664. Is there water on the moon? — JB, Edmonton, Canada
Recent radar studies of the moon's surface have indicated that water may be present at the bottoms of deep craters near the moon's north and south poles. Because sunlight never reaches into these craters, they have cooled by radiating their heat into the empty space overhead and are now extremely cold. They're so cold that water deposited there, probably by comet impacts, has remained as ice for millions of years. While the ice in your freezer slowly disappears because the water molecules sublime—become water vapor—at normal freezer temperatures, extremely cold ice barely sublimes at all and can exist in a vacuum almost indefinitely.