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Scientists discover drier mantle on moon's farside, offering potential insight on lunar evolution

Chinese scientists have discovered that the moon's mantle contains less water on the lunar farside than on the nearside, based on analysis of basalts collected by the Chang'e-6 (CE6) lunar mission.

This breakthrough research was conducted by a team led by Prof. Hu Sen from the Institute of Geology and Geophysics of the Chinese Academy of Sciences.

The team's findings, in Nature, reveal that the mantle source of CE6 mare basalts contains 1–1.5 μg.g^-1 of water, indicating that the lunar farside mantle is drier than that of the nearside. This result may offer crucial insight into the moon's formation and thermochemical evolution.

Over the past two decades, extensive studies of lunar samples from the nearside have demonstrated a highly heterogeneous distribution of water in the moon's interior, with concentrations ranging from approximately 1 to 200 μg.g^-1.

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.

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.

The team's results indicate that the parent magma of these basalts contains 15–168 μg.g^-1 of water. Additionally, the team estimated that the mantle source of the CE6 basalts has a water content of 1–1.5 μg.g^-1, significantly lower than that of the nearside mantle.

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.

This study was conducted in collaboration with Nanjing University.