The far side of the Moon appears to be significantly drier than scientists had anticipated, challenging previous assumptions about the lunar surface and raising fresh questions about the Moon’s history and suitability for future human exploration.
Understanding the presence and distribution of water on the Moon is essential — not only for upcoming crewed missions but also for shedding light on how the Moon formed and evolved. While we’ve long known the Moon isn’t completely arid, the notion that its water content might vary dramatically from one hemisphere to the other is a new and surprising development.
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Recent findings from China’s Chang’e 6 lunar lander suggest that the soil on the far side of the Moon — the hemisphere that permanently faces away from Earth — contains significantly less water than that of the near side. These results are based on fresh lunar samples collected from the South Pole-Aitken Basin, one of the largest known impact craters in the Solar System.
Professor Sara Russell, a planetary scientist at the Natural History Museum in London who studies the origin of water in the Solar System, said the findings were unexpected.
“For a long time, it was believed that the Moon was completely dry,” she noted. “Only relatively recently did measurements of lunar meteorites and Apollo samples show that water is trapped within the Moon’s minerals.”
“But I don’t think anyone expected the water not to be evenly distributed. We’ve been relying mostly on samples from the near side, and this discovery — if confirmed — could significantly alter our understanding of lunar water.”
The far side of the Moon is rugged and densely cratered, unlike the smoother near side visible from Earth. This unevenness is largely due to differences in the lunar crust — the far side has a much thicker crust, preventing ancient asteroid impacts from breaching the surface. On the near side, thinner crust allowed impacts to release molten rock, forming the dark ‘seas’ we can observe from Earth.
The disparity in water content adds another key difference. Lunar soil from the near side typically contains around 350 millilitres of water per cubic metre. In contrast, samples from the far side contain ten to one hundred times less.
The reasons for this disparity are still being investigated. One theory suggests that the near side stayed warmer for longer after the Moon formed, due to its proximity to the still-molten Earth, while the far side cooled more rapidly. This thermal difference may have influenced how and where water was retained. Alternatively, the region sampled by Chang’e 6 may simply be an unusually dry part of the far side. More missions will be needed to determine whether the entire far side shares this dryness.
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Given the Moon’s fiery birth — it was once a molten mass of rock — any original water would have been boiled off. Scientists therefore believe the water must have arrived later, possibly through meteorite impacts.
“The leading theory is that clay-rich meteorites, especially carbonaceous chondrites, delivered water to the Moon,” explained Professor Russell. “These meteorites contain significant amounts of water locked in their minerals, and their chemical signatures match what we see in lunar samples.”
A second possibility involves the solar wind — streams of hydrogen particles from the Sun — which may have reacted with oxygen in lunar minerals to create water molecules. Most of this water is now bound within rocks and soil, but some exists as ice in permanently shadowed craters, where sunlight never reaches.
This has important implications for future lunar missions. Water is a critical resource for sustaining life and for use in technologies such as fuel production. Astronauts could use local water for drinking, converting into oxygen, or producing hydrogen for propulsion.
“There are hopes to establish lunar bases in the coming years,” said Professor Russell. “Knowing where water is — and how much of it there is — will be crucial for that. The far side of the Moon, in particular, could be a valuable site for research, including radio astronomy, because it’s shielded from Earth’s signals.”
“But to make any of that happen, we need more probes, more samples, and more data. We’re still just beginning to understand our nearest celestial neighbour.”
This latest discovery underscores just how much more there is to learn about the Moon — and why its exploration continues to captivate scientists across the globe.
