The presence of frozen water beneath the surface of the Moon could make a major difference to the viability of future colonies. NASA even crashed a spacecraft into the lunar surface just to find out if there was water there. Consequently, the discovery of a mineral that only forms in the presence of water in a lunar asteroid could be a major boost to colonization plans. However, there is still a lot of guesswork about the conditions in which it formed.
The Apollo missions brought back rock samples from the Moon, but these are not the only lunar rocks we possess. When asteroids strike the Moon, they can throw up some of the surface, a small fraction of which eventually becomes meteorites that reach the Earth. These carry distinctive traces that distinguish them from meteorites of other sources.
In a study of 13 lunar meteorites found in northwest Africa, Dr Masahiro Kayama of Tohoku University, Japan, detected moganite – a form of silicon dioxide that only forms in the presence of alkaline water – in one.
The fact that out of 13 samples, only one (labeled NWA 2727) contains evidence of water indicates the lunar subsurface is probably mostly dry. That's no surprise, however. We don't need vast frozen oceans if we are to set up space colonies, just pockets of water ice sufficient to meet a small population's needs. Since the morganite makes up an average of 77 percent by weight of the unshocked silica micro-grains of NWA 2727, it appears the water presence was substantial where this meteorite was formed.
Several lines of evidence indicate it is very unlikely NWA 2727's moganite formed after the meteorite came to Earth. Consequently, Kayama and co-authors explain the moganite was likely produced near the Moon's surface from ice brought there by carbonaceous asteroids, which also carried minerals that make accompanying water alkaline. When the Moon's hot sunlight melted ice, the silica micro-grains (including moganite) precipitated out.
It's not easy to extrapolate from a single sample, but in Science Advances Kayama estimates more than 0.6 percent of the Moon's subsurface material is water ice, hopefully enough for us to mine it without having to drill to suspected water in the mantle.
The Lunar Crater Observation and Sensing Satellite (LCROSS) revealed evidence for water ice in permanently shadowed craters near the poles, where it has been less likely to be turned to gas by the Sun's heat and escape. However, that has not answered the question of whether ice exists at greater depths at other latitudes, and if so in what quantities. Nevertheless, the paper concludes that the estimated water concentration based on the moganite is consistent with what LCROSS and other satellite surveys have measured.
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