Are the icecaps of mars composed mostly of frozen water?
The ice caps in the North pole of Mars compose of frozen water and frozen carbon dioxide and the ice caps on the South Pole consists mainly of of frozen water.
No planet is composed of mainly frozen water; Mars has probably the most frozen water of all the planets after the Earth. However, older textbooks and teacher's notes may well refer to Pluto, which is now classed as only a "dwarf planet" and is composed of roughly 50-70 percent rock and 30-50 percent ice. So, although Pluto has a lot of ice, it has more rock than ice (frozen water).
Mars has quite a bit of frozen water on it, but Europa, one of Jupiter's moons, is comprised of almost entirely frozen water. Europa has a frozen core, then a layer of liquid water that is kept liquid by friction, then the thick, ice crust. But of course Europa is only a moon. Pluto is probably made of roughly 50% water ice, but Pluto is only a dwarf planet.