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Yesterday, the reporter learned from the Institute of Physics of the Chinese Academy of Sciences that the Chinese research team found a mineral crystal rich in water molecules and ammonium on the moon in the lunar samples brought back by Chang'e 5. This discovery marks the first time that molecular water has been found in returned lunar soil, and it also reveals the true existence form of water molecules and ammonium on the moon.

The discovery of hydrated minerals on the lunar surface marks a major breakthrough in the study of lunar water and ammonium, and also provides new possibilities for the development and utilization of lunar resources in the future. The relevant results have been published online in Nature Astronomy.

Whether there is water on the moon has been a core issue in lunar scientific research and resource utilization for decades. Historically, no water-containing minerals were found in the lunar soil collected by the Apollo missions, which once made the scientific community believe that the moon was a dry desert. It was not until recent years that a series of remote sensing missions found evidence of the existence of lunar water in the permanent shadow areas at the lunar poles and some illuminated areas of the moon. Using highly sensitive characterization techniques, people have successively found "water" (hydroxyl OH-) at the PPM (parts per million) level in some Apollo samples. So far, no conclusive evidence of the existence of water molecules has been found in the returned lunar soil. The existence form of molecular water on the lunar surface has also been unknown.

The research was completed by Chen Xiaolong, a researcher at the Institute of Physics, Chinese Academy of Sciences, Jin Shifeng, an associate researcher, Hao Munan, a doctoral student, in collaboration with Guo Zhongnan, an associate professor at the University of Science and Technology Beijing, Yin Bohao, an engineer at Tianjin University, and Ma Yunqi, a researcher at the Qinghai Institute of Salt Lakes, Chinese Academy of Sciences.

Through high-precision single crystal diffraction and chemical analysis, the researchers determined that the molecular formula of the mineral is (NH4, K, Cs, Rb) MgCl3·6H2O, which is a hydrated mineral.

Photos and composition of ULM-1. a. Photo of CE5 soil sample, b. Photo of ULM-1 single crystal, c. EDS spectrum, d. EPMA spectrum, e. Raman spectrum, f. IR spectrum.

Its structure contains up to six crystal waters, and the mass ratio of water molecules in the sample is as high as 41%.

Crystal structure and charge density of ULM-1.

Infrared and Raman spectroscopy can clearly observe the vibration peaks of water molecules and ammonium, and charge density analysis can distinguish the hydrogen in water molecules.

Chen Xiaolong introduced that the crystal structure of the mineral is the same as a rare crater mineral discovered on Earth in recent years. On Earth, this type of mineral is formed by the interaction between hot basalt and volcanic gases rich in water and ammonia, suggesting a close connection between lunar water and volcanic activity.

To ensure the accuracy of this discovery, the researchers conducted rigorous chemical and chlorine isotope (37Cl/35Cl) analysis. Experimental data showed that the Cl isotope composition of the mineral was significantly different from that of Earth minerals, with a δ37Cl value of up to 24‰, which is consistent with minerals on the moon.

Distribution of chlorine isotopes in different terrestrial and exoplanetary materials.

Analysis of the mineral's chemical composition and formation conditions further ruled out terrestrial contamination or rocket exhaust as the source of this hydrate. The presence of this hexahydrate mineral forms an important constraint on the composition of lunar volcanic gases. Based on thermodynamic analysis, the lower limit of water content in lunar volcanic gases at that time was comparable to that of Lengai volcano, the driest volcano on Earth today, which is of great significance for our understanding of the evolution of the moon. These findings reveal a complex history of lunar volcanic degassing.

Constraints on water fugacity in lunar volcanic gases from ULM-1 crystallization.

The discovery of this hydrated mineral also reveals a possible form of water molecules on the moon - hydrated salt. Unlike volatile water ice, this hydrate is very stable in the high-latitude areas of the moon (Chang'e 5 sampling site). This means that even in the vast sunlight-exposed areas of the moon, this stable hydrated salt may exist, providing a broader prospect for the utilization and exploration of lunar resources.

Associate Researcher Jin Shifeng and Hao Munan are the co-first authors, and Researcher Chen Xiaolong is the corresponding author. The Lunar Exploration and Space Engineering Center provided lunar soil samples (CE5C0400) for this study.

Author: Xu Qimin

Text: Xu Qimin Photos: Provided by the interviewee Editor: Xu Qimin Responsible editor: Ren Quan

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