Mars

For the Roman god of war, see Mars (god)

Mars
Mars photographed by Viking Orbiters
Symbol
Name of discoverer	Known to ancients
Name origin	Roman god of war
Orbital characteristics
Primary	Sun
Order from primary	4
Perihelion	206,620,000 km
Aphelion	249,230,000 km
Semi-major axis	227,920,000 km
Titius-Bode prediction	1.6 AU
Circumference	1,429,033,627 km
Orbital eccentricity	0.0935
Sidereal year	686.980 da
Synodic year	779.94 da
Avg. orbital speed	24.077 km/s
Inclination	1.850° to the ecliptic
Rotational characteristics
Sidereal day	24.6229 h
Solar day	24.6597 h
Rotational speed	465.11 m/s
Axial tilt	25.19°
Physical characteristics
Mass	6.4185 * 1023 kg
Density	3,933 kg/m³
Mean radius	3389.5 km
Equatorial radius	3396.2 km
Polar radius	3376.2 km
Surface gravity	3.71 m/s²
Escape speed	5.03 km/s
Surface area	144,800,000 km²
Minimum temperature	140 K
Mean temperature	218 K
Maximum temperature	300 K
Number of moons	2
Composition	Rock
Color	Red
Albedo	0.15
Magnetosphere
Magnetic dipole moment at present	2.1 * 1018 N-m/T
Magnetic dipole moment at creation	1.51 * 1023 N-m/T
Decay time	535 a
Half life	370.83 a

Mars is the fourth planet from the Sun, after Mercury, Venus, and Earth. It is called "The Red Planet" because it looks red from Earth.

The planet has been known since Antiquity. Because of this, the planet was named after the Roman equivalent of Ares, the Greek god of war.

Physical and surface features

There is a dichotomy on the surface of Mars; the northern hemisphere consists of low-lying volcanic plains, with the southern hemisphere consists of ancient, heavily-cratered highland plains. Mars also has two volcanic regions: Tharsis and Elysium. Within the Tharsis region is the largest volcano in the solar system, Olympus Mons. Olympus Mons is approximately 500 km across and is about the same size as the state of Arizona. Its peak is 24 km above the surrounding plains and it is surrounded by a vertical scarp 6 km high. A large canyon system, Valles Marineris, is located along the Martian equator east of the Tharsis region. Valles Marineris is 2-6km deep and 200-600km wide and approximately 4,500 km long (approximately the distance from San Francisco to Boston). The eastern part of Valles Marineris appears to have a tectonic origin, but the western part is characterized by features such as teardrop islands and longitudinal grooves, indicating it may have been formed by catastrophic flooding in the past. Such flooding may be possible under current climate conditions. Run-off channels in the cratered highland indicate a thicker atmosphere or warmer climate in the past that would have allowed water to be stable on the surface.

Recent satellite imagery has shown that Mars has a 3 kilometer deep ice cap around its polar regions. The northern polar region is surrounded by dune fields. During the Martian winter, a layer of carbon dioxide condenses at the poles and sublimates in the summer, leaving the water-ice and dust polar caps exposed. The ratio of dust to water-ice contained in the polar caps remains unknown.

Satellites

Mars has two moons: Phobos and Deimos (which respectively mean "Fear and "Panic" in Greek). Russian scientists in the 1950s posited that Phobos might be hollow, but these claims were discredited in the late 1960s.

Topographic map of Mars.