glass beads with large-size nano-iron protrusions at both ends. This special shape of glass beads provides an excellent "laboratory" for studying the formation and distribution of nano-iron.

The moon appears reddish during a lunar eclipse

(Image source: Wikipedia)

Solar wind irradiation: the source of small nanoparticles of iron

Scientists have also discovered a large number of smaller nano-iron particles (only a few nanometers in size) on the surface of the glass beads. These small particles are mainly distributed in the area with a depth of hundreds of nanometers on the surface of the lunar soil particles, which just corresponds to the effective injection depth of solar wind ions.

The solar wind is a stream of charged particles that is continuously ejected from the surface of the sun, mainly composed of protons and electrons. These high-energy particles continuously bombard the lunar surface, causing physical and chemical changes in the surface material. The study found that within the depth range of solar wind ion injection, there is a clear association between pores or vesicle defects caused by radiation damage and the precipitation of small-sized nano-iron particles. This phenomenon clarifies the causal relationship between small-sized nano-particles and solar wind radiation.

**This discovery overturns the previous cognition of the scientific community. **The traditional view is that nano-iron is mainly formed in the amorphous sedimentary layer on the surface of lunar soil particles. However, this study shows that small-sized nano-iron is actually distributed in the radiation damage layer below the sedimentary layer. This means that the solar wind's spectral modification of the lunar surface is far more important and profound than previously thought.

More interestingly, scientists found that when the size of lunar soil particles is smaller than the penetration depth of solar wind ions, the entire particle is filled with dense nano-iron particles. This observation further confirms the key role of solar wind radiation in the formation of small-sized nano-iron.

Formation and evolution of lunar glass beads and nano-metallic iron

(Image source: Reference 1)

New explanation for the moon's changing color

This research breaks through the traditional view that nano-iron is mainly formed by a single mechanism. Scientists have confirmed that nano-iron particles of different sizes have different formation mechanisms: large particles are mainly produced by meteorite impacts, while small particles are mainly derived from solar wind radiation.

This discovery provides a new perspective for understanding the color changes on the lunar surface. The changes in the optical properties of the lunar surface are actually the result of the combined action of these two mechanisms. Large nano-iron particles mainly affect the overall reflectivity of the lunar surface, while small nano-iron particles affect the detailed characteristics of the spectrum more.

By understanding the relative contributions of these two mechanisms, scientists can now more accurately predict changes in the optical properties of different regions of the Moon. For example, in the magnetic anomaly regions of the Moon, the influence of the solar wind may be weaker, so the color changes in these regions may be mainly affected by meteorite impacts. In contrast, in the equatorial regions of the Moon, where solar wind irradiation is stronger, the formation of small-size nano-iron particles may be more significant.

In addition, this study also provides new ideas for understanding the evolution of the lunar shadow region. In the permanent shadow region, due to the lack of solar wind radiation, the color change may be mainly dominated by meteorite impacts. This is of great significance for exploring the water ice resources in the lunar polar regions.

The moon just after the full moon

(Image source: Wikipedia)

Conclusion

This study not only reveals the mechanism of color change on the lunar surface, but also deepens our understanding of the interaction between the space environment and the surface of celestial bodies. By analyzing the formation mechanism of nano-iron of different sizes, scientists actually constructed a microscopic scale space weathering model.

This model is not only applicable to the moon, but can also be extended to other atmosphereless celestial bodies, such as asteroids and Mercury. By comparing the nano-iron features on the surfaces of different celestial bodies, we can infer the history of the space environment they have experienced, and thus better understand the evolution of the solar system.

The research results provide important reference for future lunar and asteroid exploration missions, and also open up a new direction for space weathering research.

The lunar soil samples brought back by Chang'e-5 are like opening a door to the secrets of the moon. Through the detailed study of lunar glass beads, Chinese scientists not only revealed the mystery of the moon's "color" changes, but also showed us the profound impact of the cosmic environment on the surface of celestial bodies. With the deepening of the lunar exploration project and the advancement of analytical technology, we look forward to more amazing discoveries about the moon and even the entire solar system.

References:

1. Laiquan Shen, Rui Zhao, Chao Chang, Jihao Yu, Dongdong Xiao, Haiyang Bai, Zhigang Zou, Mengfei Yang & Weihua Wang, Separate effects of irradiation and impacts on lunar metallic iron formation observed in Chang'e-5 samples