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Mars (planet)

larger image with caption
Orbital characteristics
Mean radius227,940,000 km
Revolution period686.98 days
Synodic period779.95 days
Avg. Orbital Speed24.1 km/s
Number of satellites2
Physical characteristics
Equatorial diameter6,794.4 km
Surface area144 million km2
Mass6.421 × 1023 kg
Mean density3.94 g/cm3
Surface gravity3.72 m/s2
Rotation period24.6229 hours
Axial tilt25.19°
Escape Speed5.0 km/s
Surface temp.
Atmospheric characteristics
Atmospheric pressure0.7-0.9 kPa
Carbon dioxide95.32%
Carbon monoxide0.07%
Water vapor0.03%

Mars is the fourth planet in the solar system, named for the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color.

Table of contents

Physical characteristics

Mars has always fascinated people. Its red, fiery appearance was mysterious and intriguing. Mars has only a quarter the surface area of Earth and only 1/10th the mass (though because it lacks oceans the area of Mars's accessible dry land is approximately equal to that of the Earth's dry land). Mars has two small moons, Phobos and Deimos, both small and oddly shaped, possibly captured asteroids. Mars has polar ice caps that contain frozen water and carbon dioxide. An extinct volcano, Olympus Mons, is, at 25 km, the tallest mountain in the solar system. Mars's atmosphere is very thin: the surface air pressure is only 7.5 millibars compared to an average 1013 millibars on Earth. The atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon, with only a trace of oxygen and water.

Mars has an important place in human imagination due to the old belief that life existed on Mars. This was supposed because of observations of linear features on the surface that appeared artificial and seasonal changes in the brightness of some areas that were thought to be caused by vegetation growth. This gave rise to many stories concerning Martians. The linear features are now know to be non-existent or in some cases, ancient dry watercourses. The color changes have been ascribed to dust storms. Recently analysis of meteorites thought to have come from Mars show some features that may be fossils of single-celled organisms, although this idea is controversial. There is as yet no conclusive evidence that there has ever been life on Mars.

The International Astronomical Union's Working Group for Planetary System Nomenclature (http://wwwflag.wr.usgs.gov/USGSFlag/Space/nomen/nomen) is responsible for naming Martian surface features.

See the NASA Mars Fact Sheet (http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact)

Mars' size compared to Earth (/upload/0/07/Earth-mars.jpg)

Mars's moons

Both Phobos and Deimos are tidally locked with Mars, always pointing the same face towards it. Since Phobos orbits around Mars faster than the planet itself rotates, tidal forces are slowly but steadily decreasing its orbital radius. At some point in the future Phobos will impact on Mars's surface. Deimos, on the other hand, is far enough away that its orbit is being slowly boosted instead.

Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos and Deimos in Greek mythology, sons of the Greek god Ares.

Mars's natural satellites
Name Diameter (km) Mass (kg) Mean orbital
radius (km)
Orbital period
Phobos 22.2 (27 x 21.6 x 18.8) 1.08×1016 9378 7.66 hours
Deimos 12.6 (10 x 12 x 16) 2×1015 23,400 30.35 hours

The exploration of Mars

Unmanned probes

Many robotic probes have been sent from Earth to Mars, with a few spectacular successes and a notably high failure rate. Some of the failures can be ascribed to technical incompetence, but enough others have failed for no apparent reason for researchers to half-jokingly speak of an Earth-Mars "Bermuda Triangle" or of a Great Galactic Ghoul which subsists on a diet of Mars probes. In order to understand the history of the robotic exploration of Mars it is important to note that launch windows occur at intervals of slightly over 2 years (the planet's synodic period).

Two Soviet flyby probes were launched towards Mars in October 1960 but failed to reach Earth orbit. In 1962, three more Soviet probes failed -- two remaining in Earth orbit and one losing communication with Earth en route to Mars. In 1964, there was another failed attempt to reach Mars.

Between 1962 and 1973, NASA's Jet Propulsion Laboratory designed and built 10 spacecraft named Mariner to explore the inner solar system. These spacecraft were designed to visit the planets Venus, Mars and Mercury for the first time. The Mariners, relatively small robotic explorers, were launched on Atlas rockets. Each weighed less than half a ton.

Mariner 3 and Mariner 4 were identical spacecraft designed to carry out the first flybys of Mars. Mariner 3 was launched on November 5, 1964, but the shroud encasing the spacecraft atop its rocket failed to open properly. Mariner 3 failed to reach Mars. Three weeks later, on November 28, 1964, Mariner 4 was launched successfully on an eight-month voyage to the red planet.

