Drier far side of the Moon deepens understanding of lunar evolution

Drier far side of the Moon deepens understanding of lunar evolution
by Simon Mansfield
Sydney, Australia (SPX) Apr 14, 2025

The Chang'e 6 mission has unveiled that the Moon's far side mantle holds significantly less water than its near side, offering crucial insight into the Moon's geological history. Analysis of rock samples from the mission indicates water content in the far side's mantle is as low as 1 to 1.5 micrograms per gram-less than 2 parts per million-marking the driest values ever recorded in lunar geology.

Published in Nature, the study was led by Professor Hu Sen of the Chinese Academy of Sciences' Institute of Geology and Geophysics. "Even the driest desert on Earth contains around 2,000 parts per million of water - over a thousand times more than what's found on the lunar far side surface," Hu noted.

Past estimates of lunar water content relied exclusively on near side samples, which have recorded levels as high as 200 micrograms per gram. The new far side data challenges those earlier assumptions, reaffirming the prevailing theory that the Moon formed from a high-temperature impact event 4.5 billion years ago that stripped away volatiles like water.

The Chang'e 6 probe, launched in May 2024, landed in the South Pole-Aitken Basin and returned in June with 1,935 grams of the first-ever samples collected from the Moon's hidden hemisphere. From that haul, a 5-gram subset comprising 578 particles, 28 percent of which were mare basalt fragments, was selected for this analysis.

Hu explained that this sample set provided the first opportunity to directly assess the water content of the lunar far side mantle, long suspected to differ from the near side not only in water content but also in its geologic makeup. A prior study in November 2024 had already revealed an ancient, far side volcanic event not observed in near side samples.

Geological differences between the lunar hemispheres extend to their chemical composition, surface morphology, and crustal thickness. Hu's team believes these disparities may be essential for refining models of the Moon's internal processes and its origin.

"The water content in the lunar rocks plays a crucial role in testing the hypothesis of the moon's origin," Hu emphasized. If levels had exceeded 200 micrograms per gram, current theories would be under greater scrutiny. Instead, the low water values strengthen the impact hypothesis.

Francis McCubbin, Astromaterials Curator at NASA's Johnson Space Center and a peer reviewer of the study, described it as "a landmark study on the water abundance of the lunar far side."

These findings carry broader implications for upcoming lunar missions, particularly for China's plans to establish a permanent base and conduct crewed landings before 2030. As Hu told Xinhua, varying terrain on the Moon's two hemispheres will necessitate advanced navigation and control systems. Water availability will also play a critical role in supporting long-term human presence.