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The Moon’s soil, especially in polar regions, could hold substantial amounts of water.

© ESA/NASA

The Moon may hold much more water than we think

Scientists have long suspected the Moon holds sizable reserves of water, secreted as ice in the deep cold of permanently shadowed craters near the poles. Two new studies tell us more about the possible extent of those reserves. One suggests the shadowy polar caches may cover an area equivalent to the states of Connecticut and Massachusetts combined; the other reveals traces of water elsewhere on the Moon’s surface, trapped in rocks or between the grains of lunar soil. That’s welcome news for NASA, which plans to return astronauts to the lunar surface in 2024 as a first step toward a permanent outpost and eventual journeys to Mars.

Water on the Moon would be good for more than just drinking. It can be chemically split into hydrogen and oxygen, yielding components for rocket fuel—and breathable air. Having ready supplies of water on the lunar surface would be a boon for colonists there, because it is so expensive to transport from Earth. A 2008–09 orbital expedition detected the signature of water in shadowy lunar hollows. But how much is there? 

To find out, Paul Hayne, a planetary scientist at the University of Colorado, Boulder, and his colleagues estimated the number and size of permanently shadowed polar regions, where temperatures remain below –163°C. Any water in those areas would have likely come from meteorites, comets, and other objects that once slammed into the Moon’s surface. Most water would have vaporized, but some would have drifted through the sparse atmosphere until it reached a shadowy nook, only to deposit as frost on the ever-frigid rocks and soil.

Analyzing high-resolution lunar images, the team calculated that the Moon’s polar regions host about 40,000 square kilometers of permanently shadowed areas that could contain water, from kilometers-wide craters to shallow depressions in the meteorite-gouged terrain, they report today in Nature Astronomy. About 60% of that area is in the Moon’s southern hemisphere.

Although the researchers did not estimate how much water might be present, anything in these regions should be easy to harvest, Hayne says. It might be as simple as having a lunar rover drag icy rocks and soil into a sunlit spot and collect water as it evaporates. Just last week, NASA announced a $47 million commercial contract to send an ice-seeking drill to the Moon in 2023.

A second study follows tantalizing evidence that there may be water elsewhere on the Moon. As early as 2009, scientists detected a spectral signature suggesting the presence of water in sunlit areas of the Moon. But because that signature—a particular wavelength of infrared radiation—can also be absorbed by substances other than water, it wasn’t indisputable evidence.

So Casey Honniball, a lunar scientist at NASA’s Goddard Space Flight Center, and colleagues sought a different spectral signature that could be generated only by water. They used NASA’s Stratospheric Observatory for Infrared Astronomy telescope, an infrared instrument mounted in a converted Boeing 747 that cruises at about 14,000 meters, to look for infrared light at a wavelength emitted by water molecules. Their flights found that each kilogram of lunar soil along two narrow swaths of the Moon’s surface contains between 100 milligrams and 400 milligrams of water, or about one raindrop’s worth, the team reports today in Nature Astronomy. Almost all of that water, they note, would be locked in shadowed areas between grains of lunar soil, or trapped in glassy materials created when micrometeorites smacked the lunar surface.

The team’s finding is “very exciting,” Hayne says. He adds that if the water is so trapped, it would be relatively easy to melt the glassy materials and, in essence, “mine” the water.

That mining would be a boon to future Moon missions, says Jacob Bleacher, chief exploration scientist for the Human Exploration and Operations Mission Directorate at NASA headquarters. Understanding where water is will not only help NASA decide where to send astronauts, but it could also lighten their payloads—and make more room for scientific equipment to be carried aloft.