Mariner 4 flew past Mars on July 14, 1965, providing the first close-up photographs of another planet. The pictures, played back from a small tape recorder over a long period, showed lunar-type impact craters. Some of them seemed touched with frost in the chill Martian evening.

NASA continued the Mariner program with another pair of Mars flyby probes at the next launch window. These probes reached the planet in 1969. See Mariner 6 and 7 for details. During the following launch window the Mariner program again suffered the loss of one of a pair of probes. Mariner 9 successfully entered orbit about Mars, after the launch time failure of its sister ship, Mariner 8. When Mariner 9 reached Mars, it and two Soviet orbiters, found that a planet-wide dust storm was in progress. The mission controllers used the time spent waiting for the storm to clear to have the probe rendezvous with, and photograph, Phobos. When the storm cleared sufficiently for Mars' surface to be photographed by Mariner 9, the pictures returned represented a substantial advance over previous missions. These pictures were the first to offer evidence that liquid water might at one time have flowed on the planetary surface.

In 1976 the two Viking probes entered orbit about Mars and each released a lander module that made a successful soft landing on the planet's surface. These missions returned the first color pictures and extensive new scientific information.

The Soviet probes of the Mars probe program attempted a number of landings several years before Viking, but were not nearly as successful as the Martian missions of the Mariner program.

The Mars Pathfinder spacecraft, landing on July 4, 1997, carried a tiny remote-controlled rover called Sojourner, which travelled a few metres around the landing site, exploring the conditions and sampling rocks around it. The mission website was the most heavily-trafficked up to that time.

Next came Mars Global Surveyor. This mission was the first successful one to the red planet in two decades when it launched November 7, 1996, and entered orbit on September 12, 1997. After a year and a half trimming its orbit from a looping ellipse to a circular track around the planet, the spacecraft began its primary mapping mission in March 1999. It has observed the planet from a low-altitude, nearly polar orbit[?] over the course of one complete Martian year, the equivalent of nearly two Earth years. Mars Global Surveyor recently completed its primary mission on January 31, 2001, and is now in an extended mission phase.

The mission has studied the entire Martian surface, atmosphere, and interior, and has returned more data about the red planet than all other Mars missions combined. This valuable data is archived at http://wufs.wustl.edu/missions/mgs/mola/ .


Recent findings by the Mars Odyssey probe's gamma ray spectrometer and neutron spectrometer have determined that there are vast deposits of water ice in the upper three meters of Mars's soil within 60° latitude of the south pole. Similar quantities of ice are expected to be present in the north polar region as well, but measurements will not be made until later in 2002.

On June 2, 2003, the European probe Mars Express set off from Baikonur to the Mars, expected to arrive there in December 2003.

On June 10, 2003, NASA's MER-A ("Spirit") Mars Exploration Rover was launched. It is planned to land in an area called "Gusev Crater" (believed to once being a crater lake) on January 4, 2004. It will examine rock and soil for evidence of the area's history of water. A second rover, MER-B ("Opportunity") will land on January 25, 2004 in an area called "Meridiani Planum" and will carry similar geological work.

Several other recent probes from the United States and Russia have failed upon arrival at Mars.

Manned missions

Many people, from Wernher Von Braun on, have long advocated a manned mission to Mars as the next logical step for a manned space program. As well as the undeniable romanticism of human exploration, such advocates believe that the amount of scientific work able to be performed in a human mission would be far superior than that possible with robotic explorers, thus making the large cost of a manned mission justifiable. With present technology, Mars Direct, forcefully advocated by Robert Zubrin of the Mars Society, is believed by many as the most practical and affordable plan for a manned Mars mission.

Longer term, some scientists believe Mars top be a good candidate for terraforming and human colonization, though other prominent skeptics (such as Robert Park[?]) hotly dispute its practicality.


The "Ares Vallis" area which is among the rockiest parts of Mars, as photographed by the Mars Pathfinder lander, in the 1997 mission. The "twin peaks" are seen in the distance. NASA (larger image)


Mars's closest approach to Earth is approximately 48,700,000 miles (78,390,000 kilometers).

A handful of objects are known that are surely meteorites and may be of Martian origin. Two of them may show signs of ancient bacterial activity.

See also:

Solar system:

External link


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