Researchers discover water reserves in moon rocks

Tiny glass globules in moon rocks contain water generated by solar wind. This is shown by studies of soil samples that the Chinese probe "Chang'e-5" brought back to Earth in December 2020. According to analyzes by a Chinese research team, the glass beads formed when meteorites and asteroids hit contain 20 to 200 times more water than ordinary moon rock. This would provide an easily degradable reservoir of water for exploration and colonization of the moon, the scientists write in the journal "Nature Geoscience".

"The past two decades have shown that there is a significant amount of water on the moon's surface," said Huicun He of the Chinese Academy of Sciences in Beijing and her colleagues. This does not only mean larger deposits of water ice in permanently shadowed regions at the poles of the moon. Because, as measurements by the "Lunar Prospector" and "Chandayaan-1" probes have shown, the lunar surface also contains small amounts of water everywhere, even away from the poles.

Most researchers suspect that this water is produced by the solar wind - a stream of energetic particles, primarily electrically positively charged atomic nuclei of hydrogen, emanating from the sun. When these hydrogen ions meet oxygen-containing minerals in the moon rock, water molecules can form.

So far, however, it has been unclear where this water is deposited - and thus also whether it can be used technically. Water would be a valuable resource for future moon landings; not only as drinking water, but also the oxygen it contains as breathing air and hydrogen and oxygen as rocket fuel.

Since the investigation of the soil samples brought to Earth by the Apollo astronauts, it has been known that moon rock contains several percent of micrometer-sized glass beads - formed from rock that melted when asteroids hit, ejected into space and fell back to the moon solidified in spherical form. He and her colleagues also found such glass beads in the moon rock brought to Earth by "Chang'e-5" - and examined them more precisely than ever before using the most modern methods.

To the surprise of the researchers, it turned out that water collects in the beads. While the basalt rock in the soil samples contains only a few micrograms of water per gram - i.e. millionths of a gram - the glass beads contain up to 1.5 milligrams (thousandths of a gram). In addition, the water content is greatest at the edge of the beads and decreases towards the center. For the researchers, this is a clear sign that the water is diffusing into the beads from the outside and is being deposited there.

With the help of further investigations, the team was able to measure the proportion of deuterium – i.e. heavy hydrogen – relative to normal hydrogen in the lunar water. The result: the water contains significantly less deuterium than water on Earth, in comets or on asteroids and corresponds more closely to the composition of the solar wind. "This shows us that the water in the glass beads was produced by the solar wind," the scientists conclude.

"Our results suggest that the lunar surface contains a much larger amount of water from the solar wind than previously thought," conclude He and her colleagues. This means that a water reservoir is available for future exploration of the moon, which is probably relatively easy to tap.

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