In 1994 this area was probed with radar by Clementine , a scientific spacecraft unusual in that she was built by America 's Department of Defence , rather than the civilian National Aeronautics and Space Administration ( though NASA scientists have been heavily involved ) . Clementine 's mission was to test a set of optical instruments developed for the " Star Wars " strategic defence initiative of the 1980s . Ice and rock reflect radio waves differently .

Water on the moon In a cavern , in a canyon . WHEN astronomers first turned their telescopes towards the moon they saw what they thought was a familiar pattern . The light , mountainous regions were obviously land . The dark , smooth ones were clearly water . And so they were named: Ocean of Storms ; Bay of Rainbows ; Sea of Tranquillity . Unfortunately , when Apollo 11 landed in the latter , it confirmed what had , by then , been known for a long time - - that these " seas " are actually just dusty deserts . Apart from a flutter of excitement at the discovery of some orange soil during Apollo 17 's visit ( it might have been water - containing rust , but turned out to be little beads of glass ) , there has never been the slightest indication of water on the moon .

For those romantics who would like to set up lunar colonies , this has always been something of a disappointment . Having a bit of water around would mean that much less in the way of raw materials need be lugged up from the earth . Such romantics have always hoped against hope that there might be some corner of the moon where water - - or , at least , ice - - did linger . Their best chance was that it might be found in some deep crater where the sun never shines and the temperature is therefore so low that ice would not evaporate . Now , a paper published in Science suggests that this fantasy may , just possibly , prove to be true .

The crater in question is the Aitkin basin , at the moon 's south pole . This has long been regarded as the most likely place to find ice because , at the poles , the sun is so low in the sky that the bottom of a deep hole might well be in permanent shadow . And the Aitkin basin is , indeed , deep - - about 13km ( 8 miles ) at its lowest point . Some 15,500 square kilometres of its floor is reckoned to be in perpetual darkness .

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In 1994 this area was probed with radar by Clementine , a scientific spacecraft unusual in that she was built by America 's Department of Defence , rather than the civilian National Aeronautics and Space Administration ( though NASA scientists have been heavily involved ) . Clementine 's mission was to test a set of optical instruments developed for the " Star Wars " strategic defence initiative of the 1980s . These instruments had , by the 1990s, found themselves more or less redundant for their original purpose , but the defence department was still anxious to try them out , and a mission to the moon seemed a publicity - friendly way of doing it .

Nevertheless , the ice was detected not by one of these flashy cameras , but by an experiment dreamed up as an afterthought . Since Clementine was not actually fitted with a proper radar ( or any other instrument that could have probed for ice in the unlit polar regions ) , a group led by Stewart Nozette , of the American Air Force 's Phillips Laboratory in Alexandria , Virginia , thought of using the craft 's radio transmitter as a makeshift .

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After assessing which areas seemed to be in permanent shade , Dr Nozette and his colleagues pointed the beam from the transmitter towards the moon 's surface at such an angle that the reflection could be picked up on earth by one of NASA 's Deep Space Network radio telescopes . Ice and rock reflect radio waves differently . Rock is as opaque to radio as it is to light . The reflection is therefore fairly clean . Ice , however , is partly transparent . Some of the radio waves are reflected straight back while others bounce around inside it a bit first . This smears the signal .

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Such a smeared signal was detected only from the part of the zone in permanent shadow . Nearby areas exposed to the sun 's rays , and similar areas at the moon 's north pole , looked , as expected , like bare rock . On the other hand , the smeared signal looks very like that from icy comets , and like that from similarly unilluminated areas of the planet Mercury , which are also thought to contain pockets of ice . Dr Nozette 's results suggest that about 1% of the unilluminated area in the Aitkin basin is covered by ice - - possibly to a depth of several metres .

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Where such ice ( if ice it be ) might have come from is unclear . It almost certainly did not leak out of the moon itself - - the rocks brought back by the Apollo missions suggest that the moon has always been bone dry . Most probably it was plastered on to the surface by collisions with comets , and has simply stayed there .

This opens up an interesting possibility . If the moon has stayed put in its orbit , the ice could have been building up at the bottom of the Aitkin basin since its formation , about 4 billion years ago . Each comet that landed nearby would have added a new layer to this particular ice - cream sundae . Examining the area with a drill , then , could yield a valuable record of how comets may have changed during the history of the solar system .

Despite the noises emanating from the publicists at NASA and the defence department , though , the discovery is not of much use in any plan to build manned bases on the moon . To be useful , such bases would best be sited at the moon 's equator , where the maximum amount of sunlight is available to power them . Dragging snow balls from the pole to feed them would not be much easier than making water from the local rocks , or even bringing it from earth . It is one of nature 's many little jokes that the only place on the moon where there could possibly be any water is the one place where it is of almost no use at all .

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