mars

"Fourth planet" redirects here. For other uses of "Fourth planet", see Fourth planet (disambiguation). For other uses of "Mars", see Mars (disambiguation).

Mars

Computer-generated view based on a Mars Global Surveyor image mosaic (1999-04). At left, orographic water ice clouds are suspended over the shield volcanoes Olympus Mons, Alba Mons and the Tharsis Montes. The north polar summer (water) ice cap is at top, incised by Chasma Boreale. At lower right, Valles Marineris stretches east-west over 4000 km. Dark areas on the right are lacking in surface dust; the bright area at the lower right limb is the impact basin Argyre.
Designations
Pronunciation	i/ˈmɑrz/
Adjective	Martian
Orbital characteristics [2]
Epoch J2000
Aphelion	249,209,300 km 1.665 861 AU
Perihelion	206,669,000 km 1.381 497 AU
Semi-major axis	227,939,100 km 1.523 679 AU
Eccentricity	0.093 315
Orbital period	686.971 days 1.8808 Julian years 668.5991 sols
Synodic period	779.96 days 2.135 Julian years
Average orbital speed	24.077 km/s
Mean anomaly	19.3564°
Inclination	1.850° to ecliptic 5.65° to Sun's equator 1.67° to invariable plane[1]
Longitude of ascending node	49.562°
Argument of perihelion	286.537°
Satellites	2
Physical characteristics
Equatorial radius	3,396.2 ± 0.1 km[a] [3] 0.533 Earths
Polar radius	3,376.2 ± 0.1 km[a] [3] 0.531 Earths
Flattening	0.005 89 ± 0.000 15
Surface area	144,798,500 km2 0.284 Earths
Volume	1.6318×1011 km3[4] 0.151 Earths
Mass	6.4185×1023 kg[4] 0.107 Earths
Mean density	3.9335 ± 0.0004[4] g/cm³
Equatorial surface gravity	3.711 m/s²[4] 0.376 g
Escape velocity	5.027 km/s
Sidereal rotation period	1.025 957 day 24.622 9 h[4]
Equatorial rotation velocity	868.22 km/h (241.17 m/s)
Axial tilt	25.19°
North pole right ascension	21 h 10 min 44 s 317.681 43°
North pole declination	52.886 50°
Albedo	0.170 (geometric)[5] 0.25 (Bond)[6]
Surface temp. min mean max Kelvin 130 K 210 K[6] 308 K Celsius −143 °C[8] −63 °C 35 °C[9]
Apparent magnitude	+1.6 to −3.0[7]
Angular diameter	3.5–25.1"[6]
Atmosphere[6][13]
Surface pressure	0.636 (0.4–0.87) kPa
Composition	(mole fractions) 95.32% carbon dioxide 2.7% nitrogen 1.6% argon 0.13% oxygen 0.08% carbon monoxide 210 ppm water vapor 100 ppm nitric oxide 15 ppm molecular hydrogen[10] 2.5 ppm neon 850 ppb HDO 300 ppb krypton 130 ppb formaldehyde 80 ppb xenon 18 ppb hydrogen peroxide[11] 10 ppb methane[12]

 

Animation of Mars's rotation from the vantage of an observer who moves south, then north, to hover over both poles, showing the planet's major topographic features.

Mars is the fourth planet from the Sun and the second smallest planet in the Solar System. Named after the Roman god of war, it is often described as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance.^[14] 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. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the second highest known mountain within the Solar System (the tallest on a planet), and of Valles Marineris, one of the largest canyons. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.^[15][16] Mars has two known moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids,^[17][18] similar to 5261 Eureka, a Martian trojan asteroid.
Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet's 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 were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unmanned missions suggest that Mars once had large-scale water coverage on its surface.^[19] In 2005, radar data revealed the presence of large quantities of water ice at the poles^[20] and at mid-latitudes.^[21][22] The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.^[23]
Mars is currently host to five functioning spacecraft: three in orbit – the Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter – and two on the surface – Mars Exploration Rover Opportunity and the Mars Science Laboratory Curiosity. Defunct spacecraft on the surface include MER-A Spirit and several other inert landers and rovers such as the Phoenix lander, which completed its mission in 2008. Observations by the Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.^[24]
Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −3.0,^[7] which is surpassed only by Jupiter, Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 km (186 miles) across when Earth and Mars are closest because of Earth's atmosphere.^[25]