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Scientists have made a surprise discovery about our moon. It is generally thought to be bone dry but now, three new studies show that actually, the lunar dirt holds stores of hidden water.

NPR's Nell Greenfieldboyce reports.

NELL GREENFIELDBOYCE: Nobody thought the surface of the moon was covered with water. Even the scientist who saw the first hints of water didn't believe it.

Professor CARLE PIETERS (Geological Sciences, Brown University): Like any normal person, you'd say, well, that's ridiculous. You know, it can't be there. It must be calibration problems.

GREENFIELDBOYCE: Carle Pieters says her team thought their instrument must be out of whack. They spent months looking at their data.

Prof. PIETERS: Arguing amongst ourselves what could be real, what could not be real - and it would not go away.

GREENFIELDBOYCE: Pieters works at Brown University. She heads up research with a NASA instrument called the moon mineralogy mapper, which went up on a space craft built by India to orbit the moon. To make sure this mapper really had picked up water, the scientists got data from two other spacecrafts that flew by the moon. It turns out those probes detected signs of water, too. But Pieters says it's not the kind of water we normally think about here on Earth.

Prof. PIETERS: There's no oceans. There's no clouds. There's no lakes. There's no puddles. This is completely different.

Ms. JESSICA SUNSHINE (Senior Research Scientist, University of Maryland): You know, it's not your grandmother's water on the moon.

(Soundbite of laughter)

Ms. SUNSHINE: It isn't. You sort of have to throw out everything you think of by that phrase.

GREENFIELDBOYCE: Jessica Sunshine is a researcher at the University of Maryland in College Park.

Ms. SUNSHINE: It's not liquid water.

(Soundbite of laughter)

Ms. SUNSHINE: It's not frozen water and it's not gaseous water. OK? It's none of those things. What it is is a very thin film, maybe several layers of molecules thick.

GREENFIELDBOYCE: She says if you could scrape all the water molecules off an area about the size of a football field, you'd get maybe a quart.

Ms. SUNSHINE: And it could be a lot less. I think our understanding is not great.

GREENFIELDBOYCE: Another thing that's not yet understood is the source of the water. It seems to come and go over the course of the lunar day, as temperatures change. The new findings are being published this week by the journal Science. Carle Pieters says it was amazing to discover a previously unknown, dynamic process on our nearest celestial neighbor.

Prof. PIETERS: Nature surprises us. And in this case, the moon completely surprised us. This was something that we were not expecting.

GREENFIELDBOYCE: One reason space enthusiasts are so interested in water on the moon is because of hopes that it could come in handy if astronauts ever go back. NASA has been working on new rockets and space capsules for a return by 2020. There's even talk of a lunar base. Those plans are all currently under review by the Obama administration. And at a press conference this afternoon, experts pointed out that the moon's surface still looks pretty dry despite these new findings. Jim Green is director of NASA's planetary science division.

Dr. JIM GREEN (Director, Planetary Science Division, NASA): Even the driest deserts in the Earth have more water than are at the poles and the surfaces as we've presented here on the moon.

GREENFIELDBOYCE: Still, NASA hopes to find more water next month. Scientists had long assumed if the moon had any water at all, it would be frozen at the bottom of dark, cold craters. So in October, NASA will put a spent rocket part on a collision course with that kind of crater to see if the impact kicks up evidence of buried ice.

Nell Greenfieldboyce, NPR News.

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