Scientists, authors, and dreamers have for decades propounded on the possibility of building human colonies on the moon.
Their visions may have taken a huge step toward reality with the discovery of frozen water on a body thought to have been more arid than a desert. A lake-like mass of ice the size of four football fields has been detected at the bottom of a 10 mile-deep crater on the moon's dark south pole by Clementine, a US satellite built to test technology for a space-based antiballistic missile defense system.
The discovery of the lake, first made in 1995 and revealed this week, holds profound implications for the future of lunar exploration. The ice it contains, thought to be "tens of feet deep," could potentially be used to sustain a human colony by providing it with a source of drinking water, irrigation for crops, and oxygen for breathing.
"The fact that there is a potential resource there increases clearly the moon's desirability as a way station in space," says Wes Huntress, who heads the National Aeronautics and Space Administration's office of space science. "If it can't be used as a way station, it could be used for humans to live and train for life for planetary bodies."
There is a further possibility that the hydrogen and oxygen from the water could be processed into rocket fuel for return trips to Earth. "This would eliminate the need to carry a round-trip supply [of fuel]," says Rick Lehner, a spokesman for the Pentagon's Ballistic Missile Defense Organization, popularly known as star wars. "It could end up that you might be able to harvest this water."
The discovery comes only months after NASA announced that scientists had found evidence life may have existed more than 3.6 billion years ago on Mars, where there are huge amounts of frozen water, a basic necessity for life. Strong evidence of liquid water on Jupiter's moon Europa has fueled speculation about the existence of primitive life below that satellite's icy surface. But there's no speculation now that the presence of water on the moon holds a similar potential. "Scientists ... don't seem to have looked at the possibility of life within the water, but mainly at its [future] application," says Mr. Lehner.
The discovery comes at a time of renewed interest in lunar exploration, not just by US scientists, but by Europe and Japan, which has what Boston University astrophysicist Michael Mendillo calls "a very aggressive program" to explore the moon and exploit its resources. Next September, NASA will launch Lunar Prospector, a $63 million mission to map 80 percent of the moon, as well as record its mineral distribution. It will also look for signs of water.
Clementine's discovery involved some serendipity. The $75-million spacecraft was using the moon as a target for a research radar, and scientists were intrigued by readings from the crater's depths. During lengthy analysis, the signals were compared with those from frozen water masses detected on other planets and found to be similar. Although there was a lot of debate at first, now "there seems to be a consensus ... that it is water ice," says Lehner.
The ice could be 3.6 billion years old. The moon's age is estimated at about 4 billion years. "The theory is that the ice came as a result of the tail of a comet or the collision of an asteroid or meteor with the surface of the moon," Lehner says. An early theory that water could exist on the moon was dispelled by the geological specimens brought back by the six Apollo missions.
Now, the prospect of using the moon's water as a basis for human colonies depends on where it is and how easily it can be recovered. Researchers still may find that importing water from Earth would be cheaper than taking chips of the moon's blocks. Yet the discovery is significant, Dr. Mendillo says, if for no other reason "than it reminds us once again that the early solar system was a very violent place."