Recent data from the Clemetine mission supports the possiblity of the presence of water ice at the south pole of the Moon. In permanently shadowed regions on the surface of the Moon, the temperature is a constant -233 degrees centigrade. These temperatures are cold enough to permanently trap water molecules that have been introduced by comets or water-bearing meteoroids. Areas on the Moon where permanently shadowed regions might exist are at the lunar poles. The Sun circles low around the polar horizon because the rotation axis of the Moon is nearly perpendicular to the ecliptic plane. Any deep depression near the poles would therefore remain in permanent shadow. Image mosaics constructed for the Moon's north and south poles, display the lighting conditions for a period of two lunar days (e.g., 69 Earth days). At the north pole there are no appreciable permanently shadowed areas. In contrast, the south pole contains over 15,000 square kilometers of permanent shadow area. It is in this region that water ice could exsist.
In the quest for detecting ice on the Moon, the radio transmitter onboard Clementine was used to bounce radio waves off the surface of the Moon to a station listening on the Earth. The energy of the reflected radio waves is a function of the compositional properties of the surface from which they are reflected. Rocky surfaces scatter radio waves randomly, while icy surfaces reflect radio waves coherently like a bicycle reflector. By distinguishing between weak radar returns and strong radar returns, it is possible to detect deposits of ice. Radar waves were bounced off of the Moon's north and south pole as well as areas around the poles. The radar ground tracks, shown in green, are illustrated on the south polar mosaic (the permanently dark regions are shown in red). The radar return from orbit 235, non-dark region, displays the characteristic radar return from a rocky silicate surface. Only orbit 234, which passed directly over the permanently dark regions at the south pole, has the radar reflectance properties of ice. Radar data collected from the north pole and other regions without permanent darkness do not have the ice signature.