Water abundance in the lunar farside mantle

The water contents of the lunar interior record important clues for understanding the formation and subsequent thermochemical evolution of the Moon¹. The Chang'e-6 (CE6) mission returned samples from the South Pole-Aitken impact basin of the lunar farside^2-4, providing an opportunity to study the water contents of the farside mantle. Here we report the water abundances and hydrogen isotope compositions of apatite and melt inclusions from CE6 mare basalt, derived from partial melting of the lunar mantle. The parent magma of CE6 mare basalt is estimated to have a water abundance of 15-168 μg g^-1 with a δD value of -123 ± 167‰. Our estimate of water abundance of 1-1.5 μg g^-1 for the mantle source indicates that the farside mantle is potentially drier than its nearside counterpart. This contrast thus suggests that the distribution of water in the interior of the Moon may exhibit a hemispheric dichotomy similar to numerous surface features⁵. The new estimate for the lunar farside mantle represents a landmark for estimating the water abundance of the bulk silicate Moon, providing critical constraints on the giant impact origin hypothesis^6-8 and the subsequent evolution of the Moon for which the role of water is central^1,9.