PROVIDENCE, R.I. [Brown University] — Researchers have made the first definitive detection of water ice exposed at the uppermost surface of the Moon. The deposits, found near the lunar poles, are hidden away on crater floors and other areas that are permanently shadowed from direct sunlight.

Depending upon how much ice is there, the researchers say, the deposits could be a resource for future lunar exploration.

The research, published in Proceedings of the National Academy of Sciences, was led by Shuai Li, a postdoctoral researcher at the University of Hawai‘i at Mānoa who earned his Ph.D. and did prior postdoctoral work at Brown University. The detection was made using data primarily from the Moon Mineralogy Mapper (M3), an imaging spectrometer launched in 2008 aboard India’s Chandryaan-1 spacecraft.

“This was a really surprising finding,” Li said. “While I was interested to see what I could find in the M3 data from [permanently shadowed regions], I did not have any hope to see ice features when I started this project. I was astounded when I looked closer and found such meaningful spectral features in the measurements.”

The M3 data were bolstered by data from Lunar Orbiter Laser Altimeter (LOLA), the Lyman-Alpha Mapping Project (LAMP), and the Diviner instrument onboard America’s Lunar Reconnaissance Orbiter (LRO) mission.

“It had previously been hypothesized that water ice may be stable in permanently shadowed regions of the lunar surface,” said Ralph Milliken, an associate professor in Brown’s Department of Earth, Environmental and Planetary Sciences and a study co-author. “But evidence from different instruments didn’t always agree and it wasn’t clear if there was truly water ice exposed at the surface or if it was buried beneath dry lunar soil. In this study, we were able to look at reflected sunlight at near-infrared wavelengths to demonstrate, for the first time, that there is indeed water ice exposed at the uppermost surface.”

Making a definitive detection of anything in permanently shadowed regions (PSRs) is no easy task, Milliken said. Spectrometers like M3 measure the light that bounces off a planetary surface. By looking at which wavelengths are absorbed or reflected, scientists can get an idea of what minerals and other compounds are present. But PSRs don’t receive any direct light and therefore don’t reflect any direct sunlight, which makes these regions a challenge to study remotely.

For this study, Li and co-authors looked at the edges of the PSRs and examined the light that spills into them from the surrounding terrain. Li developed a method for making sense of the weak signal from “terrain-scattered” light, and teasing out signatures consistent with ice.  

The study showed that ice deposits had patchy but widespread distributions on both the Moon’s north and south poles. Milliken said it’s hard to tell at this point just how much total ice there is or whether there’s enough to be used in future lunar exploration.

“What we’re able to see in this study is right at the optical surface — the upper millimeter or less,” Milliken said. “But if the ice is stable at the surface in these areas, then it is likely that it is also present in greater abundances at depth. We still need more information about these areas to determine if there is enough water to make it a viable resource for human exploration, whether it is for drinking water or production of rocket fuel, et cetera. I would consider the maps produced by this study as an important guide to know how to plan future missions, whether orbital or landed, to explore these deposits in more detail.”

The findings are part of a growing body of research suggesting that the Moon isn’t nearly as dry as researchers had long assumed. The first hint that water is present on the surface of the Moon came in 2009, when a research team led by Brown planetary scientist Carle Pieters showed trace amounts of water (or a related molecule, hydroxyl) embedded in the lunar soil. Milliken, Li and other colleagues have since expanded on that work, creating a global map of water abundance in lunar soil. Milliken and colleagues also reported significant concentrations of water trapped in lunar volcanic material, suggesting that the Moon’s interior contains water as well.

It had long been assumed that, over time, water might migrate to permanently shadowed regions, where it would be stable in high concentrations as surface ice. This study shows, at long last, that is indeed the case.

In addition to showing that water ice can exist on the lunar surface, Milliken says the research could expand the possibilities of future lunar research.

“I think a major outcome of this work is that it demonstrates that using terrain-scattered light is a viable method for detecting ice in the edges of shadowed regions,” he said. “Now that we know this, a future instrument could be sent to the Moon that is really tailored to these low-signal conditions, and such an instrument could provide much more information about the spatial distribution and perhaps abundance of water ice in these regions.”