Planetologists have focused their attention recently on a family of minerals known in the jargon as the ‘planetary ices.’ A small part of this special interest derives from the expectation of Comet Halley in 1985. Comets are often classified by planetary petrologists as ‘dirty snowballs.’ In large part, however, interest in the ices stems from the exciting results of the Voyager spacecraft missions. Most of Saturn's rings, indeed most of the moons of Saturn and Venus, have turned out to be icy. The interiors of the giant planets are made up of ices. Pluto has ice; so has its large moon, Charon.