In 1971, the Mariner 9 spacecraft arrived in orbit around Mars - to find it entirely engulfed in a planet-wide dust storm. The only features clearly visible were four dark spots (now known to correspond to the Tharsis volcanoes and Olympus Mons).
Carl Sagan was so fascinated by these that his colleagues at Cornell University jokingly called them "Carl's Marks"; Sagan, in his turn, nicknamed the four spots Harpo, Groucho, Chico, and Zeppo.
