In a cosmic twist that could rewrite our understanding of lunar history, Chinese scientists have unearthed compelling evidence that the Moon’s farside mantle is significantly drier than its nearside counterpart. This discovery, published in Nature, emerges from the Chang’e-6 (CE6) mission—China’s ambitious and historic venture to retrieve samples from the mysterious lunar farside.
A Tale of Two Hemispheres
The Moon has always shown one face to Earth, but it’s the hidden, rugged terrain of the farside that has long captured scientific curiosity. Thanks to CE6, we now have direct evidence that the mantle beneath this elusive hemisphere holds markedly less water than the nearside—a finding that opens new chapters in lunar science.
Led by Professor Hu Sen of the Institute of Geology and Geophysics at the Chinese Academy of Sciences, the research team discovered that the mantle source of CE6 mare basalts—lava-like rocks formed from ancient volcanic activity—contains just 1 to 1.5 micrograms of water per gram (μg/g). In stark contrast, similar samples from the Moon’s nearside have shown mantle water concentrations ranging from 1 to 200 μg/g, painting a picture of a hydrologically lopsided Moon.
Why This Matters: Water, Origins, and Evolution
Water is more than a life-giving molecule—it’s also a cosmic storyteller. The way it behaves during planetary formation and volcanic processes can reveal hidden details about a planet or moon’s interior and evolutionary journey. On the Moon, water behaves as an incompatible element, meaning it resists being incorporated into solid minerals during magma crystallization. Instead, it prefers to stay in the melt, concentrating in the residual magma.
Here’s where things get interesting: the nearside of the Moon, particularly the Procellarum KREEP Terrane, is known for its high concentrations of thorium and other incompatible elements, including water. Meanwhile, the farside—especially the South Pole–Aitken (SPA) Basin, where CE6 touched down—shows lower concentrations of these elements.
This chemical discrepancy hints at a deeper geophysical divide. “Our findings strongly suggest that the Moon’s mantle has a hemispheric water distribution—a fundamental and previously unconfirmed asymmetry,” Prof. Hu explained.
Digging Deep: The Science Behind the Discovery
To unlock these secrets, researchers focused on melt inclusions and apatite crystals within the CE6 basalt samples. Melt inclusions are tiny pockets of molten rock trapped inside crystals, preserving snapshots of the magma’s original chemistry. Apatite, a phosphate mineral that naturally incorporates water and volatile elements into its structure, acts like a mineral time capsule.
Through careful analysis of water content and hydrogen isotopic ratios, the team was able to reconstruct the water budget of the parent magma. Their estimates—ranging from 15 to 168 μg/g of water in the magma itself—combined with advanced geochemical modeling, led to the conclusion that the mantle source had only a fraction of that water, at 1–1.5 μg/g.
Rewriting Lunar History
The implications of this dry-versus-wet mantle contrast stretch beyond chemistry—they ripple into theories about the Moon’s formation and internal dynamics. The Moon is believed to have formed following a giant impact between the early Earth and a Mars-sized body known as Theia. This cataclysmic event likely led to the formation of a magma ocean on the Moon, from which its crust and mantle crystallized.
If one hemisphere of the Moon ended up with significantly less water, it could indicate that the early magma ocean was not uniformly mixed, or that the impact itself delivered materials asymmetrically. Either scenario challenges the simplistic models of the Moon as a homogenous, bone-dry body and underscores the complexity of planetary formation.
Clues Hidden in the Highlands
The team’s findings also dovetail with previous research into the Moon’s surface geology. The Feldspathic Highlands and the SPA Basin, both on the farside, are older and more heavily cratered than the smoother, lava-flooded terrains of the nearside. The lack of thorium and other incompatible elements in these regions has long puzzled scientists.
Now, with water added to the list of scarce resources in the farside mantle, a clearer picture begins to emerge—one that points to a geochemical and thermal dichotomy dating back billions of years. This dichotomy could explain not only differences in volcanic activity between the two hemispheres but also variations in crust thickness, surface composition, and even tectonic history.
A Mission Beyond the Surface
The Chang’e-6 mission is China’s first to return samples from the Moon’s far hemisphere and only the second sample-return mission in the 21st century, following Chang’e-5’s nearside success in 2020. Launched with collaboration from Nanjing University and other institutions, CE6 has pushed the boundaries of robotic space exploration and planetary science.
Its triumph underscores the importance of sample return missions. Remote sensing can hint at surface compositions, but nothing matches the resolution and richness of hands-on (or rather, robotic-arm-on) geochemistry. As nations increasingly set their sights on the Moon for exploration, resources, and perhaps even habitation, understanding its internal structure becomes more than academic—it’s strategic.
Looking Ahead: What This Means for Future Research
This landmark study offers a more nuanced view of the Moon’s internal water budget. For scientists modeling the evolution of the bulk silicate Moon—the silicate-dominated portions excluding the core—these new water constraints offer crucial boundary conditions.
They also raise provocative new questions:
- Could the Moon have experienced hemispheric-scale convection patterns that redistributed water and heat?
- Was the giant-impact hypothesis more chaotic or asymmetric than previously thought?
- Might future farside missions uncover even drier or unexpectedly hydrated materials?
Final Thoughts: A Dry Discovery, a Flood of Insight
In the grand narrative of lunar exploration, the Chang’e-6 mission has delivered more than just rocks—it has provided a rare and precious window into the Moon’s inner workings. The discovery that the farside mantle is drier than the nearside adds a rich, new layer to the evolving portrait of our celestial companion.
As China and other spacefaring nations continue their push toward deeper lunar exploration, one thing is certain: the Moon still has many secrets to reveal, and its silent farside may be the key to unlocking them.
Reference: Sen Hu, Water abundance in the lunar farside mantle, Nature (2025). DOI: 10.1038/s41586-025-08870-x. www.nature.com/articles/s41586-025-08870-x