Britannica Sources used in this answer

Why is water ice found in some lunar polar craters?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

near and far sides of Earth's Moon — Moon (Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.

Water ice is found in some lunar polar craters because these regions remain permanently in shadow, allowing the ice to survive just beneath the surface without sublimating.123

Scientists long hypothesized that such conditions could preserve water ice in these shadowed areas. This was confirmed in 2009 by the LCROSS mission, which deliberately crashed a rocket stage into the floor of Cabeus, a crater near the Moon’s south pole. Subsequent analysis of the resulting impact plume revealed that the lunar soil at that location contained 5.6 percent water ice.132

While water molecules have been detected across much of the lunar surface—potentially formed by solar wind hydrogen ions interacting with oxygen in the soil—the permanently shadowed polar craters provide the unique, cold environment necessary for ice to persist in significant quantities.13