In 1996, researchers working with radar data from the orbiting lunar spacecraft Clementine reported indications of ice at the south pole of the moon. However, in 1997 researchers, Campbell among them, published a paper in Science reporting on Arecibo's radar imaging of the lunar poles that showed no evidence of ice. Both the Clementine and Arecibo radars would only have detected ice if it had been in the form of large chunks or slabs. Last year, the neutron spectrometer aboard the Lunar Prospector orbiter, launched in January 1998, detected significant deposits of hydrogen at the moon's north and south poles. This was interpreted as indicating the presence of water ice, since hydrogen in water molecules is thought to be the most likely source of the element at the poles.
Neutrons are subatomic particles that are continually ejected from the lunar soil by cosmic rays.
More information . With these first detailed altimetry data for the polar regions, the Cornell researchers have been able to map out the positions of most of these cold traps. Because the sun's limb rises less than two degrees above the horizon at the south pole, the floors of impact craters and other low areas can be in permanent shadow. In contrast, the radar beam from Goldstone reaches up 6 to 7 degrees above the horizon, allowing many polar features hidden from the sun to be imaged by radar. The floors of five large craters in the south polar region are hidden from the sun, the researchers say.