Blast some particles from the sun onto the seemingly dry moon and what do you get? Water, says a team of researchers who probed 43-year-old samples returned from the moon by Neil Armstrong and other Apollo astronauts.
The finding was not really a surprise, notes lead researcher Yang Liu, with the University of Tennessee in Knoxville, but it was a long time coming.
Analysis of the Apollo moon samples, which began in the 1970s, previously had uncovered the presence of hydrogen inside volcanically produced glass beads in the soil. In 2008, scientists found hydrogen in a phosphate mineral in lunar rocks, and last year found it again inside another mineral, olivine.
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Three robotic probes, including NASA's LCROSS experiment, also have found evidence for water ice on the moon. But where the water came from has been a mystery.
Using two new techniques to dig down into chemistry of hydrogen inside lunar soil grains, Liu and colleagues determined that most of it came from the solar wind, a steady stream of charged particles from the sun that permeates and defines the boundaries of the solar system.
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"The energy of these particles is enough to cause damage of the surface of a grain, but is low enough to be embedded and bound with oxygen on the surface," Liu wrote in an email to Discovery News.
Later, when asteroids or other bodies smashed into the lunar surface, some of the soil melted, forming glass beads laced with hydroxyl, a seemingly "dry" mineral, comprising one oxygen atom and one hydrogen, that actually can store water.
Earth's mantle, for example, holds at least one ocean's worth of water in hydroxyl minerals.
"So far, the only generally accepted way of delivering water to the inner solar system is collision with water-rich asteroids or comets that originate from the outer solar system," notes geochemist Marc Chaussidon, with Université de Lorraine in France.
"Liu and colleagues present evidence that the lunar surface contains water that originates from the solar wind -- as well as the water delivered by comets, as previously proposed."
It's too early to say how much of the moon's water can trace its origin to the solar wind.
"We have largely worked with one soil. There is about 80 ppmw (parts per million by weight) water generated by this process. For 3-meter (9-foot) deep soil covering one football field, this (translates) to about 3 metric tons of water if it could be fully extracted," Liu said.
"We really cannot apply this to the whole moon since we do not know the distribution of water on the moon," she added.
The research also suggests that other airless bodies, such as the asteroid Eros, Mars' moon Deimos and the protoplanet Vesta, may have a way to make water as well.
"Our concept of the presence of 'water' needs to be considered for new places in the inner solar system," Liu wrote.
The research appears in this week's Nature Geoscience.