Remote sensing detection has found the prevalence of water (OH/H2O) on the lunar surface, but the origin and distribution of lunar surface water has been controversial due to the lack of direct sample analysis evidence. Recently, the team of Tang Hong and Li Xiongyao, members of the Center for Excellence in Comparative Planetology of the Chinese Academy of Sciences and the Institute of Geochemistry of the Chinese Academy of Sciences, conducted research on the Chang’e-5 lunar soil sample, and through infrared spectroscopy and nanoion probe analysis, found that there is a large amount of solar wind genesis water in the mineral surface of Chang’e-5, and estimated that the water content contributed by solar wind proton injection into the Chang’e-5 lunar soil is at least 170 ppm. Combined with transmission electron microscopy and energy spectroscopy analysis, it is revealed that the formation and preservation of water due to solar wind are mainly affected by the exposure time of minerals, crystal structure and composition. The study confirms that lunar surface minerals are an important “reservoir” of water, providing an important reference for the distribution of water in the latitude region of the lunar surface.
The infrared spectrometers on board the Cassini, Deep Impact, and Chandrayaan-1 missions all detected widespread solar wind genesis water on the lunar surface, and analysis of Apollo samples found only that water in cementitious glass, volcanic glass, and plagioclase was associated with the source of the solar wind. However, at present, the formation and preservation characteristics of solar wind genesis water in different minerals on the lunar surface are insufficiently understood, and in-depth analysis is also needed in conjunction with lunar samples.
China’s Chang’e-5 mission collected 1.731 kg of lunar soil samples in the northeastern lunar storm ocean (43.06°N, 51.92°W), which is higher than the previous Apollo and Luna missions. In addition, isotopic dating results indicate that the Chang’e-5 sample is about 2 billion years old, making it the youngest lunar sample ever obtained. Compared with the Apollo sample, the Chang’e-5 sample was sampled at a unique location and formation age, providing a new window for exploring the content and distribution of lunar surface water. Pyroxene, plagioclase and olivine, as the main constituents of the Chang’e-5 lunar soil, are the best carriers for studying the water reserves of solar wind genesis. In this study, the pyroxene, olivine and plagioclase minerals in the Chang’e-5 lunar soil samples were studied, and the genesis, content and storage status of water in different minerals were analyzed, thereby evaluating the water content of the Chang’e-5 lunar soil and the water distribution in the latitude region of the lunar surface.
The water content of the Chang’e-5 mineral is isotope ratio to hydrogen
Infrared reflection spectroscopy (Figure 1) analysis found that the Chang’e-5 mineral generally has a wide absorption peak at 3,200 to 3,800 cm-1, indicating the presence of OH. Among them, a small absorption peak of 3,300 cm-1 and a wide absorption peak of 1,640 cm-1 were found in CE-PL2 plagioclase, indicating the presence of H2O. Based on the irred reflection spectroscopy of the Earth sample, the overall water content in the Chang’e-5 mineral was determined, including the water content of olivine (CE-OL1, CE-OL2, CE-OL3) was 152 ± 14 to 311 ± 30 ppm, the water content of plagioclase (CE-PL1, CE-PL2, CE-PL3) was 231 ± 16 to 385 ± 27 ppm, and the water content of pyroxene (CE-PY1, CE-PY2) was 134 ± 19 to 199 ± 28 ppm. The NanoSIMS analyzed the water content and hydrogen isotope ratio in the mineral surface layer of about 200 nm, with a water content of 916 ± to 4483 ± 314 ppm, a water content of 1798 ± 81 to 4476 ± 142 ppm in the surface layer of pyridine, and a water content of 3471 ± 166 to 5962 ± 335 ppm. Hydrogen isotope analysis showed that these waters were significantly H-poor D-rich (δD = –773 ± 188 to –945 ± 384 ‰), close to the hydrogen isotope ratio of the solar wind (δD ≈ – 1,000 ‰). The difference in water content obtained by infrared reflection spectroscopy and NanoSIMS analysis is mainly attributed to differences in test depth, indicating that the water in the mineral is mainly distributed within the polar surface layer. The characteristics of water concentration in minerals within the surface and high D-depletion strongly indicate the source of the solar wind for water, and water is mainly present in the form of OH, and a small part may be in the form of H2O.
