The discovery of widespread but small amounts water on the surface of the moon, announced Wednesday, stands as one of the most surprising findings in planetary science.
Three spacecraft picked up the signature of water, not just in the frigid polar craters where it has long been suspected to exist, but all over the lunar surface, which was previously thought to be bone dry.
"Widespread water has been detected on the surface of the moon," said planetary geologist Carle Pieters of Brown University in Rhode Island, who led one of the studies detailing the findings.
While the findings, detailed in the Sept. 25 issue of the journal Science, don't mean there are pools of liquid water sitting on the moon, it does mean that there is — entirely unexpectedly — water potentially tied up or mixed in the minerals that make up the lunar dirt.
"What we're detecting is completely unexpected," Pieters said. "The moon continues to surprise us."
The moon dirt would be akin to soil from an arid environment like Arizona — it wouldn't feel wet to the touch, but there's certainly water bound up in it, Pieters told SPACE.com.
This discovery may well revolutionize our understanding of the nature of the moon's surface, experts say, and it has geologists eager to go back to the moon and dig up some lunar dirt.
"I rank this as a game changer for lunar science," said University of Colorado astrophysicist Jack Burns, chair of the science committee for the NASA Advisory Council. Burns was not involved in the new findings. "In my mind this is possibly the most significant discovery about the moon since the Apollo era."
Samples of lunar rocks brought back to Earth by Apollo astronauts had never shown any signs of water, leading scientists to presume that the moon was bone dry, except for possible pockets of water ice in permanently shadowed craters at the moon's south pole.
But the new observations, from India's Chandrayaan-1 satellite, NASA's Cassini spacecraft and NASA's Deep Impact probe call into question 40 years of assumptions on the make-up of the lunar surface.
"If it stands, then that really changes our understanding of the lunar surface," said Ray Arvidson, a planetary scientist at Washington University in Saint Louis who also was not involved in the new studies.
All three spacecraft detected the spectral signature of water (the wavelengths of light that it reflects) across the lunar surface. The signal, or fingerprint of water, was strongest at the lunar poles and varied in strength depending on the time of day, with the most robust signals coming early in the morning and the lowest at midday.
The signal actually indicates the presence of both water molecules and hydroxyl — an oxygen molecule and hydrogen molecule bonded together, or essentially water missing one of its hydrogens. Hydroxyl is more reactive than water. What minerals bear the hydroxyl isn't clear, though examples of hydroxyl-bearing minerals on Earth are the various clays.
How much of each type of molecule exist on the surface can't be determined from the data.
The exact form that the water takes on the lunar surface isn't clear with the data the scientists have, either, though they have several ideas: The water could be mixed in to the lunar surface or could be a part of altered minerals present in the lunar dirt.
"We do not know precisely," Pieters said.
It also remains a mystery how the water and hydroxyl formed, though some of the scientists who made the discovery suggest it could be from the interaction of the solar wind with the lunar surface.
The findings come just as NASA's Lunar Reconnaissance Orbiter and LCROSS impactor are set to explore the the lunar south pole craters looking for water ice.
"Much to our surprise, we found [water] elsewhere," Pieters said.
What the new observations are seeing is "water in a different mode on the surface altogether," she added, as the polar crater water ice is thought to be in solid clumps on the surface, while the newly detected water is attached to surface minerals.
The new findings, though, could help explain how the water ice got to the lunar poles. The fluctuating signals seen throughout the day could indicate that the water is migrating across the surface toward colder, higher latitudes and eventually the poles.
"If the water molecules are as mobile as we think they are — even a fraction of them — they provide a mechanism for getting water to those permanently shadowed craters," Pieters said.
But whether the signals actually show moving water or just water being created and destroyed daily can't be determined with the current data and would take a dedicated orbiter to tease apart, Pieters said.
"We need to go back obviously," she said.