The far side of the moon has a drier mantle than researchers previously believed, giving scientists more insight into its water content and the way that the moon has evolved.
“Water abundance is crucial for understanding the origin and subsequent evolution process of the moon,” says Sen Hu, an astrophysicist with the Institute of Geology and Geophysics of the Chinese Academy of Sciences.
A better understanding of the moon’s water content — though minimal — may also point to potential resources available for any bases built up there in the future, Hu says.
Exploring the Moon’s Mantle and Dark Side
The moon’s mantle is the layer between the core and the crust. Most of the mantle used to be liquid magma during the earlier formation of the Earth’s satellite when the moon was active volcanically. This magma eventually solidified, though the mantle seems to have gone through some tumultuous times in its early formation — including a possible flip of material.
The lunar mantle also contains some water. Some samples returned from the moon’s nearside revealed about 1 to 200 micrograms of water per gram of other material, Hu says.
However, no one has examined if the far side of the moon has different water quantities, until now. In a new study published in Nature, Hu and his colleagues analyzed a mantle sample brought back by the China National Space Administration’s (CNSA) Chang’e 6 mission in 2024 — a robotic moon exploration on the dark side of the moon.
Analysis of mare basalts, believed to originate in the moon’s mantle, revealed that these far-sided materials had only 1 microgram to 1.5 micrograms of water per gram of other material.
Read More: The Water on the Moon May Trace Back to Early Earth — and Comets
A Drier Lunar Mantle
The moon’s near side and far side are quite different in terms of their composition. The rock types between the two sides vary, as well as the topography and thorium concentration, Hu says.
The thorium was a clue to potential differences in water content, since both thorium and water stand out from other materials in magma. The differing levels of thorium found on the crust of the moon may also mean differing levels of water stored in the mantle.
The Chang’e 6 mission brought material from the South Pole – Aitken (SPA) basin — a massive impact crater that hit the far side of the moon 4.2 billion years to 4.3 billion years ago. Hu says that it’s possible that the impactor may have caused this difference in water between regions of the moon, among other geological differences.
One more possibility, Hu says, is that the moon’s mantle just has different amounts of water at different depths. A deeper mantle may have less water than a mantle closer to the surface, no matter whether it’s on the near side or the far side. As a result, the sample from the deep crater could just be characteristic of a deeper layer of lunar mantle.
Read More: Scientists Are Still Pondering These Mysteries of the Moon
More Research From the Far Side of the Moon
To confirm either of these theories, more research would be needed on samples from more of the far side of the moon. This could help researchers better understand if this sample just represents the SPA basin, or if dryness there is represented more uniformly across the far side of the moon.
An answer to these questions could provide more clarity about the origin of the moon and its early evolution — including how massive impactors like the one that formed the SPA basin affected its geology.
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Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.
