Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman (Roman mythology) god of war, Mars (Mars (mythology)). It is often described as the "Red Planet", as the iron oxide (iron(III) oxide) prevalent on its surface gives it a reddish appearance (Mars surface color). Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. Mars? rotational period and seasonal cycles are likewise similar to those of Earth. Mars is the site of Olympus Mons, the highest known mountain in the Solar System, and of Valles Marineris, the largest canyon. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Unlike Earth, Mars is now geologically and tectonically inactive.

Until the first flyby of Mars occurred in 1965, by Mariner 4, many speculated about the presence of liquid water on the planets surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations (Striation (geology)) were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, yet of all the planets in the Solar System other than Earth, Mars is the most likely to harbor liquid water, and thus to harbor life. Geological evidence gathered by unmanned missions suggest that Mars once had large-scale water coverage on its surface, while small geyser-like water flows may have occurred during the past decade. In 2005, radar data revealed the presence of large quantities of water ice at the poles, and at mid-latitudes. The Phoenix Lander directly sampled water ice in shallow martian soil on July 31, 2008.

Mars has two moons, Phobos (Phobos (moon)) and Deimos (Deimos (moon)), which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Martian Trojan asteroid. Mars is currently host to three functional orbiting spacecraft: Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. On the surface are the two Mars Exploration Rovers (Spirit (Spirit rover) and Opportunity (Opportunity rover)) and several inert landers and rovers, both successful and unsuccessful. The Phoenix Lander completed its mission on the surface in 2008. Observations by NASAs now-defunct Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.

Mars can easily be seen from Earth with the naked eye. Its apparent magnitude reaches ?2.91, a brightness surpassed only by Venus, the Moon, and the Sun, although most of the time Jupiter will appear brighter to the naked eye than Mars. Mars has an average opposition (opposition (planets)) distance of 78 million km but can come as close as 55.7 million km during a close approach, such as occurred in 2003.