Scientists reached this conclusion after digging through extensive data from Earth- and space-based observatories.

The daring experiment, which destroyed the spacecraft, took place when the probe had reached the end of its useful life.

The impact site

Proving the existence of water on the Moon would make dreams of a Moon base less fanciful, as an in-situ source of water would be available.

Scientists suspect that ancient cometary impacts on the Moon delivered ice that is now hidden in permanently shadowed regions at the Moon's poles. Large amounts of hydrogen measured indirectly from lunar orbit by Lunar Prospector during its mapping mission support this idea.

Research group leaders from the University of Texas at Austin announced their results on Wednesday at an astronomers' meeting in Padua, Italy.

Near miss?

"There are several possible explanations why we did not detect any water signature, and none of them can really be discounted at this time," said Dr Ed Barker, assistant director of the university's McDonald Observatory.

These explanations include:

• the spacecraft might have missed the target area;
• the spacecraft might have hit a rock or dry soil at the target site;
• water molecules may have been firmly bound in rocks as hydrated minerals as opposed to existing as free ice crystals, and the crash lacked enough energy to separate water from the minerals;
• no water exists in the crater and the hydrogen detected by the Lunar Prospector spacecraft earlier is simply pure hydrogen;
• the telescopes used to observe the crash, which have a very small field of view, may not have been pointed correctly;
• water and other materials may not have risen above the crater wall

The failure to find water is disappointing, but the chances of detection had been assessed to be less than 10%. It does not rule out that water is present.

The $63 million Lunar Prospector was launched in January 1998. In March 1998, mission scientists announced the first suggestions of the presence of water ice. They later estimated that up to six billion metric tonnes of water ice may be buried in these craters, under about 18 inches of soil.