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Luna, our moon technically forms a
double planet system with the Earth. Although, as can be seen in Taylor (1975, 1982), oxides are plentiful on the Moon's surface, H2O is not one of them. Water is effectively absent from the Moon and any trace of it (the unique "rusty" rock sample comprised
of geothite FeO OH) appears to be terrestrial contamination. This led some to declare that the
Moon is waterless "millions times more than the Sahara Desert", although, on one occasion, the Apollo 15 mission detected an unexplained cloud of the water vapour near its surface. Chyba (1987) thinks that several sources of lunar water during the past 2 billion years (including recent cometary impacts) probably each supplies 10-100 km3 of water: its complete absence testifies to a high efficiency of ejecta, atmospheric blow-off, photo dissociation and destruction by solar wind (although some water may remain in polar cold traps). A first lunar private mission, planned for 1992, wants to find out from the Moon's polar orbit whether the deep polar craters hold water ice which has evaporated from the lunar surface. Any quantity of water on the Moon would be welcome for both human settlements and as a source of the rocket fuel. However, frozen, uncharted oceans begin only when we reach the icy moons of the gaseous giants.
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Evidence from the Clementine probe, which flew to the Moon in 1994, revealed an unusual radar reflection from regions near the lunar south pole which
might be explained by the effects of scattering from deposits of ice. The evidence from the Clementine was called into question when detailed observations made by the Arecibo radar system in 1992 using the same radio
frequency as Clementine found similar reflection patterns on parts of the lunar surface that were not permanently shadowed. A number of small areas (less than 1 km) were found containing high radar backscatter, suggestive
of the presence of ice, but these areas were shown to be in sunlight. Water ice would quickly evaporate in these areas due to the warmth of the sunlight, therefore the backskatter was attributed to roughness on the
surface.
Both the Clementine and Arecibo were looking only into zones of darkness visible from Earth. Neither study looked at the more promising areas of true permanent darkness, such as that in a ten-kilometre-wide
crater at the Moon's south pole. The Lunar Prospector launched on January 8, 1998 was designed to investigate the possibility of polar ice deposits through a low polar orbit search of the lunar surface. On March 5, 1998
it was announced that data returned by the Lunar Prospector spacecraft indicated that water ice is present at both the north and south lunar poles, in agreement with
Clementine results for the south pole reported a few years earlier. The lunar ice could comprise an area of 10 000 - 50 000 square km around the north pole and 5 000 - 20 000 square km around the south pole. The Lunar Prospector was equipped with a Neutron Spectrometer which was designed to detect minute amounts of water ice on the Moon by measuring the speed of neutrons that skitter into space when cosmic rays strike the Moon's surface. Since neutrons are slowed down by hydrogen, comparing the proportion of fast, hot neutrons to the slower, cold ones will give a measure of the amount of hydrogen on the Moon's surface. So far, the data from the Prospector's show a strong indication that water is present in both the north and the south polar areas due to the significant amount of hydrogen measured at both locations.
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