The moon was dry as dust -- even more so -- they would have told you. For sure, the moon was probably pummeled with comets -- which are largely ice -- billions of years ago, when the solar system was young and filled with debris. But on an airless world, baked by the sun, any water would probably have vaporized and escaped into space.
"It was a question we thought we didn't have to address," said Carle Pieters, a space scientist at Brown University, in an interview with ABCNews.com. She is a member of the team whose work is making headlines around the world today.
Since the 1990s, scientists have been reporting indirect evidence of water, or at least its chemical components, from readings taken by passing spacecraft. The prevailing theory has been that there may be pockets of ice, mixed with soil in craters near the lunar south pole, where the sun never shines and the ice is always hundreds of degrees below zero.
But now Pieters and her colleagues have delivered a surprise. They say they found evidence of water -- in small concentrations, but enough to add up to billions of gallons -- virtually all over the lunar surface.
The data came from three different robot space probes:
One was India's Chandrayaan-1, which has been orbiting the moon since late last year.
An American ship, called Deep Impact, has mostly been used to examine comets, but pointed its instruments at the moon in June.
And NASA's Cassini spacecraft, now orbiting Saturn, took some long-distance measurements of the moon on the way out from Earth in 1999 -- though nobody thought to analyze the data at the time.
"We've been debating amongst ourselves for months, because it seemed outlandish," said Pieters. "It's impossible. It took a long time for us to convince all the members of the team."
The scientists worried that the instruments on the spacecraft were badly calibrated -- but three different ships, launched in different years from different countries, couldn't all be wrong in precisely the same way.
"Deep Impact nailed it," said Pieters.
What would the lunar water be like? To ask whether it is ice or vapor, or even liquid, is almost meaningless. The researchers paint a picture of individual molecules mixed in the lunar dirt.
Jim Green, NASA's planetary science chief, said today it may only amount to a quart per ton of soil. But since the moon is more than 2,000 miles in diameter, it would add up.
"Finding water on the Moon in daylight is a huge surprise, even if it is only a small amount of water and only in the form of molecules stuck to soil," said Jessica Sunshine of the University of Maryland, who worked on the Deep Impact data.
So what now? Engineers at NASA, mandated by the Bush administration to build a lunar base, talk about how much easier that would be if future astronauts don't have to bring water with them. Water -- H2O -- can be used for drinking and making food. It can also be broken down into its components, to supply oxygen for breathing and hydrogen for fuel.