Figure 1 Mineral water content and hydrogen isotope ratio
The structural characteristics of the mineral polar surface layer and the water distribution characteristics of solar wind genesis
Transmission electron microscopy (TEM) images show that the polar surface layers of olivine, plagioclase, and pyroxene are all partially amorphized and/or fully amorphized ring bands (Figure 2), and energy spectroscopy shows that these amorphous ring bands are consistent with the underlying crystals, demonstrating that the formation of amorphous ring bands is caused by solar wind injection. The thickness of the amorphous ring band is mainly in the range of 40 to 100 nm, and the plagioclase (CE-PL1) with the highest water content is significantly different from other samples, with a fully amorphized ring band with a thickness of about 100 nm on its surface and a partial amorphized ring band with a thickness of about 400 nm in the lower part. Combined with TEM and NanoSIMS analysis, the depth profile of solar wind genesis water in the Chang’e-5 mineral was drawn, as shown in Figure 3, the water content in the minerals was the highest, and the water content in the amorphous ring belt decreased rapidly with the increase of depth, and slowly decreased in the lower crystals. The longitudinal distribution characteristics of solar wind genesis water indicate that solar wind genesis water is mainly distributed in the amorphous ring belt, and a small amount can diffuse into the internal crystal.
Fig. 2 TEM diagram of the surface microstructure of the Chang’e-5 mineral
Figure 3 Depth profile of the mineral water content of Chang’e-5
Solar wind is a major influencing factor in the formation of water
The churning of the lunar surface causes the lunar soil particles to be exposed to the solar wind at different times, that is, the total amount of solar wind protons injected into the minerals is different, resulting in different amounts of solar wind genesis water. Solar wind particle injection can cause damage to the surface structure of the mineral, so the degree of amorphism of the ring band can assess the exposure time of the mineral. Comparing the microscopic structure of the surface layer and the water content analysis, it was found that the surface water content of the mineral was generally positively correlated with the thickness and amorphous ring band and the degree of crystallization, and was consistent with the radiation trace density trend in the lower crystal, indicating that the solar wind exposure time of the mineral was the most important factor affecting the solar wind genesis water content. However, the CE-OL1 olivine sample, despite having the highest water content, had a low degree of amorphism and radiation trace density due to the higher mg content of the sample relative to other olivines and the relatively difficult to destroy the crystal structure at the same exposure time. The chemical composition of plagiosite samples in this study is similar, among which CE-PL1 is significantly higher than other plagioclase amorphism and water content, mainly due to the different directions of the crystal surface injected by solar wind protons.
Implications for the distribution of water on the surface of the lunar solar wind
Combined with the mineral composition of the Chang’e-5 lunar soil, this study estimates that the solar wind genesis water content in the lunar soil of Chang’e-5 is at least 170 ppm, which is significantly higher than that of the lunar interior water, so this study believes that solar wind proton injection is the main source of water in the lunar soil of Chang’e 5. In addition, the lunar soil maturity analysis of Chang’e 5 indicates its relative immaturity, considering that the solar wind genesis water in the lunar surface latitude area found by remote sensing is positively correlated with the lunar soil maturity, this study shows that in the lunar surface latitude area, such as the northern storm ocean and the rainy sea basin, the lunar soil maturity is similar to that of the Chang’e 5 region, and there may be an approximate content of solar wind genesis water; The lunar soil in the highlands on the northwest side of the Storm Ocean is relatively mature, and there may be higher levels of solar wind genesis water in the lunar soil in this region. This study reveals the high content of solar wind genesis water in lunar soil minerals, evaluates the distribution of solar wind genesis water in the latitude regions of the lunar table, provides an important basis for the future utilization of lunar surface water resources, and also provides an important reference for the formation mechanism of solar wind genesis water in the solar system without atmosphere bodies (such as Mercury and asteroids).
The above research results were published in the international authoritative journal Nature Communications. The first author of the paper is Zhou Chuanjiao, ph.D. candidate of the Institute of Geochemistry, Chinese Academy of Sciences, and the corresponding authors are Associate Researcher Tang Hong and Researcher Li Xiongyao of the Institute of Geochemistry, Chinese Academy of Sciences. The research has been funded by the Strategic Pilot Science and Technology Project of the Chinese Academy of Sciences (XDB 410000000), the National Natural Science Foundation of China (41931077), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2018435), the Advance Research on Civil Aerospace Technology (D020201) and the Frontier Science Key Research Program of the Chinese Academy of Sciences (ZDBS-SSW-JSC007-10, QYZDY-SSW-DQC028). (Source: Institute of Geochemistry, Chinese Academy of Sciences)
Related paper information:https://doi.org/10.1038/s41467-022-33095-1
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