Chinese study reveals lower water content in lunar farside mantle

Published in Nature, the team's findings reveal that the mantle source of the CE6 mare basalts contains only 1 to 1.5 micrograms of water per gram. This is substantially lower than the mantle water content inferred from previous nearside samples, where concentrations have varied from 1 to as high as 200 micrograms per gram over the past two decades of lunar research.

The Procellarum KREEP Terrane (PKT), located on the lunar nearside, features elevated thorium levels compared to other major geochemical regions like the Feldspathic Highlands and the South Pole-Aitken (SPA) Basin. Because both thorium and water are incompatible elements during magmatic differentiation, they tend to accumulate in melts rather than in crystallized minerals. This geochemical behavior supports the idea that the mantle under the SPA Basin on the farside may be inherently drier.

To test this hypothesis, scientists examined melt inclusions and apatite minerals within the CE6 mare basalts, the first-ever samples returned from the SPA Basin. The results revealed that the basalts' parent magma had water concentrations ranging from 15 to 168 micrograms per gram. Using these values, the researchers estimated the water content of the source mantle to be just 1 to 1.5 micrograms per gram.

This difference in water content strengthens the case for a hemispheric divide in the Moon's interior hydration. It echoes other pronounced asymmetries observed between the Moon's two hemispheres and has significant implications for theories regarding the Moon's formation, especially the giant-impact hypothesis.

The discovery offers a more refined estimate of the bulk water inventory in the Moon's silicate portion and enhances our understanding of how water influenced the Moon's geological development.

This research was conducted in partnership with Nanjing University and received support from the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and other institutions.

Research Report:Water abundance in the lunar farside mantle