A large portion of the moon's surface may be covered with water. That is the surprising finding of a trio of spacecraft that have turned up evidence of trace amounts of the substance in the lunar soil.
Many scientists suspect water ice might lurk in permanently shadowed craters at the moon's poles, which play host to some of the coldest known regions in the solar system.
But new findings suggest that a small amount of water clings to lunar soil across the moon's surface. The first detection was made by India's Chandrayaan-1 probe. The spacecraft, which failed in August after less than 10 months in orbit, was the first lunar orbiter to carry an instrument capable of measuring how much light is absorbed by water-bearing minerals.
"There's nothing else it could be," says Carle Pieters of Brown University in Providence, Rhode Island, leader of the Chandrayaan-1 instrument team that made the detection.
Chandrayaan-1 found hints of water across the lunar surface when it measured a dip in reflected sunlight at a wavelength absorbed only by water and hydroxyl, a molecule that contains one atom of hydrogen and one atom of oxygen.
But the team was not convinced they had found water. "We spent literally months digging up anything we could find that could possibly explain this feature, simply because we didn't think it was there on the surface," Pieters says.
To help verify the signature, team members turned to data collected by NASA's Cassini probe, which buzzed the moon in 1999 on its way to Saturn, and NASA's Deep Impact spacecraft, which flew past the moon in June 2009 en route to an encounter with the comet Hartley 2. Both spacecraft also showed evidence of water and hydroxyl, molecules that are likely both present on the moon.
But seemingly not in great quantities. Harvesting water from a baseball-field-sized swathe of soil might field "a nice glass of water", Pieters told New Scientist. Nonetheless, it might provide a resource for future lunar explorers.
Finding water on the surface changes the bone-dry picture of the moon that had been built up since the days of the Apollo missions. "If you had told anybody three weeks ago that there was even a minuscule amount of water on the moon, they would have laughed at you," says Jennifer Sunshine of the University of Maryland at College Park and the deputy principal investigator for Deep Impact's extended mission.
Chandrayaan-1's measurements suggest that the water sits in the upper few millimetres of the lunar surface. As a result, Pieters and colleagues favour a scenario in which the water is created when hydrogen atoms carried by the solar wind slam into oxygen-rich materials in the lunar surface, combining to form hydroxyl and water.
"It's a fascinating and interesting and useful result," says Paul Spudis of the Lunar and Planetary Institute in Houston, Texas. "Basically it's opened up a whole new field of study ... that has a whole lot more questions than answers."
There is also evidence to suggest the water might be on the move. Deep Impact's observations suggest water might be more prevalent during the colder parts of the month-long lunar day, near sunrise and sunset. That indicates the water might be actively created and destroyed, or else may be migrating as sunlight heats it enough to release it from the minerals it was originally stuck to.
If water on the surface is mobile, it could provide a different source of water for the permanently shadowed polar craters, whose main water source is thought to have been water-bearing comets that slammed into the moon.
"Even if it takes a couple of hops or a thousand hops or a million hops, ultimately [the water] could accumulate in a nice happy place like these permanently shadowed areas, and once it gets in there it's not going to come out," says Pieters.
But there is active debate on whether water lurks in the moon's dark craters. Radar signals reflected off polar craters have shown some ice-like signals. And neutrons detected by NASA's Lunar Prospector in 1998 suggested the presence of hydrogen, although it was not clear whether the atoms were locked up in water ice or in some other form.
NASA's Lunar Reconnaissance Orbiter, which launched in June, is now hunting for similar signatures.
NASA's LCROSS, which is set to collide with a crater on the moon's south pole on 9 October, could potentially help resolve the question. The spacecraft and the spent rocket stage it is currently shepherding will throw up plumes of debris that the spacecraft, LRO, and telescopes will scrutinise for signs of water ice.