The latest effort by astronomers to seek water on the moon has dampened hopes that it has much, if any, with which to establish a lunar colony. A moon base is a long-term goal of the U.S. space program as a departure point for human missions deeper into space.
The moon appears barren and dry, but an early suggestion that it might have water came from a U.S. satellite mapping it in 1996. The satellite sent radar waves to a lunar south pole crater that is in permanent darkness. The return signal was characteristic of the way the waves would bounce off water molecules in ice.
Two years later in 1998, another U.S. satellite using different technology detected a component of water - hydrogen atoms - being displaced from the moon's surface by gamma ray bombardment from space. Researchers speculated that the hydrogen could be coming from ice deposits one meter below crater floors at both lunar poles.
But a new study using a huge radar dish in Arecibo, Puerto Rico suggests that if there is any water on the moon, the amount is insignificant.
A team including Bruce Campbell of the Smithsonian Institution in Washington says the results imply that ice is likely to be present only as grains embedded in rock.
"People have suggested that if there were ice in thick slabs, then perhaps future lunar exploration would be made easier by going to those areas and using that as a resource," Campbell said. "The results that we obtained are not very helpful for that. You probably would not want to use this small amount of ice spread through the dust as a resource."
The radar signals from Earth penetrated below the lunar surface.
The resolution or sharpness of the images was higher than any previous radar study - down to 20 meters. The researchers looked for a particular signature of ice in and around the crater and found it.
But they also got the same response from sunlit areas where temperatures can reach nearly 120 degrees Celsius. As they report in the journal Nature, this suggests that the signals could actually be reflections off rocky terrain and crater walls, not off ice.
So if the moon has no water, what was the source of hydrogen measured by the 1998 U.S. satellite mission?
A scientist who saw that data says the hydrogen could have been implanted by the solar wind, a powerful and continuous blast of electronically charged atomic particles from the sun.
Director Alan Binder, of the Lunar Research Institute in Arizona, points out that future visitors to the moon could use this hydrogen to make water because there is also a lunar source for oxygen, the other component of water. Since both elements are rocket fuels, the moon could be used as a base to launch missions deeper into the solar system.
"Rocks, lunar materials, have about 40 to 43 percent oxygen in them, so we have always planned on breaking the rocks down to get the metals and to get out the oxygen," saidBinder.
Therefore, a lack of ice on the moon would not deter future lunar operations. But Mr. Binder notes that it is important to know what form the hydrogen takes before embarking on such ventures.
"If we do not know what the form is and we go there thinking we are going to harvest ice with all kinds of ice harvesting equipment and it turns out to be solar wind implanted hydrogen, we have got the wrong equipment," he said.
Scientists say the only way to know for sure if ice lays beneath the moon's surface is to send a robotic explorer to dig down.
The U.S. space agency NASA plans to do this in 2008. It will dispatch a satellite to crash land on the moon to look for ice below the surface. In the same launch, it will send a second satellite that will orbit the moon to sense ice and other useful resources in addition to creating high-resolution surface maps and seeking landing sites for future missions.