The moon is not the dry, dull place it seems. Traces of water lurk in the dirt unseen.

Three different space probes found the chemical signature of water all over the moon's surface, surprising the scientists who doubted the unexpected measurement until it was confirmed independently and repeatedly.

It is not enough moisture to foster homegrown life on the moon. But if processed in mass quantities, it might provide resources — drinking water and rocket fuel — for future moon-dwellers, scientists said. The water comes and goes during the lunar day.

It is not a lot of water. In a two-litre soda bottle of lunar dirt, there probably would be a medicine dropper full of water, said University of Maryland astronomer Jessica Sunshine, one of the scientists who discovered the water. Another way to think of it is that a drink of water would require 730 square metres of dirt to produce, said team leader Carle Pieters of Brown University.

"It's sort of just sticking on the surface," Sunshine said. "We always think of the moon as dead, and this is sort of a dynamic process that's going on."

The discovery, with three studies being published in the journal Science on Thursday and a NASA briefing, could refocus interest in the moon. The appeal of the moon waned after astronauts visited 40 years ago and called it "magnificent desolation."

The announcement comes two weeks before a NASA probe will be smashed near the moon's south pole to see whether it can kick up buried ice. Over the last decade, astronomers have found some signs of underground ice on the moon's poles. This latest discovery is quite different. It finds unexpected and pervasive water clinging to the surface of soil, not absorbed into it.

"It is drier than any desert we have here," Sunshine said.

Found by 3 probes

The water was spotted by spacecraft that either circled the moon or flew by. All three ships used the same type of instrument, which looked at the absorption of a specific wavelength of light that is the chemical signature of only two molecules: water and hydroxyl. Hydroxyl is one atom of hydrogen with one atom of oxygen, instead of the two hydrogen atoms in water.

Because of the timing during the daylight when some of that wavelength disappears and some does not, it shows that both hydroxyl and water are present, Sunshine said.

This light wavelength was discovered first by an instrument on the Indian lunar satellite Chandrayaan-1, which stopped operating last month. Scientists initially figured something was wrong with the instrument because everyone knew the moon did not have a drop of water on the surface, Pieters said.

"We argued literally for months amongst ourselves to find out where the problem was," Pieters said. Sunshine, who was on the team, had a similar instrument on NASA's Deep Impact probe, headed for a comet but swinging by the moon in June. So Deep Impact looked for the water-hydroxyl signature and found it.

Scientists also looked back at the records of NASA's Cassini probe, which is circling Saturn. It has the same kind of instrument and whizzed by the moon 10 years ago. Sure enough, it had found the same thing.

The chance that three different instruments malfunctioned in the same way on three different spaceships is almost zero, so this confirms that it is water and hydroxyl, Pieters said.

"There's just no question that it's there," Pieters said. "It's unequivocal."

Scientists testing lunar samples that astronauts brought back to Earth did find traces of water, but they had figured it was contamination from moisture in Earth air, Pieters said.

Unequivocal

Three scientists who were not part of the team of discoverers said the conclusion makes sense, with Arizona State University's Ron Greeley using the same word as Pieters: unequivocal.

Lunar and Planetary Institute senior scientist Paul Spudis called it exciting and said it raises the logical question: Where did that water come from?

Pieters figures there are three possibilities: It came from comets or asteroids that crashed into the moon, those crashes freed up trapped water from below the surface, or the solar wind carries hydrogen atoms that binds with oxygen in the dirt. That final possibility is the one that Sunshine and Pieters both prefer.

If it is the solar wind, this also means that other places without atmosphere in Earth's solar system, such as Mercury or asteroids, can also have bits of water, Sunshine said.