The discovery, detailed in the journal Nature Astronomy, was made possible by NASA's research aircraft SOFIA, short for Stratospheric Observatory for Infrared Astronomy.
Researchers previously found concentrations of hydrogen on the moon's sunlit surface, but were unable to determine their origin.
"Today, we're announcing the previously detected hydrogen found on the surface of the moon is located in water molecules," Paul Hertz, director of the Astrophysics Division in the Science Mission Directorate at NASA Headquarters, told reporters Monday during a teleconference.
SOFIA measured water concentrations of 100 to 412 parts per million. For comparison, the Sahara Desert hosts water concentrations 100 times greater.
But the discovery raises new questions about the abundance and distribution of water on the moon, which scientists previously believed to be exclusively locked up in polar ice caps and at the bottom of only the moon's deepest craters.
Because the moon has such a thin atmosphere, any unprotected water on the sunlit surface of the moon should be quickly lost to space, and yet, scientists have not found irrefutable evidence that the water molecules are there.
How it gets there and what keeps it there remain open questions, researchers said.
NASA's announcement was made in conjunction with the publication of a second study in Nature Astronomy, showing tiny permanently shadowed cold traps may house small patches of water ice all over the moon's surface.
NASA scientists are keen to understand the moon's hydrological dynamics as they prepare for human missions to the moon. Astronauts need water to drink, of course, but water can also be used to synthesize oxygen, make fuel, water plants and more.
If there is a sustainable source of water on the moon, that makes packing for long stays on the moon a lot easier.
"It's far easier to travel when don't have to carry everything with you that you might need once you're there," Jacob Bleacher, chief exploration scientist for NASA's Human Exploration and Operations Mission Directorate, said during the teleconference. "You can be much more efficient with what you pack."
"Water is heavy, therefore is expensive to launch from the surface if we don't have take water with us," Bleacher said. "We have an opportunity to take other things with us, for instance, payloads to do more science."
NASA plans to continue using SOFIA's instruments to look for water on sunlight portions of the lunar surface, but to solve the mysteries of the moon's water supply, it's likely more direct lunar exploration will be necessary, the scientists said.
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