A bright surface layer of ice (either frozen water or some other volatile compound) covering the poles of a planetary body, usually varying in size during the year. On Earth, the polar caps are made of water ice, whereas on Mars they consist mostly of carbon dioxide with a layer of water ice underneath that persists from year to year. The transient Martian polar caps of carbon dioxide can extend down to latitude 60° in winter, but retreat back to 85–87° latitude at the height of summer, revealing the permanent cap of water ice. Of the other bodies in the Solar System only Triton has a definite polar cap, which appears to consist of solid nitrogen and may vary seasonally.
Subjects: Astronomy and Astrophysics.