Up to 22 percent of the surfaces of Shackleton Crater, located near the moon’s south pole, may be water ice, new findings from NASA’s Lunar Reconnaissance Orbiter show.

That may sound like a lot, but it adds up to only about 100 gallons of water inside the 12-mile wide, two-mile deep crater.

WATCH VIDEO: NASA smashes the LCROSS and spent Centaur rocket into the moon in a search for water on the lunar surface.

“It’s not a lot,” lead research Maria Zuber, with the Massachusetts Institute of Technology, told Discovery News. “It’s certainly not an amount of water that could facilitiate human exploration or allow hydrogen mining or that kind of thing.”

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Lunar colonists shouldn’t give up hope yet. The findings, which were painstakingly crafted together from more than 5 million elevation measurements made by LRO, are only surface-deep.

“What you don’t actually know is what’s below the surface. You don’t know how much, if any, water ice is below,” Zuber said.

The new research is based on measurements made with LRO’s laser altimeter, which bounces laser light onto the ground to determine its reflectivity. Scientists then use that information to figure out surface elevation and composition.

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Because of the moon’s tilt, the sun doesn’t shine into Shackleton crater, leaving it cold and dark and presumably an ideal place for any of the moon’s remnant water to collect.

Previous studies of Shackleton using different wavelenghts of light found no surface brightening — i.e. no water. Compared to that, “22 percent seems like a lot,” Zuber added.

Also of note, LRO found bright spots not just on Shackleton’s floor, an indication of highly reflective water ice, but also even brighter areas on the crater’s walls, an unexpected and puzzling finding.

“We found that extremely interesting,” Zuber said. “If the only explanation of what was in the crater was water ice then one would expect the ice to be concentrated in the coldest, darkest part of the crater, which would be the floor. So it suggests to us that there was some space weathering going on, with material sliding down the walls.”

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One theory is that the moon may have periodic “moonquakes” from meteorite impacts or gravitational tugs by Earth which caused the crater walls to shake and shed their older material, revealing fresher, brighter soil.

Scientists also determined that the crater, which is believed to be more than 3 billion years old, is very well preserved, with its floor the same age as its rim. That suggests that very little has changed as far as what has been deposited at the bottom of the crater.

The research appears in this week’s Nature.

Images: Top: False color image of Shackleton crater, located near the moon’s south pole, was made with more than 5 million elevation measurements from NASA’s Lunar Reconnaissance Orbiter’s laser altimeter instrument (NASA/GSFC/SVS); Bottom: False-color elevation map of the crater. Blue areas are lowest; red and white are highest (NASA/Maria Zuber, et al Nature)