Period of rotation (equatorial): 24 hr. 37 min. 23 sec.
Mean orbital velocity: 15 miles (24.1 km) per second
Inclination of axis: 23.98 degrees
Orbital eccentricity: 0.093
Orbital inclination: 1.85 degrees
Diameter at equator: 4,222 miles (6,794 km)
0.547 x Earth's diameter
Oblateness: 0.006
Mass: 6.42 x 10^23 kg
0.107 x Earth's mass
Volume: 1.63 x 10^11 km^3
0.15 x Earth's volume
Density: 3.94 g/cm^3
0.712 x Earth's density
Surface gravity: 3.72 m/s^2
0.38 x Earth's gravity
Escape velocity: 3.2 miles (5.0 km) per second
Mean surface temperature: -9 F / -23 C / 250 K
Maximum apparent diameter as seen from Earth (seconds of arc): 25.7╙
Albedo: 0.16
Number of known satellites: 2
Mean diameter of Sun as seen from the planet: 21' 00"
Satellite data
Phobos
Mean distance from center of Mars: 5,830 mi. (9,380 km)
Inclination of Axis: 0 degrees
Rotation period: 0.319 days
Orbital period: 0.319 days
Orbital eccentricity: 0.018
Orbital inclination to Mars' equator: 1.1 degrees
Diameter: 17 x 14 x 11 mi. (27 x 22 x 18 km)
Mass: 9.6 x 10^15 kg
Density: 1.9 g/cm^3
Escape velocity: 0.01 miles (0.016 km) per second
Maximum apparent diameter as seen from Mars: 12' 3"
Deimos
Mean distance from center of Mars: 14,600 mi. (23,500 km)
Inclination of Axis: 0 degrees
Rotation period: 1.262 days
Orbital period: 1.262 days
Orbital eccentricity: 0.003
Orbital inclination to Mars' equator: 2.8 degrees
Diameter: 6 x 7 x 9 mi. (10 x 12 x 15 km)
Mass: 2.0 x 10^15 kg
Density: 2.1 g/cm^3
Escape velocity: 0.005 miles (0.008 km) per second
Maximum apparent diameter as seen from Mars: 1' 7"
Planet composition
Researchers are not certain whether Mars has a core, but they speculate that it would be iron-rich and smaller than Earth's. Surrounding the core is a thick mantle, composed of silicates. The thin crust is covered by reddish oxides, which give the planet its color. Rocks on the surface are volcanic in origin and are composed mainly of iron (18%) and silicon (45%). Traces of magnesium, sulfur, and aluminum are also present.
The youngest surface on Mars lies at the poles, which are covered with white ice caps that wax and wane with the seasons. The northern pole is covered with water ice, while the southern pole has dry, carbon dioxide ice.
Mars is smaller and less dense than Earth. In fact, Mars has the lowest density of all the terrestrial planets (3.9). Its diameter (6,749 km) is half the Earth's and twice the Moon's.
Core: radius of 1500 to 2000 km
Mantle: 1300 to 1800 km thick
Crust: 25 to 40 km thick
Topography
Evidence suggests that the planet had a denser atmosphere in the past that allowed water to flow on the planet. Physical features closely resembling shorelines, gorges, riverbeds, and islands suggest that great rivers once marked the planet.
The surface of Mars is covered with craters resulting from volcanic activity and impact. The southern hemisphere of the globe is more densely cratered than north. The two largest craters in south are Hellas (1,800 km in diameter) and Argyre (800 km). Helles is the deepest on Mars, lying 3 km below the planet surface.
The northern hemisphere hosts several gigantic shield volcanoes that rise from the broad Tharsis "bulge" and from a smaller raised area in Elysium to the west. Extending eastward from Tharsis is an immense canyon system, Valles Marineris. The largest surface feature on the planet, Valles Marinaris stretches over 3,000 miles and reaches widths up to 450 miles. Radiating outward are arrays of tension faults (graben). Valles Marineris ends in an huge region of chaotic terrain between Chryse Planitia and Margaritifer Sinus. North of the canyons are outflow channels that appear to be the result of catastrophic flooding between 3,500 and 3,000 million years ago.
