Mars

Mars, the fourth planet from the Sun, with a mean solar distance of 141.6 million mi (227.9 million km) and a mean diameter of 4,223 mi (6,796 km). Mars takes about 687 earth-days to orbit the Sun. The planet's temperature ranges from −191° to 81°F (−124° to 27°C), and its tenuous atmosphere consists mainly of carbon dioxide. The distinctive Martian polar caps are composed of frozen carbon dioxide and water ice.

Telescopically, Mars appears as an ocher-red disk marked by extensive dark areas; these latter have in the past been erroneously termed maria (seas). Several observers in the past reported sighting networks of straight lines on the Martian surface—the famous canals—but observations with large telescopes and the photographs sent back by the United States' Mariner (1965, 1969, 1971) and Viking (1976) space probes showed these to be an optical illusion. Mars actually has a cratered surface marked with canyons, ancient volcanoes, and jumbled terrains.

No probe has yet found evidence that life ever existed on the planet. Mars has 2 satellites (moons), Phobos and Deimos. In 1996 scientists of NASA, Lockheed Martin, and universities in Montreal, Georgia, and Stanford announced they had found fossil traces on a meteorite from Mars. These traces could be the result of early biological activity on Mars. Although this discovery was not seen as evidence of life on Mars, it led to other meteorites from Mars being studied more closely.

See also: Planet; Solar System.