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Mars is the fourth planet from the Sun and is known as the Red Planet due to its reddish appearance as seen from Earth. The planet is named after Mars, the Roman god of war. A terrestrial planet, Mars has a thin atmosphere and surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It has the highest mountain in the solar system, Olympus Mons, and the largest canyon, Valles Marineris. Mars' rotational period and seasonal cycles are also similar to those of the Earth.
Until the first flyby of Mars by Mariner 4 in 1965, it was speculated that there might be liquid water on Mars. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, which looked like seas and continents. Also, long dark striations were interpreted by some observers as irrigation channels of liquid water. These straight line features were later proven not to exist and were instead explained as optical illusions. Still, of all the planets in our solar system other than Earth, Mars is the most likely to harbor liquid water, and perhaps life.
Mars is currently host to three functional orbiting spacecraft: Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. This is more than any planet except Earth. The surface is also home to the two Mars Exploration Rovers (Spirit and Opportunity). Geological evidence gathered by these and preceding missions suggests that Mars previously had large-scale water coverage, while observations also indicate that small geyser-like water flows have occurred in recent years. Observations by NASA's Mars Global Surveyor shows evidence that parts of the southern polar ice cap have been receding.
Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids similar to 5261 Eureka, a Mars Trojan asteroid. Mars can be seen from Earth with the naked eye. Its apparent magnitude reaches −2.9, a brightness surpassed only by Venus, the Moon, and the Sun, though for much of the year Jupiter may appear brighter to the naked eye than Mars.
Mean distance from Sun: 1.524 AU
(228,000,000 km/141,700,000 mi)
Diameter: 6,792 km (4,220 mi)
Length of year: 687 days
Rotation period: 24 hr 37 min
Mean orbital velocity: 24.14 km/sec (15 mi/sec)
Inclination of axis: 25.2°
Mean density: 3.95 grams/cm³
Inclination to ecliptic: 1.85°
Number of observed satellites: 2
Comparisons with Earth:
Average distance from Sun: 1.52 x Earth
Diameter: 0.532 x Earth
Mass: 0.108 x Earth
Density: 0.7 x Earth