Chinese scientists analyzing samples from the Moon’s farside have uncovered that its mantle holds significantly less water than the nearside—a finding that hints at a deep, internal divide within our celestial neighbor.
This discovery, based on basalt samples returned by China’s Chang’e-6 mission, offers new insight into the Moon’s mysterious formation and evolution. It could even reshape theories about the Moon’s violent origins and how water shaped its interior over billions of years.
Drier Mantle on the Lunar Farside
Chinese scientists have found that the Moon’s mantle contains significantly less water on the farside than on the nearside, based on a new analysis of volcanic rocks collected by the Chang’e-6 lunar mission.
The research, led by Prof. Hu Sen from the Institute of Geology and Geophysics at the Chinese Academy of Sciences, was published today (April 9) in Nature. The team examined basalts from the lunar farside and found that their mantle source contains only 1 to 1.5 micrograms of water per gram, far lower than levels typically found on the nearside. This discovery could provide important clues about how the Moon formed and how its interior evolved over time.
Water Variation Across the Moon
Previous studies of nearside lunar samples have shown that water in the Moon’s interior is unevenly distributed, with concentrations ranging from about 1 to 200 μg/g.
Notably, the crust exposed at the surface of the Procellarum KREEP Terrane on the lunar nearside has a higher thorium (Th) concentration than the other two primary lunar geochemical provinces: the Feldspathic Highlands and the South Pole–Aitken (SPA) Basin on the lunar farside.
Incompatible Elements and Lunar Dryness
Both Th and water are considered incompatible elements during magmatic processes, meaning they preferentially remain in the melt rather than becoming incorporated into crystallizing minerals. This geochemical behavior suggests that the mantle beneath the SPA Basin, on the lunar farside, may contain a lower abundance of water.
To confirm this hypothesis, the research team focused on analyzing water content and hydrogen isotopes in melt inclusions and apatite within CE6 mare basalts—the first samples returned from the farside SPA Basin.
Measuring Moon’s Hidden Water
The team’s results indicate that the parent magma of these basalts contains 15-168 μg/g of water. Additionally, the team estimated that the mantle source of the CE6 basalts has a water content of 1-1.5 μg/g, significantly lower than that of the nearside mantle.
A New Look at Lunar Origins
This disparity points to a potential hemispheric dichotomy in the Moon’s internal water distribution, mirroring many of the asymmetrical features observed on the lunar surface.
This new estimate of the lunar farside mantle’s water content marks a significant step forward in refining our understanding of the bulk silicate Moon’s water inventory. It provides important constraints on the giant-impact hypothesis of the Moon’s origin and underscores the role of water in the Moon’s long-term evolution.
Reference: “Water abundance in the lunar farside mantle” by Huicun He, Linxi Li, Sen Hu, Yubing Gao, Liang Gao, Zhan Zhou, Mengfan Qiu, Disheng Zhou, Huanxin Liu, Ruiying Li, Jialong Hao, Hejiu Hui and Yangting Lin, 9 April 2025, Nature.
DOI: 10.1038/s41586-025-08870-x
This study was conducted in collaboration with Nanjing University and supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and other funding agencies.
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