By Paul Rincon
Science Reporter, BBC News
Hopes that the Moon's south pole has significant water ice deposits that could be used to set up a lunar base appear to be unfounded, a study says.
Nasa hopes to return humans to the Moon by 2020
Hypothesised deposits of lunar water-ice have figured in Nasa's planning for future Moon landings.
This resource would be invaluable for supplying bases and making fuel for propelling spacecraft beyond the Moon.
The study in Nature journal suggests radar echoes thought to be from frozen water could be from rocky debris.
The first evidence for water-ice deposits came from radar observations made by the US Moon orbiter Clementine, launched in 1994.
According to mission scientists, values for a radar signature called the "circular polarisation ratio (CPR)" indicated frozen water below the dust in craters near the lunar south pole that were shaded from the Sun.
The Clementine researchers also admitted that this radar signature could be created by echoes from rough terrain and walls of impact craters.
In the latest study, Donald Campbell of Cornell University in Ithaca, New York, and colleagues suggest the latter explanation is the more likely.
They collected new radar images of the Moon's south pole using the Arecibo telescope in Puerto Rico. These images achieved a resolution of 20m (65ft), the highest resolution yet of this region. They paid special attention to Shackleton Crater, which has generated most interest.
"These radar images are by far the highest radar measurements made of the Moon. This allowed us to measure the distribution of this radar polarisation signature with much more accuracy," Professor Campbell told the BBC.
The authors also detected the CPR signature from areas of the surface that were exposed to bright sunlight, and where ice deposits could not persist.
"It's the same kind of signature that's found near Shackleton Crater; it cannot possibly be related to ice," co-author Jean-Luc Margot, from Cornell University, told the BBC.
Shackleton Crater has attracted much interest in relation to the US space agency's (Nasa) lunar exploration programme.
This is because of the suggested presence of water-ice on its Earth-facing inner slope, and because a few locations on its rim may be permanently sunlit for periods of up to 200 days each year, making them favourable for human habitation.
"The simplest explanation is that we're looking at a signature due to [impact debris] from the crater and not some strange signature due to water-ice," Dr Margot added.
Clementine gathered radar data on the Moon's south pole
But Dr Paul Spudis, who was a scientist on the Clementine mission, said: "We both see the same thing, what we disagree on is what it means.
"We knew CPR was not uniquely diagnostic of ice and we pointed out there could be other explanations in the paper we published in Science 10 years ago. But because we got this response in the dark areas near the pole and we did not get it from the lit areas, we interpreted it as ice.
Dr Spudis, from the Johns Hopkins Applied Physics Laboratory in Maryland, added: "When I look at their paper, I see them setting up a straw man, which they demolish. But it's a straw man of their own devising. The title of their paper is 'No evidence of thick deposits of ice at the lunar south pole'. No one ever claimed that there was.
"We claimed that we saw certain areas that had reflection characteristics that were set in the polar darkness that were consistent with water-ice."
The authors of the Nature paper suggest that if any ice does exist at the south pole, it is likely to be as small, widely scattered grains rather than thick deposits.
"The Nature paper does not preclude the existence of ice that would be mixed in with the lunar regolith ('soil') or that would be buried very deep," said Jean-Luc Margot.
But the team found no evidence of "pure" water-ice that could be mined or extracted easily.
As part of US President George W Bush's vision for space exploration, unveiled in 2004, the White House charged Nasa with using lunar exploration "to further science, and to develop and test new approaches, technologies, and systems, including use of lunar and other space resources, to support sustained human space exploration to Mars and other destinations."
Searching for near-surface water ice in polar cold traps on the Moon is one mission priority for Nasa's Lunar Reconnaissance Orbiter probe, due to launch in 2008.
"If there is ice, it will make exploration a lot easier. At one level, we never expected ice to be there, so we're not worse off. But we would be better off if there were deposits of ice there," said Dr Margot.
"If there were thick, buried deposits laid down over time and we could extract a core, that would provide a fantastic window on the history of the Solar System."