Image Credit: NASA and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: J. Bell (Cornell U.)










Mars, the fourth planet from the Sun in the Solar System. The last of the four terrestrial planets. Based on orbital observations and the examination of the Martian meteorite collection, the surface of Mars appears to be composed primarily of basalt. Some evidence suggests that a portion of the Martian surface is more silica-rich than typical basalt.
Mars has no intrinsic magnetic field, the planetary dynamo ceased to have functioned and caused the planet's magnetic field to fade away. Observations show that parts of the planet's crust have been magnetized and that alternating polarity reversals of its dipole field have occurred. Current models of the planet's interior imply a core region approximately 1,500 kilometres in radius, consisting primarily of iron with about 15% sulfur.
This iron sulfide core is partially fluid, and has twice the concentration of the lighter elements than exist at Earth's core. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but now appears to be inactive. Mars has the highest known mountain in the solar system (26 km). It is an extinct volcano, which contains several other large volcanoes. Mars is scarred by a number of impact craters. Due to the smaller mass of Mars, the probability of an object colliding with the planet is about half that of the Earth. However, Mars is located closer to the asteroid belt, so it has an increased chance of being struck by materials from that source. Mars is also more likely to be struck by short-period comets. Mars lost its magnetosphere 4 billion years ago, so the solar wind interacts directly with the Martian ionosphere, keeping the atmosphere thinner than it would otherwise be by stripping away atoms from the outer layer.  The scale height of the atmosphere is about 11 km. The atmosphere on Mars consists of carbon dioxide,  nitrogen, argon, and contains traces of oxygen and water. The atmosphere is quite dusty which give the Martian sky a red brown color when seen from the surface.
If Mars had an Earth-like orbit, its seasons would be similar to Earth's because its axial tilt is similar to Earth's. However, the large eccentricity of the Martian orbit has a significant effect, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder than would otherwise be the case. Mars has the largest dust storms in the Solar System. The polar caps at both poles consist primarily of water ice. However, there is dry ice (Frozen carbon dioxide ) present on their surfaces.
Mars’ average distance from the Sun is about 230 million km and its orbital period is 687 (Earth) days. The solar day on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35 seconds. A Martian year is equal 1 year, 320 days, and 18 hours. Mars' axial tilt is 25.19 degrees, which is similar to the axial tilt of the Earth. As a result, Mars has seasons like the Earth, though on Mars they are about twice as long given its longer year.
Mars has two moons, Phobos and Deimos, which are small and irregularly shaped, which orbit very close to the planet and are thought to be captured asteroids. Both satellites were discovered in 1877 by Asaph Hall.
The ability of Mars to develop and sustain life, requires that it has liquid water on its surface. This requires that the orbit of the planet lies within the habitable zone, which for the Sun is currently occupied by Earth. Mars orbits half an astronomical unit beyond this zone and this, along with the planet's thin atmosphere, causes water to freeze on its surface. The past flow of liquid water, however, demonstrates the planet's potential for habitability.
The lack of a magnetosphere and extremely thin atmosphere of Mars are a greater challenge: the planet has little heat transfer across its surface, poor insulation against bombardment and the solar wind, and insufficient atmospheric pressure to retain water in a liquid form.

