The discovery of widespread but small amounts water on the surface of the Moon, announced Sept. 23, stands as one of the most surprising findings in planetary science.

“I rank this as a game changer for lunar science,” said University of Colorado astrophysicist Jack Burns, chair of the NASA Advisory Council’s science subcommittee. 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.”

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, do not mean there are pools of liquid water sitting on the Moon, it does mean that there is — entirely unexpectedly — water potentially mixed in with the minerals that make up the lunar dirt.

“What we’re detecting is completely unexpected,” Pieters said. “The Moon continues to surprise us.”

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 the NASA-built Moon Mineralogy Mapper on 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 St. Louis who also was not involved in the new studies.

All three spacecraft detected the spectral signature of water across the lunar surface. The signal, or fingerprint of water, was strongest at the lunar poles. The signal also 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 detection from Chandrayaan-1 came first and took the team members by surprise; they first thought it was error in the data that would have to be calibrated for. But no matter what errors they accounted for, the signal still showed up. The data from Cassini and Deep Impact clinched the discovery.

“The rest is history now. It is completely conclusive,” said Pieters, the Moon Mineralogy Mapper’s principal investigator.