Signs of water ice have been found in craters near the Moon's poles. The ice appears limited to smaller areas of the Moon than many expected, and its purity isn't great. Nevertheless, there is enough to make a major difference to the viability of a lunar base.

The Apollo astronauts brought back barely a trace of water in the rocks they collected. If we wanted to establish a colony on the Moon, most people assumed we were going to have to take water there, or at least make it ourselves through chemical reactions. More recently, however, this has come into question.

Without a protective atmosphere, water ice on the Moon's surface would turn to gas when sunlight hit it, and quickly be lost to the Moon's weak gravity. Consequently, Dr Shuai Li of the Hawaii Institute of Geophysics and Planetology looked at the bottom of craters near the lunar north and south poles – the only places direct sunlight never reaches.

Unfortunately, these spaces, called cold traps, are also the hardest parts of the Moon to study – not least because they are in permanent shadow. In Proceedings of the National Academy of Sciences, Li describes using light scattered off the walls of craters or nearby mountains and captured by NASA’s Moon Mineralogy Mapper to investigate these areas.

The paper reports thousands of pixels, equivalent to 280-by-280-meter (760-by-760-foot) spaces, in which the light carried a clear spectroscopic signal of water ice, indicating its presence in the first few millimeters of the lunar surface. All lie within 20 degrees of a pole. Only about 3.5 percent of cold traps produced a water ice signal, but some areas may be up to 30 percent water, which should support well-placed bases. 

Ice has also been found on Ceres and, astonishingly, near Mercury's poles, despite the hellish heat of the majority of the innermost region of the planet. Consequently, Li's discovery is not a total surprise, and indeed the lunar ice is less extensive than the ice on Ceres or Mercury. There have been hints before, such as in an ejecta plume. However, as the paper notes; “Direct evidence for water ice exposed at the lunar surface has remained elusive.”

Volcanic glass beads on the lunar surface have been interpreted as indicating the presence of large amounts of water deep within the Moon. Encouraging as that finding was, future Moon missions would probably prefer to find water lying around the surface, even if in limited locations, than to have to drill deep or extract molecules trapped in other minerals.

The authors speculate the reason most cold traps don’t have ice may be a result of “impact gardening” by asteroids, and the changing orientation of the poles. Whether the ice came from within the Moon, or was delivered by comets, remains unclear.