The surface, even the parts that receive sunlight, is sprinkled with traces of water. This water arises when charged hydrogen atoms from the sun strike the lunar regolith, split oxygen bonds in the soil, and join with them to produce water. The final product is only a few molecules deep, though—no use for thirsty astronauts.
The stuff that future astronauts really want lies in the shadows, inside craters where the sun never shines. Like Earth, the moon was probably bombarded with water-bearing asteroids and comets in its early days. Without an atmosphere to disintegrate them, the objects smashed into bits on the surface. Particles of water ice, newly exposed, scattered. The ice that drifted into darkened craters, or even small shadows cast by boulders, survived. “Once a molecule got into one of these areas, it could never get out,” says Carle Pieters, a planetary scientist at Brown University who oversaw a mineralogy instrument on an Indian robotic mission to the moon in 2008.
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NASA and commercial space companies have set their sights on these mysterious, enduring reservoirs at the south pole, and they have based their estimates of the supply on the past decade of exploration. Bridenstine’s estimate, according to his office, likely comes from NASA’s chief scientist, Jim Green. Several scientists tell me the estimate is not outlandish, but they stress that it is merely a range. “If you take 10 scientists in a room, you get multiple answers,” says Thomas Zurbuchen, the NASA associate administrator who leads the agency’s science programs.
So what if astronauts were on the moon right now, and they could rappel into one of these craters and see the water ice for themselves? They would probably find something like “a dirty snowbank,” says Lindy Elkins-Tanton, a planetary scientist at Arizona State University. “It’s going to be a mess,” she says. “It’s going to be water and sulfur oxides and ammonia, and it’s going to be shards of rock and glass from the moon, and then it’s going to be a lot of organic materials.”
Astronauts would have to figure out how to extract lunar dust, metals, and other materials that could be harmful if consumed. “There’s been a little bit of an assumption that we can use the water-purification systems that we use here on Earth,” Elkins-Tanton says. “The water we’re going to find on the moon is not like any water we’ve ever had to process on Earth.”
To pin down the nature of water ice on the moon, scientists need fresh data from new missions. NASA wants to send landers and rovers to probe the silvery terrain for answers.
But even the most advanced rovers could miss signs of water, says Rick Elphic, a planetary scientist at Ames who is working on science instruments for the moon program. Spacecraft observations have shown that while total darkness provides the right conditions for water ice, it doesn’t guarantee the presence of water ice. And direct hits from meteors can scramble the shadowed regions that do have ice.