Dozens of spacecrafts, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, and Japan to study the planet's surface, climate, and geology. Roughly two-thirds of all spacecraft destined for Mars have failed in one manner or another before completing or even beginning their missions. This high failure rate can eventually be ascribed to technical problems.
The first successful fly-by mission to Mars was NASA's Mariner 4, launched in 1964. The first successful objects to land on the surface were two Soviet probes, Mars 2 and Mars 3 from the Mars probe program, launched in 1971, but both lost contact within seconds of landing. Then came the 1975 NASA launches of the Viking program, which consisted of two orbiters, each having a lander; both landers successfully touched down in 1976 and remained operational for 6 and 3 years, for Viking 1 and Viking 2 respectively. The Viking landers relayed the first color pictures of Mars and also mapped the surface of Mars so well that the images are still sometimes used to this day. The Soviet probes Phobos 1 and 2 were sent to Mars in 1988 to study Mars and its two moons, unfortunately Phobos 1 lost contact on the way to Mars, and Phobos 2, while successfully photographing Mars and Phobos, failed just before it was set to release two landers on Phobos's surface. Following the 1992 failure of the Mars Observer orbiter, NASA launched the Mars Global Surveyor in 1996. This mission was a complete success, having finished its primary mapping mission in early 2001. Contact was lost with the probe in November 2006 during its third extended program, spending exactly 10 operational years in space. Only a month after the launch of the Surveyor, NASA launched the Mars Pathfinder, carrying a robotic exploration vehicle Sojourner, which landed in the Ares Vallis on Mars. This mission was another big success, and received much publicity, partially due to the many spectacular images that were sent back to Earth.
In 2001 NASA launched the successful Mars Odyssey orbiter, which is still in orbit as of June 2007.
In 2003, the ESA launched the Mars Express craft, consisting of the Mars Express Orbiter and the lander Beagle 2. Beagle 2 failed during descent and was declared lost in early February 2004.
Also in 2003, NASA launched the twin Mars Exploration Rovers named Spirit and Opportunity. Both missions landed successfully in January 2004 and they have achieved all their targets. Among the most significant scientific returns has been conclusive evidence that liquid water existed at some time in the past at both landing sites. Martian dust devils and windstorms have occasionally cleaned both rovers' solar panels, and thus increased their lifespan.
On August 12, 2005 the NASA Mars Reconnaissance Orbiter probe was launched toward the planet, arriving in orbit on March 10, 2006 to conduct a two-year science survey. The orbiter will map the Martian terrain and weather to find suitable landing sites for upcoming lander missions. It also contains an improved telecommunications link to Earth, with more bandwidth than all previous missions combined.
The NASA Phoenix Mars lander launched on August 4, 2007 and is scheduled to arrive on the north polar region of Mars on May 25, 2008. The lander has a robotic arm with a 2.5 m reach and is capable of digging a meter into the Martian soil. The lander will be in an area with an 80% chance of ice being less than 30 cm below the surface, and has a microscopic camera capable of resolving to one-thousandth the width of a human hair.
Phoenix will be followed by the Mars Science Laboratory in 2009, a bigger, faster (90 m/hour), and smarter version of the Mars Exploration Rovers. Experiments include a laser chemical sample that can deduce the make-up of rocks at a distance of 13 m.
The joint Russian and Chinese Phobos-Grunt sample-return mission, to return samples of Mars' moon Phobos, is scheduled for a 2009 launch.
In 2012 the ESA plans to launch its first Rover to Mars, the ExoMars rover will be capable of drilling 2 m into the soil in search of organic molecules.
NASA and Lockheed Martin have begun work on the Orion Crew Exploration Vehicle (CEV), which is currently scheduled to send a human expedition to Earth's moon by 2020 as a stepping stone to an expedition to Mars thereafter.
The European Space Agency hopes to land humans on Mars between 2030 and 2035.
This will be preceded by successively larger probes, starting with the launch of the ExoMars probe and a Mars Sample Return Mission.

