When scientists announced last March that the Lunar Prospector spacecraft had detected evidence of as much as 300 million tons of water at the moon's poles, advocates of space travel were ecstatic, envisioning moon colonies with ready-made reservoirs. Now, with more data and more analysis, the news for would-be lunar explorers gets even better: the moon may harbor 20 times as much water as thought, more than 6 billion tons, much of it possibly in solid chunks of ice rather than the dusty frost earlier reports suggested.
To find the water, which astronomers believe came from comet impacts billions of years ago, Prospector has searched the lunar poles since January. The orbiter's instruments find hydrogen--evidence of water--by measuring the number of neutrons produced by cosmic rays hitting the moon. Neutrons lose predictable amounts of energy when they bounce off water's hydrogen atoms. The result: Fewer neutrons of certain energies make it into space. Prospector indeed saw fewer such neutrons near the lunar poles, a sign that water was present.
"The data indicate there are higher concentrations, and the higher concentrations you get, the more big chunks you're going to have," says space physicist David Lawrence of Los Alamos National Laboratory, who helped analyze the data. Most of the water, he says, sits 16 inches below the lunar surface.
Solid pieces of ice would be much easier for astronauts to extract than frost would be, says Lawrence. They're also better for research. "That ice is really a piece of a comet. So you're learning a lot about different parts of the solar system and how things are formed and how they evolved."
Lawrence, who suspects the ice is partly chunks and partly frost, speculates that the pieces of ice could be as small as a potato or as big as a large pool trapped in a shaded crater. "You're really going to have to put a rover down on the moon in those regions and look."