Scientists have been looking for water on the Moon for several years, but the latest effort using radar signals from Earth has dampened hopes that much, if any, is there. Yet the Moon does have the separate elements for water.
The Moon appears barren and dry, but the first suggestions it might have water came from a U.S. satellite mapping it in 1996. The satellite bounced radar waves off a permanently shaded crater at the Moon's frigid south pole. 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 rays 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 Puerto Rico suggests that if there is any water on the Moon, there is not much. A team led by Bruce Campbell of the Smithsonian Institution in Washington says the results imply that ice is likely to be present only as grains or thin layers 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," he said. "The results that we obtained are not very helpful for that. You probably wouldn't want to use this small amount of ice spread through the dust as a resource."
The radar signals from Earth penetrated more than five meters below the lunar surface, deeper than previous radar inspections. Mr. Campbell's team reported in the journal Nature that if significant ice deposits were present, the radar reflection would have been stronger, appearing nearly white on a viewing screen. But he says the reflection was weak, unlike the situation, for example, on the planet Mercury.
"We see very, very bright reflections from the floors of craters around the poles of Mercury because there is ice that has been trapped there after comets have hit the surface," said Bruce Campbell. "We were looking for the same kind of bright signatures in polar craters on the Moon, but we don't see them."
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 data from that mission says the hydrogen could have been implanted by the solar wind, a powerful and continuous blast of electronically charged atomic particles from the sun. Alan Binder, director 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. And 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," said Alan Binder.
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 don't know what the form is and we go there thinking we're going to harvest ice with all kinds of ice harvesting equipment and it turns out to be solar wind implanted hydrogen, we've 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. Although the U.S. space agency, NASA, has plans to do this on Mars, there is no similar plan for the Moon.
Mr. Binder laments this because he says the Moon can be a source of minerals, for example, or a site where solar energy can be sent by microwave to Earth.
"There are many economic reasons for going back to the Moon, but NASA is just not interested, period," noted Mr. Binder.
Fortunately, the radar methods developed to assess ice on the Moon are not wasted. The Smithsonian's Bruce Campbell says Mars was on his group's mind all along.
One reason we did the lunar experiment was to help us work out the kinds of techniques we might want to do from orbit around Mars," he said.