Mars: Facts & Figures  
Discovered By
  Known by the Ancients
Date of Discovery
Average Distance from the Sun
  Metric: 227,936,640 km
English: 141,633,260 miles
Scientific Notation: 2.2793664 x 108 km (1.523662 A.U.) 
By Comparison: 1.524 x Earth
Perihelion (closest)
  Metric: 206,600,000 km
English: 128,400,000 miles
Scientific Notation: 2.066 x 108 km (1.381 A.U.) 
By Comparison: 1.404 x Earth
Aphelion (farthest)
  Metric: 249,200,000 km
English: 154,900,000 miles
Scientific Notation: 2.492 x 108 km (1.666 A.U.) 
By Comparison: 1.638 x Earth
Equatorial Radius
  Metric: 3,397 km
English: 2,111 miles
Scientific Notation: 3.397 x 103 km
By Comparison: 0.5326 x Earth
Equatorial Circumference
  Metric: 21,344 km
English: 13,263 miles
Scientific Notation: 2.1344 x 104 km
  Metric: 163,140,000,000 km3
Scientific Notation: 1.6314 X 1011 km3
By Comparison: 0.150 x Earth
  Metric: 641,850,000,000,000,000,000,000 kg
Scientific Notation: 6.4185 x 1023 kg
By Comparison: 0.10744 x Earth
  Metric: 3.94 g/cm3
By Comparison: 0.714 x Earth
Surface Area
  Metric: 144,100,000 km2
English: 89,500,000 square miles
Scientific Notation: 1.441 x 108 km2
By Comparison: 0.282 x Earth
Equatorial Surface Gravity
  Metric: 3.693 m/s2
English: 12.116 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mars.
Escape Velocity
  Metric: 18,072 km/h
English: 11,229 mph
Scientific Notation: 5.02 x 103 m/s
By Comparison: Escape velocity of Earth is 25,022 mph.
Sidereal Rotation Period (Length of Day)
  1.026 Earth days 
24.62 hours 
By Comparison: Earth's rotation period is 23.934 hours.
Sidereal Orbit Period (Length of Year)
  1.8807 Earth years 
686.93 Earth days 
Mean Orbit Velocity
  Metric: 86,871 km/h
English: 53,979 mph
Scientific Notation: 24,130.9 m/s
By Comparison: 0.810 x Earth
Orbital Eccentricity
By Comparison: 5.59 x Earth
Orbital Inclination to Ecliptic
  1.8 degrees
Equatorial Inclination to Orbit
Orbital Circumference
  Metric: 1,366,900,000 km
English: 849,400,000 miles
Scientific Notation: 1.3669 x 109 km
By Comparison: 1.479 x Earth
Minimum/Maximum Surface Temperature
  Metric: -87 to -5 °C
English: -125 to 23 °F
Scientific Notation: 186 to 268 K
Atmospheric Constituents
  Carbon Dioxide, Nitrogen, Argon
Scientific Notation: CO2, N2, Ar
By Comparison: CO2 is responsible for the Greenhouse Effect and is used for carbonation in beverages.
N2 is 80% of Earth's air and is a crucial element in DNA. Ar is used to make blue neon light blubs.

Mars: Moons: Phobos: Facts & Figures  
Discovered By
  A. Hall
Date of Discovery
Average Distance from Mars
  Metric: 9,378 km
Equatorial Radius
  Metric: 13.4 x 11.2 x 9.2 km
  Metric: 10,630,000,000,000,000 kg
Scientific Notation: 1.063 x 1016 kg
Rotation Period (Length of Day)
  1.026 Earth days 
24.62 hours 
By Comparison: Synchronous with Mars
Orbit Period (Length of Year)
  0.31891023 Earth days 
Orbital Eccentricity
Orbital Inclination to Ecliptic
  1 degree
Mars: Moons: Deimos: Facts & Figures  
Discovered By
  A. Hall
Date of Discovery
Average Distance from Mars
  Metric: 23,459 km
Equatorial Radius
  Metric: 7.5 x 6.1 x 5.2 km
  Metric: 2,380,000,000,000,000 kg
Scientific Notation: 2.38 x 1015 kg
Rotation Period (Length of Day)
  1.026 Earth days 
24.62 hours 
By Comparison: Synchronous with Mars
Sidereal Orbit Period (Length of Year)
  1.2624407 Earth days 
Orbital Eccentricity
Orbital Inclination to Ecliptic
  0.9 - 2.7 degrees


A Planet


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