惑星科学：月の裏側の水の存在量の評価

The far side mantle of the Moon might be drier than its nearside counterpart, a paper published in Nature suggests. The findings are based on an analysis of samples obtained during the Chang’e-6 mission.

Understanding the abundance of water in the lunar mantle can provide insights into the formation and evolution of the Moon. Over the past two decades knowledge about the levels of water in the mantle has increased considerably, which has led researchers to shift away from the once traditional ‘bone dry’ concept to the idea of a relatively ‘wet’ Moon. On 25 June 2024 the Chang’e-6 mission returned samples from the South Pole–Aitken impact basin on the far side of the Moon, providing the opportunity to examine the water abundance in the lunar mantle in this region for the first time.

Sen Hu and colleagues analysed samples from the South Pole–Aitken impact basin and report water abundances and hydrogen isotope compositions of the mineral apatite and melt inclusions from basalt derived from partial melting of the lunar mantle. They estimate that the water abundance for the mantle is 1–1.5 micrograms per gram, which is on the dry end of measurements made on the lunar nearside (1–200 micrograms per gram). The authors suggest the difference in the distribution of water in the Moon’s mantle could potentially represent a hemispheric division, which mirrors differences in surface features.

The authors conclude that their findings are important for estimating the total water abundance on the Moon and provide constraints for the giant impact hypothesis for the formation of the Moon and subsequent lunar evolution.