MARS, CANALS OF

apparent systems of rectilinear markings on the surface of Mars that are now known to be illusions caused by the chance alignment of large craters and other features of the Martian surface. They were the subject of much controversy in the late 19th and early 20th centuries. The Italian astronomer and statesman Giovanni Virginio Schiaparelli observed about 100 of them, from 1877, and described them as canali (Italian: channels). Others had earlier noted similar markings, but Schiaparelli's writings first drew wide attention to the subject. The U.S. astronomer Percival Lowell became the leader of those who believed the markings to be bands of vegetation, kilometres wide, bordering irrigation ditches dug by intelligent beings to carry water from the polar caps. Lowell and others described canal networks, studded with dark intersections called oases and covering much of the surface of the planet. Occasionally the lines were perceived as doubled; i.e., two parallel lines became visible where only a single canal had been seen before. Most astronomers could see no canals, and many doubted their reality. Experiments with untrained observers showed that disconnected features in diagrams or drawings might be perceived as straight-line networks when viewed at the proper distance. Photography through the Earth's atmosphere offered no solution because the lines were near the limit of resolution of the human eye and beyond that of the camera. The controversy was finally resolved only when pictures were made from several hundred kilometres above the surface of Mars by the Mariner 6 and 7 spacecraft in 1969. These showed many craters and other features but nothing resembling a network of channels.