'Dry side' of the Moon reveals new details about lunar evolution

Recent findings indicate that the moon’s farside may contain far less water in its mantle than the side that perpetually faces Earth. 

Scientists studying rock and soil returned from this lesser-seen region have uncovered an intriguing possibility that water may be distributed unevenly throughout the lunar interior, shedding new light on how the moon formed and evolved over billions of years.

An intriguing hemispheric difference

Soil and rocks recovered from the moon’s mysterious farside suggest it may be drier than the side constantly facing Earth, according to a team of Chinese scientists.

The samples were gathered by China’s Chang’e-6 (CE6) mission and analyzed by researchers at the Chinese Academy of Sciences. 

Although missions over the decades have detected anywhere from 1 to 200 micrograms of water per gram of lunar material on the near side, the new measurements indicate the farside may be on the exceptionally dry end of the spectrum, pointing to an intriguing hemispheric difference.

A landmark lunar mission

China became the first nation to land a spacecraft on the farside last year. The CE6 probe touched down in the South Pole-Aitken basin, a massive impact crater on the lunar farside that is one of the largest in the solar system. 

By returning five grams of volcanic rock and soil, the mission provided scientists with their first opportunity to characterize water content beneath the less-explored hemisphere. 

Within those five grams, 578 tiny particles were selected for close analysis with electron microscopes.

This work builds on more than a decade of research into lunar water, which first began shifting scientific opinion away from viewing the moon as “bone dry” to recognizing that it contains small but meaningful amounts of water locked in its mantle.

Exceptionally dry lunar rocks

The CE6 sample analysis focused on mineral grains – especially apatite, a phosphate mineral known to trap hydrogen, and tiny pockets of ancient molten rock called melt inclusions.

These inclusions, often encased in minerals like olivine or ilmenite, can record the chemistry of the original magma that melted deep in the mantle and rose to the surface. Apatite and melt inclusions therefore serve as invaluable windows into the moon’s interior.

The researchers estimate that the water content of the parent magma for these far-side basalts could have ranged from about 15 to 168 micrograms per gram. 

After careful modeling of how water-bearing minerals crystallize and release their hydrogen over time, they concluded that the mantle source for these rocks might contain just 1 to 1.5 micrograms of water per gram – among the lowest estimates so far for any region of the moon.

Significance of the moon’s dry side

A better grasp of water distribution in the lunar interior can help explain how the moon formed, whether through a giant impact or another process, and how its mantle evolved as it cooled. 

The near side, especially the Procellarum KREEP Terrane, holds more thorium and appears richer in water.

By contrast, the farside’s low thorium and reduced water content suggest a deep-seated imbalance, possibly caused by the enormous collision that created the South Pole-Aitken basin. That event could have relocated water-bearing minerals, leaving the farside comparatively depleted.

Upcoming moon missions

Even if confirmed by further samples, these results do not upend existing mission plans. Astronauts will likely continue targeting the near side, particularly near the south pole, where ice may be abundant in permanently shadowed craters. 

NASA aims to send four astronauts around the moon next year as part of its Artemis program, followed by a landing no earlier than 2027. China has also announced plans for its own crewed lunar landing by 2030. 

Both space agencies have strong motivation to explore and possibly utilize ice deposits for life support and rocket fuel, making the relatively water-poor farside less of a priority for immediate human presence.

Mystery of the moon’s interior

The new estimate of dryness from the farside underlines how much remains unknown about the moon’s interior and its hemispherical differences. Further sampling will be crucial before scientists can assert that the entire farside is as arid as these initial fragments suggest. 

Future missions by NASA’s Artemis program and additional Chinese projects aim to collect still more diverse samples, potentially revealing whether the dryness is widespread or limited to one particular region.

Nevertheless, the potential for a drier farside does not diminish the scientific significance of exploring this untouched terrain.

Ongoing study of the moon’s dry side

The quest to understand lunar water continues to reshape models of the moon’s origin and development, guiding how researchers plan future missions, target landing sites, and search for the best places to establish a lasting foothold on Earth’s only natural satellite. 

However, the researchers behind the CE6 findings emphasize that more data – and more samples – are key.

Study co-author Sen Hu, an astrophysicist at the Chinese Academy of Sciences, concluded that more samples are required to further investigate how much water there is on the farside.

The study is published in the journal Nature.

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