Genuine moon water has been found for the first time in rocks that were brought back to Earth during NASA's historic Apollo missions 40 years ago. The water is similar to that detected in comets, suggesting that the moon's scarce supply got there through the impacts of these icy bodies.
To determine where this moon water came from, researchers studied thin slices of rocks collected by astronauts in the 1970s using several kinds of microscopes. Under examination was the ratio of hydrogen — two atoms of which bind with oxygen to make a water molecule —to a rare version of hydrogen called deuterium.
James Greenwood, a planetary scientist at Wesleyan University and lead author of a new paper describing the results, said he and his colleagues discovered that the levels of the deuterium isotope in moon water are double that of Earth's and "not from this planet."
"This is cometary water and not the same-old water we have on Earth," said study co-author Lawrence Taylor, a planetary geochemist at the University of Tennessee.
The Apollo moon rock water is the latest in a recent string of discoveries of water and ice on the lunar surface.
Last week, scientists announced that more than 600 million tons of moon water ice is lurking at the bottom of dark craters at the lunar north pole. Water vapor was also spotted during the intentional crash of two NASA probes into similarly shadowed craters at the moon's south pole in October.
Observations from several NASA and international probes have also found the chemical signature for water across other regions of the moon. But the first detection of lunar water, though in soil rather than inside a rock, came two years ago in volcanic glass beads.
Exposure of those samples to the solar wind, or particles from the sun, on the moon's surface renders their deuterium-hydrogen ratio difficult to determine, Greenwood said, making a stab at the origin of their water guesswork.
Traces of water had turned up in Apollo rocks previously, but Taylor and others had shown this water to be a result of contamination from terrestrial water.
Impacts from comets likely dumped unearthly water on the moon some four billion years ago when many of the moon's craters were gouged out. Earth also took a beating, and a small amount of our planet's water came from the skies as well.
An alternative scenario for the new hydrogen readings is that some regular hydrogen blew off the moon as it formed, depleting amounts of it compared to deuterium, Greenwood noted, but that it is too soon to tell.
Moon's wet history
The new finding of bona-fide lunar water in this first set of Apollo rocks will help update historical water estimates for the moon, according to the study.
And continued work could have big implications for lunar geology which may explain puzzling density differences of lunar minerals compared to those on earth. "Just a little water," Taylor said, can dramatically change how rocks form and age. "This is a precursory examination that has shown startling things," Taylor added.
This announcement of water in lunar rocks follows on the heels of its recent detection by orbiting probes including India's Chandrayaan-1 and by NASA's LCROSS mission last fall.
"I think we are embarking into a new era of looking at a wet moon," Greenwood told SPACE.com. "Everything will have to be rethought."
The research was presented at the 41st Lunar and Planetary Science Conference in Texas last week.