Mars has the largest volcanoes known in the solar system. Olympus Mons, the largest on Mars , is 25 km high and has a volume between 50 and 100 times that of Mauna Loa, the largest volcano on Earth. At its summit lies an enormous caldera at least 80 km in diameter.
Atmosphere
Like Earth, the Martian atmosphere is composed of gases released from the interior through volcanic activity. Mars' atmosphere is 95% carbon dioxide, 2% nitrogen, and 1.6% argon. In addition, traces of water vapor, carbon monoxide, oxygen, ozone, krypton, neon, and xenon are present. Unlike Earth, the Mars has no ozone layer, which means the planet's surface is fully exposed to ultraviolet radiation.
The atmosphere of Mars is tenuous; the surface pressure is less than 1% of the Earth's. Since the atmosphere is so thin, it cannot scatter shorter-wavelength radiation from the Sun (the phenomenon that makes the Earth's sky blue). As a result, the Martian sky is pink.
Because of the tilt of its axis, Mars also exhibits a seasonal pattern similar to Earth, although it is much colder.
Temperatures at the equator range from a summer high of approximately 4 F / -20 C / 253 K in the afternoon to -168 F / -111 C / 162 K just before sunrise. The large range in temperature contributes to causing frequent dust storms, which sometimes engulf the entire planet.
Magnetic field and gravity
Mars has only a slight magnetic field. Though the field is stronger than Venus', Mars' ionosphere and bow shock are much weaker because of its sparse atmosphere.
The surface gravity on the planet is 0.38 times the Earth's gravity.
Orbit and rotation
Compared with Earth, Mars has an eccentric orbit. Its period of revolution is 687 days.
Moons (and/or rings)
Mars has two satellites: Phobos and Deimos. Both were discovered by Asaph Hall in 1877. Researchers believe that the moons were originally asteroids that ventured too close to Mars and were captured by its gravity.
Phobos is heavily cratered. The largest is crater, Stickney, is 10 km in diameter. Deimos is relatively smooth in comparison. It has no craters larger than 3 km) The two most prominent craters are named Swift and Voltair. Both satellites are covered with a thin layer of dust.
Mythology
Its blood-like color earned the planet the name Mars, for the Roman god of War. It's satellites, Phobos (fear) and Deimos (terror), were named after mythical characters who drove the chariot of Mars.
Discovery and exploration
Of all the planets, Mars has long been considered the solar system's prime candidate for harboring extraterrestrial life. Astronomers studying the red planet through telescopes saw what appeared to be straight lines cross-crossing the surface. These observations (later determined to be optical illusions) led to the popular notion that intelligent beings had constructed a system of irrigation canals on the planet. This belief was further propagated by the writings of Jules Verne in the late 1800's and by early 20th-century movies. Another reason for scientists to expect life on Mars had to do with the apparent seasonal color changes on the planet surface. This phenomenon led to speculation that conditions might support bloom of Martian vegetation during the warmer months and cause plant life to become dormant during cooler periods.
The planet has been heavily researched in modern times. In 1965, the Mariner 4 probe passed Mars at 10,000 km. Mariner 6 and 7 passed by in 1969, and Mariner 9 orbited the planet from October 1971 - 1972. Based on photos returned by Mariner 9, researchers were able to create first reliable map of the Martian surface. In September 1975, NASA launched two Viking probes launched with the goal of landing one on each hemisphere of Mars. The landers reached Mars in June and August 1976 and both landings were successful. Initial test results on samples taken from surface seemed to indicate biological activity, but later tests confirmed that the activity was inorganic. Despite the inconclusive results of the Viking biology experiments, we know more about Mars than any other planet except Earth.
