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The moon is hiding water just under its surface, but it is slowly drying out.

We’ve known for a decade now that there was some water buried just below the dry lunar soil, but we haven’t been able to figure out much about it. NASA’s LADEE spacecraft, the Lunar Atmosphere and Dust Environment Explorer, spent six months in 2013 and 2014 orbiting just 20 to 60 kilometres above the lunar surface, and it has shed some light on the matter.

Mehdi Benna at NASA’s Goddard Space Flight Center in Maryland and his colleagues used LADEE data to track water being released from the moon’s surface. They found that the timing of 29 puffs of water matched known meteor showers, implying that as the meteorites fell to the ground, the resulting shock waves forced water beneath the surface to evaporate.

Four other water releases didn’t match with any known meteor showers, which may mean they represent falling rocks on the moon that we didn’t know about before.

“It’s like if you’re cleaning a big rug by hitting it with a stick,” says Benna. “Every time you hit it, you get a puff of dust over the whole rug because of the shock wave from you hitting one spot with the stick.”

By matching the data with theoretical models, the researchers found that the wet dirt was probably buried under a dry layer about eight centimetres thick. They also found that meteorite impacts cause the moon to lose up to eight grams of water every second. Because of that, the moon must have been full of water for a long time, with impacts slowly chipping away at its liquid.

If meteorites can pull water out of the lunar soil, so can we. “Understanding how much water we can use, whether for extended human exploration or for fuel production, is important,” says Benna. “The total amount of water is minute, but it’s easy to release and that’s to our advantage.”