IT House news on March 5, according to the Chinese Academy of Sciences website, on December 17, 2020, the mainland Chang'e-5 probe collected lunar samples located in the northern part of the Storm Ocean (43.06 °N, 51.92 °W) and returned to Earth. Isotope chronology analysis results have proved that the Chang'e-5 sample has the youngest known basalt age (~2 billion years), and combined with the results of previous research, it can be seen that the formation age and spatial exposure history of the surface lunar soil in the Chang'e-5 sampling area are much smaller than those of the Apollo lunar soil. Therefore, the Chang'e-5 sample may have retained information about the formation mechanism of elemental metal iron in the early stages of lunar soil formation and evolution.
Under the guidance of the above ideas, and combined with the previous meteorite research results, the research team of Li Yang of the Institute of Geochemistry of the Chinese Academy of Sciences carried out the in-situ micro-region electron analysis of iron-rich olivine in the powder sample of the lunar soil taken by Chang'e 5. The experimental results show that the edges of the iron olivine particles in the Chang'e-5 lunar soil sample generally have the characteristics of coexistence of porous nano-metallic iron, amorphous silicon-rich components and magnesium-rich layers (Figure 1), and thermodynamic calculations and electron loss energy spectroscopy (EELS) analysis show that nanoscale vesicles inside the nanometallic iron may be formed by O2 and SiO gases (Figure 2).
Figure 1.The composition characteristics of the outermost ring band of iron olivine particles in Chang'e 5 lunar soil are mainly composed of porous nano-metal iron (v-npFe0), magnesium-rich layer (Mg-layer) and silicon-rich component (Si) | Source: Chinese Academy of Sciences website
▲ Figure 2.Electron Energy Loss Spectroscopy (EELS) Line Scan and Transmission Electron Microscopy Spectrometer (EDS) Surface Scan Result containing stomatal elemental metal iron | Source: Chinese Academy of Sciences website
Based on the above analysis results, the study determined the formation mechanism and related products of iron olivine decomposition to form nano-metallic iron in the lunar soil. The absence of the molten and evaporative deposit layers on the surface of the iron olivine indicates that the decomposition reaction occurs under sub-solid phase conditions, and the heat source of the reaction may come from the friction during mineral fragmentation or the local thermal effects produced by low-speed micrometeorite bombardment. The nanometallic iron produced by the decomposition of iron olivine on the surface of the lunar soil particles usually has a medium particle size range (10-35 nm), and the effect of the nanometallic iron of this particle size on the spectrum is different from that of the ultra-fine nanometallic iron formed by evaporative deposition (~3 nm), and the contribution of the nanometals generated by the decomposition of magnesium ferrosilicate on the lunar surface to the spectral modification of the lunar soil needs to be further considered.
The study confirms the new genesis mechanism of elemental metal iron in the lunar soil, provides a reference for the study of the formation and evolution process of the lunar soil in the Chang'e-5 landing area, and puts forward new ideas for the analysis of subsequent returned samples of the moon and asteroids. IT House understands that the results of the research were published in Geophysical Research Letters.