Mars

1. Fourth planet from the Sun. Diameter 4,220 miles. Called the Red Planet from its colour as seen through a telescope; colour varies from butterscotch to dark brown. Much of this is from iron oxide (rust) in surface rocks. Surface gravity 38 percent that on Earth, about the same as Mercury, an effect jointly of Mars' larger size and lower density. The least dense of the rocky terrestrial planets in the system. One tenth of Earth's mass. Atmosphere mostly carbon dioxide, surface pressure varies by location and season between about 5 and 7 millibars. Surface features include Mariner Valley, a canyon system that would stretch across the United States on Earth, and four large shield volcanoes on the highland area known as the Tharsis Bulge, the largest of which is Olympus Mons, the largest mountain on any major planet in the system, three times the height of Everest and covering an area about the size of Romania. Has been visited by numerous space probes, including the Viking landers, the Sojourner rover and the Spirit and Opportunity Rovers. Currently being orbited by the Odyssey, Express and Reconnaissance Orbiters, making it the planet with the most artificial satellites beyond Earth. Although the surface is almost certainly sterile, Mars has often been imagined as an abode of life, appearing as such in works by, among others, C.S. Lewis, H.G. Wells and Edgar Rice Burroughs. There is some evidence of liquid surface water in its early history, although the atmosphere has grown too thin to allow this any more. Appears in some ways earthlike, with dust storms (especially at perihelion passage, the closest passage to the sun), growing and shrinking (largely carbon dioxide) ice caps and even, at 24 hours and 40 minutes, the most earthlike length of day of any other planet in this system. Two moons, Phobos and Deimos, both asteroids, circle the planet, the former the lowest-orbiting moon of any major planet in the system and set to run smack into Mars in about another 40 million Earth years. 2. The fourth planet's namesake, the ancient Roman god of war. Bit of a meathead, but then it was his job. Had an affair with Venus ... well, who wouldn't? Greek equivalent was Ares. 3. Chunky nougat-caramel-chocolate bar, or the company that makes them.