There is almost certainly ice water on the surface of the moon, hiding in the cold, dark places near the north and south poles, a new study shows.
Scientists had already thought there was water up there, but now we have some of the most definitive proof to date. It appears that this ice — very muddy ice, mixed with a lot of lunar dust — exists inside craters where direct sunlight does not reach it.
But we still do not know how deep it goes, or how exactly it got there.
The authors of the study, published on Monday in Proceedings of the National Academy of Sciences, say the findings are exciting because they call for further exploration of our rocky satellite. The ice could even be a resource for human visitors — perhaps to be used for drinking water, or even to make rocket fuel.
Shuai Li, the lead author and a planetary scientist at the University of Hawaii at Manoa, said that despite decades of lunar research, scientists have had trouble exploring the polar regions, in part because the craters are so dark.
“So there aren’t too many measurements,” he said. “But a lot of things are going on there.”
Researchers estimate that the exposed ice covers only 3.5 percent of the craters’ shadowy areas. They don’t know whether the water runs deep, like the tips of buried icebergs, or is as thin as a layer of frost.
The data used by Dr. Li and his team was not new. It had been collected by NASA’s Moon Mineralogy Mapper, which hitched a ride on Chandrayaan 1, India’s first lunar probe, in 2008 and 2009.
The instrument was able to map most of the moon’s surface, but data from the permanent shadows — inside some of the craters near the poles — was a little bit patchy, and hard for researchers to work with.
So Dr. Li and his team were creative, and patient. They peered into dark craters using traces of sunlight that had bounced off crater walls. They analyzed the spectral data to find places where three specific wavelengths of near-infrared light were absorbed, indicating ice water. They performed rigorous statistical analysis to make sure their results were uncorrupted by coincidental anomalies or instrument errors.
Ralph E. Milliken, a study author and an associate professor in the department of earth, environmental and planetary sciences at Brown University, said he “had a healthy dose of skepticism” when Dr. Li approached him with the idea of sifting through old data to look for clues in infrared. But he soon came around.
“I consider this to be the most convincing evidence that you actually do have true water ice at the uppermost surface — what we call the optical surface — of the moon,” he said of the study’s results.
Scientists have researched extraterrestrial water before — on Mercury, for example, or the large asteroid Ceres. But the moon has been difficult. Radar can be unreliable when the ice water is muddied by sediment, and some spectroscopic analyses couldn’t necessarily distinguish between water and plain old hydrogen.
Rachel L. P. Klima, a senior staff scientist at the Johns Hopkins University Applied Physics Laboratory who was not involved with the study, said Dr. Li’s results were impressive.
“We’ve had all of these kind of circumstantial things that hinted at ice on the moon — different data sets — but there really was not a robust observation that could only be attributed to ice,” she said. “This is, in my opinion, the first evidence that there’s really no other way to explain.”
Dr. Li hopes to see more lunar exploration in the near future. In fact, this new evidence of ice could make such exploration more likely. After all, scientists still have questions about how deep this water goes, whether it could be useful to human visitors, and where it came from — was it delivered by comets and asteroids? And if so, when?
The use of near-infrared light could help scientists find new answers to those questions.
“We’re really pushing the boundaries,” Dr. Milliken said of the study methodology. “I could imagine, now that we know this does work, you could easily go and design an instrument that is tailored to these more difficult conditions and that would open even more doors.”