Recent analysis of Moon dust collected by China's Chang'e-5 lander and brought to Earth revealed a mineral containing so much water that it makes up 41% of the mineral's weight.

This mineral is similar to novograblenovite, identified a few years ago in basaltic rock from Russia's Kamchatka Peninsula. Both the lunar and terrestrial versions share the chemical formula (NH4)MgCl3·6H2O and have similar crystalline structures.

Since novograblenovite can be studied on Earth, the discovery of an almost identical mineral on the Moon provides clues about where lunar water is hiding and how it got there, as well as insights into the history of lunar H2O.

The origin, presence, and distribution of water on the Moon remain mysterious. Scientists aim to understand this because the Moon's moisture is a crucial component of the Earth-Moon system's history. Additionally, knowing where water is located is essential for future lunar exploration missions since humans need water for survival.

Water has been found in older lunar samples, trapped in tiny glass beads produced when surface material melts and forms spherules. Water signals detected in the spectrum of light reflected from the Moon's surface suggest there's plenty more water up there, somewhere.

One prevailing theory is that water is bound up in minerals comprising the lunar regolith. However, previous studies suggested that the hydrogen and oxygen in Moon dirt could be in the form of other hydroxyl molecules – compounds made up of hydrogen and oxygen in proportions different from water.

When Chang'e-5 landed on the Moon in December 2020, it made a breakthrough – the first in situ detection of what appeared to be water in a lunar boulder. However, it was unclear whether this detection was molecular water or another hydroxyl molecule, requiring closer analysis than a robotic lander could provide.

Physicists Shifeng Jin and Munan Hao of the Chinese Academy of Sciences conducted this analysis, subjecting samples sent to Earth by the Chang'e-5 mission to X-ray crystal diffraction and chemical isotope analysis to determine whether the lunar regolith contains water or something else.

Their efforts revealed the presence of molecular water, with the mineral (NH4)MgCl3·6H2O containing up to six water molecules.

Novograblenovite rarely forms on Earth, emerging from the interaction of hot basalt with volcanic gases rich in water and ammonia. The lunar mineral isn't quite the same – the chlorine isotope within it has a different composition compared to terrestrial chlorine isotopes – but its formation mechanism is likely similar.

This suggests that both water and ammonia existed on the Moon during its volcanic past.

"Thermodynamic analysis shows that the lower limit of the water content in the lunar volcanic gas at that time was comparable to that of the driest volcano on Earth today, Lengai Volcano," the Chinese Academy of Sciences writes in a statement. "This reveals a complex history of lunar volcanic degassing, which is of great significance for exploring the evolution of the Moon."

The discovery also suggests a previously unknown source of water on the Moon – hydrated salts. These salts are much more stable than water ice, indicating they may be available even in areas frequently bathed in sunlight, reducing future reliance on water ice sequestered in shadowed craters at the lunar poles.

Sources:

Published 16 July 2024 in Nature; Evidence of a hydrated mineral enriched in water and ammonium molecules in the Chang’e-5 lunar sample

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