Mars is the fourth planet from the sun and often referred to as the red planet. It is often used as an alien planet in science fiction, for example, H G Wells' The War of the Worlds. The reddish colour is due to iron (III) oxide in the surface minerals.

Mars started out as a Roman god of agriculture, but became the god of war, because of affiliation with the Greek Ares.

Mars' orbit is slightly elliptical, not so strongly as Mercury or Pluto, but enough to make large temperature variations, between 140K and 220K (average 218K).

Mars has two moons, Phobos and Deimos, both discovered by Asaph Hall in 1877.

Mars has been visited on several occasions, first by Mariner 4 in 1965. Then by the USSR's Mars 2 lander in 1971 and the Viking landers in 1976. More recently, in 1997, Mars Pathfinder landed and the Expedition Rovers in 2004. The rovers, Spirit and Opportunity, have far outlasted expectations and are still running as of 2006. In addition, Mars Global Surveyer, Mars Odyssey and ESA's Mars Express are still in orbit. Also, Mars Reconnaissence Orbiter arrived in March 2006. Another lander, Phoenix, is planned to launch in 2007 to descend to Mars in May 2008.

The Martian atmosphere is thin and consists mostly of carbon dioxide. By composition, it is; carbon dioxide 95.3%, nitrogen 2.7%, argon 1.6%, oxygen 0.15% and water 0.03%. There are sometimes dust storms that can last for several months. The atmospheric pressure at the surface is only 1% of that on Earth.

Mars has some incredible surface features, such as Olympus Mons; a mountain over 24km higher than its surroundings and with a 250km radius (slightly under the distance between Paris and Brussels). Also, the Valles Marineris; a vast set of canyons 7km deep at its maximum, spreading 4000km. Recent expeditions sent back images and data suggest that Mars had large amounts of liquid on its surface at some point in its past. The poles of Mars are capped with water ice and frozen carbon dioxide.