The idea that there was ice on the Moon tantalized astronomers for years, even before NASA’s Apollo mission to send astronauts to the lunar surface began. When the NASA spacecraft Clementine brought sent back data that hinted at the possibility that there was ice on the surface of the Moon in 1994, it stirred excitement once again. The trouble was, the measurements recorded weren’t definitive.

Since then, scientists have been plugging along for proof of lunar surface ice. Today (Aug. 20), a team of scientists led by researchers at the University of Hawaii found the first direct evidence of frozen water on the Moon’s poles (paywall). The discovery is based on data gathered by the Moon Mineralogy Mapper, a NASA instrument that flew to the Moon back in 2008. Reanalyzing these data today, the researchers found tiny patches of ice mixed with rock on the surface of certain craters at the northernmost and southernmost points on the Moon. Previous work had hinted that there could be various combinations of hydrogen and oxygen—the elemental components of water—on the Moon, but this is the first to identify actual frozen water molecules on the surface.

“This is very exciting news, and provides significant impetus for future international landings in the polar regions to drill and return samples of this ice,” says Jim Head, a geologist studying planetary formation at Brown University, who was not involved in the study. ”If ice is at the surface, this means that much more could be buried at depth and covered and preserved below insulating soil, or diffused into and frozen in the soil layers.”

The Moon Mineralogy Mapper gathered data on how various parts of the Moon’s surface reflected beams of near-infrared light. Because different molecules send back different reflections, scientists can accurately determine what sorts of chemicals are found on any given surface that has been assessed in this way. The team also used data from other exploratory missions to the Moon, like the Lunar Reconnaissance Orbiter (LRO), which was a separate NASA mission launched in 2009, to confirm their findings. The LRO identified compounds based on the ways they reflected ultraviolet light

Combining these separate measurements of specific patterns of reflection with different types of light allowed the team to identify ice’s chemical signature mixed in with the dusty, rocky surface of the Moon in its dark, polar craters.

Shuai Li, a geologist at the University of Hawaii who worked on the study, says the data can’t tell us where the ice originally came from. However, Li adds, it’s likely that it came from comets that smashed into the Moon years ago. Collisions with other space objects, like meteorites and comets, gave the Moon its pockmarked surface, and could have easily brought a foreign substance like ice along with them. Ice on the lunar surface could also be a result of gases coming out of the rock below. It could also be due to solar winds—energetically charged ions emanating from the sun—bombarding the Moon’s surface to cause the chemical reactions needed to make frozen water. However, to truly understand the ice’s origins, Li hopes to get a rover onto the Moon to take actual samples of the frigid lunar ground and its ice.

On Earth, the presence of water gave rise to life. On other planets, it hints at the prospect of finding new life, or the ability for us to survive somewhere other than our own planet. Evidence of water on the surface of the Moon, Head says, could be useful if we ever decided to try to build lunar bases, or explore space even further.