The Moon had long been thought to be dry and barren. Then, the discovery of ice on the planet Mercury raised the possibility that ice may exist in permanently shadowed regions of the Moon's poles. The Clementine spacecraft used radar observations of the lunar south pole in 1994 to detect water, but those data were not conclusive. The Lunar Prospector returned data in 1998 that proved more conclusively that water ice deposits exist on the Moon. Such deposits would be significant for any future human exploration and development of the Moon.