The first evidence of water on the moon was sent back by a joint Defense Department-NASA spacecraft called Clementine in 1994. Clementine used radar to detect the signature of water in the permanently-shadowed South polar region, which scientists thought was the most likely place to look for water. Although tantalizing, these data were not conclusive. Lunar Prospector, which was launched on January 6 as part of NASA's low budget Discovery program to study the composition of the moon, has apparently put doubts to rest in the first weeks of its planned two-year mission. In the following months, Prospector will continue to send back more detailed data, eventually dropping from its present orbit 62 miles above the moon's surface to a mere six miles for an even closer look.
Scientists think that impacting comets delivered any water that now exists on the moon.
Even the water in a comet striking the lighted regions of the moon would quickly evaporate in the daytime temperatures, which exceed 250 Fahrenheit. The only places in which water molecules could remain is in the shade of craters and perpetually dark areas of the moon, where temperatures stay at a chilling minus 280 Fahrenheit. The new findings, made by Prospector's neutron spectrometer , confirm that water is trapped in the frigid regions of the moon. Neutrons from the solar wind continually bombard the moon. But if a neutron collides with an atom of hydrogen, it looses speed.