On Sept. 17, a joint research team led by Li Chunlai of the National Astronomical Observatory of the Chinese Academy of Sciences and Hu Hao of the China Center for Lunar Exploration and Space Engineering, and Yang Mengfei of the Beijing Institute of Control Engineering published the first research paper on the samples returned by Chang'e-6 in the journal National Science Review (NSR), describing the mineralogical and geochemical physical characteristics of the returned samples.
Typical images of samples returned from Chang'e 6. (a) Selected portion of rock chip particles larger than 1 mm from the Chang'e 6 shovel sample. (b-e) Backscattered images (BSE) of basalt chips with different structural characteristics, (f-g) breccia and (h) cemented rock. Microscope photographs of typical basalt (i and j), cemented rock (k), light-colored rock chips (l) and glassy material (m and n). Credit: Chunlai Li, Hao Hu, Meng-Fei Yang, Jianjun Liu, Qin Zhou, Xin Ren, Bin Liu, Dawei Liu, Xingguo Zeng, Wei Zuo, Guangliang Zhang, Hongbo Zhang, Saihong Yang, Qiong Wang, Xiangjin Deng, Xingye Gao, Yan Su, Weibin Wen, Ziyuan Ouyang, Nature of the lunar farside samples returned by the Chang'E-6 mission, National Science Review, 2024;, nwae328, https://doi.org/10.1093/nsr/nwae328
The paper points out that the Chang'e 6 return sample has a lower density, indicating a looser structure and higher porosity. Particle analysis showed a bimodal distribution of particle sizes in the lunar soil, suggesting that the sample may have experienced mixing of different material sources. Compared with the Chang'e-5 sample, the plagioclase content of this sample increased significantly, while the olivine content decreased significantly, suggesting that the lunar soil in this region was obviously influenced by non-metamorphic materials.
In addition, the rock debris fragments collected by Chang'e 6 mainly consisted of basalt, breccia, cemented rock, light-colored rock and vitreous material. Among them, basalt fragments accounted for 30% to 40% of the total, and their minerals were dominated by pyroxene, plagioclase and ilmenite, with very low olivine content. The breccia and cemented rocks consist of basalt fragments, glass beads, glass fragments, and a small amount of light-colored rocky detrital material such as plagioclase and sillimanite, which further reveals the complexity of the sample's origin.
Mineralogical and geochemical characteristics
Mineralogical analyses showed that the main phase compositions of the Chang'e 6 lunar soil samples are plagioclase (32.6 percent), pyroxene (33.3 percent) and glass (29.4 percent), with the glass content close to the lower limit of the Apollo samples. In addition, a small amount of pyroxene plagioclase was detected in the sample, suggesting the presence of nonmetamorphic material.
The geochemical analyses also revealed that the Chang'e 6 samples contain high levels of aluminum oxides (Al₂O₃) and calcium oxides (CaO), and relatively low levels of iron oxides (FeO), which is consistent with the characteristics of the mixture of basalts and plagioclase from the Lunar Sea. In addition, the samples contain significantly lower levels of trace elements such as thorium (Th), uranium (U) and potassium (K) than the KREEP basalts, which show a significant difference from the samples from the Apollo and Chang'e 5 missions, which are found in the Kreep stormy oceanic geoid on the lunar frontal surface.
A new chapter in lunar science?
In previous lunar explorations, more than 380 kilograms of samples have been reported from the lunar surface, particularly through six Apollo missions, three Moon missions and one Chang'e-5 mission. However, all of these samples came from the front side of the Moon. The Chinese Chang'e 6 mission accomplished the feat of human sampling from the back side of the Moon for the first time, bringing back 1,935.3 grams of valuable samples. The landing site of this sampling mission is located in the South Pole-Aitken Basin (SPA), on the back of the Moon. The sampling site is on the edge of the Apollo impact crater within the SPA basin, an area of extremely thin lunar crust, which should reveal primitive material from the early impact basins on the back of the Moon.
Credit: Chunlai Li, Hao Hu, Meng-Fei Yang, Jianjun Liu, Qin Zhou, Xin Ren, Bin Liu, Dawei Liu, Xingguo Zeng, Wei Zuo, Guangliang Zhang, Hongbo Zhang, Saihong Yang, Qiong Wang, Xiangjin Deng, Xingye Gao, Yan Su, Weibin Wen, Ziyuan Ouyang, Nature of the lunar farside samples returned by the Chang'E-6 mission, National Science Review, 2024;, nwae328, https://doi.org/10.1093/nsr/nwae328
The Chang'e 6 samples include not only basalt, which records the history of volcanic activity, but also a mixture of non-basalt materials from other regions. These samples, like the “messengers” from the ancient times of the Moon, provide important first-hand information for us to study the early impact history of the Moon, the volcanic activities on the backside of the Moon, and the composition of materials inside the Moon.
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