The water was spotted by spacecraft that either circled the moon or flew by. All three ships used the same type of instrument that 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 two hydrogen atoms in water.
Because of the timing during the daylight when some of that wavelength disappears and some doesn't, it shows that both hydroxyl and water are present, Sunshine said.
This light wavelength was first discovered 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 type instrument and whizzed by the moon ten 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 zilch, so this confirms that it's water and hydroxyl, Pieters said.
"There's just no question that it's there," Pieters said. "It's unequivocal."
Scientists testing lunar samples returned to Earth by astronauts did find traces of water, but they had figured it was contamination from moisture in Earth air, Pieters said.
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, that also means that other places without atmosphere in our solar system, such as Mercury or asteroids, can also have bits of water, Sunshine said.
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