Mars

The planet that is fourth outward from the Sun. Mars has a mean heliocentric distance (semimajor axis) of 1.524 astronomical units, equivalent to 141.6 × 106 mi (227.9 × 106 km). Its orbital eccentricity of 0.093, one of the largest of the major planets, causes the distance between Mars and the Sun to vary from 128 × 106 mi (207 × 106 km) at perihelion to 155 × 106 mi (249 × 106 km) at aphelion. The mean diameter of Mars is 4212 mi (6780 km), or about 53% that of the Earth. The planet has a mass of 7.08 × 1020 tons (6.42 × 1023 kg), about 11% of Earth's. The sidereal rotation period is 24h 37m 22.7s, corresponding to a mean solar day of 24h 39m 35.2s.

Mars appears to the unaided eye as a bright, slightly reddish star. Having a mean (synodic) period between oppositions of 780 days, Mars is in conjunction with the Sun every other year. Viewed through a telescope, Mars usually appears as a bright reddish disk marked by complex, semipermanent dark regions and variable white polar caps.

Interior

Accurate measurements of the planet's gravitational field made by orbiting spacecraft have shown that Mars has a dense core and thus is differentiated into a core, mantle, and crust. The crust, composed of silicate rocks, enriched in silicon and aluminum and deficient in magnesium, is believed to be about 30 mi (50 km) thick. Over geologic time, partial melting of the crust or upper mantle has caused lower-density materials to float to the surface, creating the observed lava plains and volcanic structures.

At present Mars does not have a global magnetic field of appreciable strength. However, early in its history Mars must have had a more substantial magnetic field, because its near-surface rocks exhibit strong remanent magnetism. See also Rock magnetism.

Geology

The Martian surface has been modified extensively by the processes of impact cratering, volcanism, faulting, and fluvial erosion. The terrain in the southern hemisphere is very heavily cratered and thus quite old, having formed some 3.8 × 109 years ago. Much of it stands 1–2 mi (1–4 km) higher than the planet's mean radius. By contrast, the northern hemisphere is dominated by vast, lava-covered plains with relatively few impact craters and an average elevation of about 3 mi (5 km) below the mean radius.

The Tharsis rise, or bulge, is an uplifted portion of the surface that stands several kilometers above the mean elevation of the planet. Most of Mars's tectonic features are associated with Tharsis, which affects approximately one-quarter of the entire surface.

The Martian surface exhibits several enormous shield volcanoes. The largest and perhaps the youngest is Olympus Mons, which is nearly 370 mi (600 km) across at its base and stands approximately 16 mi (26 km) above the surrounding terrain. Three other large shield volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons, lie along the nearby Tharsis rise. In shape and structure the Martian shield volcanoes bear a strong resemblance to their Hawaiian counterparts. At the summit of each shield is a complex of calderas, collapsed craterlike features that were once vents for lava.

Perhaps the most spectacular features on the Martian surface are the huge canyons located primarily in the equatorial regions. Valles Marineris, actually a system of canyons, extends for over 3000 mi (5000 km) along the equatorial belt. In places, the canyon complex is as much as 300 mi (500 km) wide and drops to more than 4 mi (6 km) below the surrounding surface. Dwarfing the Earth's Grand Canyon, Valles Marineris is comparable in size to the great East African Rift Valley. In general, the walls of the canyon are precipitous, have well-defined edges, and show evidence of slumping and landslide activity (Fig. 1).

An image from the Mars Global Surveyor resolves features as small as 20 ft (6 m) in a 10-km-wide portion of Coprates Chasma, which is located in the middle of Mars's vast Valles Marineris canyon complex. The high-standing plateau is underlain by multiple rock layers that may be volcanic or sedimentary in nature. (NASA/Malin Space Science Systems)
An image from the Mars Global Surveyor resolves features as small as 20 ft (6 m) in a 10-km-wide portion of Coprates Chasma, which is located in the middle of Mars's vast Valles Marineris canyon complex. The high-standing plateau is underlain by multiple rock layers that may be volcanic or sedimentary in nature. (NASA/Malin Space Science Systems)

An astonishing class of features on the Martian surface is a widespread network of channels that bear a very strong resemblance to dry river beds. Ranging in size from broad, sinuous features nearly 40 mi (60 km) wide to small, narrow networks less than 300 ft (100 m) wide, the channels appear to have been created by water erosion. The largest channels must have been formed by enormous torrents of water, presumably released in some catastrophic manner (Fig. 2).

Nanedi Vallis, a long, water-cut canyon on Mars, which is 1.5 mi (2.5 km) wide. (a) Canyon and surrounding terrain. (b) Detailed view of area in white
Nanedi Vallis, a long, water-cut canyon on Mars, which is 1.5 mi (2.5 km) wide. (a) Canyon and surrounding terrain. (b) Detailed view of area in white rectangle in part a. Rocky outcrops jut from the canyon's walls, and a secondary rivulet is seen in its floor. (NASA/Malin Space Science Systems)

Surface exploration

Five spacecraft have landed safely on the Martian surface and returned significant scientific data about its properties. Two American Viking landers arrived in 1976. Mars Pathfinder which landed in 1997, carried a small, instrumented rover, Sojourner, which was directed from Earth via remote control to several rocks and fine-grained drifts in the lander's immediate vicinity. The Mars Exploration Rovers, Spirit and Opportunity, which landed in 2004, had far greater mobility.

Meteorites from Mars

A small number of meteorites have reached Earth after being violently ejected from the planet Mars during the collision of sizable asteroids or comets. The meteorites are thus samples from Mars itself. The first of these Martian meteorites was recognized in 1982, and 14 such samples were known by the end of 1999. They are sometimes collectively termed the SNC meteorites, representing the first letters of their three subclasses: shergottites, nakhlites, and chassignites. The Martian meteorites contain a distinctive ratio of the isotopes of oxygen found nowhere on Earth, on the Moon, or in other meteorites. With one exception, they crystallized from molten rock within the last 1.3 × 109 years, far more recently than the usual 4.6 × 109 years (the age of the solar system) found in all other meteorites. The lone exception to date, designated ALH 84001, crystallized 4.5 × 109 years ago during the planet's infancy. See also Meteorite.

Atmosphere

The Martian atmosphere is very thin, with surface pressure averaging about 6.5 millibars (650 pascals), or less than 1% of the pressure at the Earth's surface. It is composed principally of carbon dioxide (95.3%), but contains nitrogen, argon, oxygen, and a trace of water vapor, totaling 4.7%. Both carbon dioxide and water form clouds in the Martian atmosphere.

Dust storms

Localized dust storms appear quite frequently on Mars. Because the Martian atmosphere is extremely thin, wind velocities greater than 90–110 mi/h (40–50 m/s) are needed to set surface dust grains in motion. Some dust storms develop with such intensity that their total extent may be hemispheric or even global.

Water inventory

The seasonal cycle of growth and decay of the bright polar caps has long been taken as evidence of the presence of water on Mars. However, evidence from the Mariner spacecraft identified carbon dioxide as the principal constituent of the polar snow, with lesser amounts of water ice also present. Formed during autumn and winter by condensation and deposition of the icy mist covering the polar regions, the polar caps at the end of winter cover a vast area extending down to latitude 60° in the southern hemisphere and 70° in the northern hemisphere. The residual caps, which never completely disappear, are believed to be composed almost entirely of water ice. It is estimated that if all the water present in the polar caps were distributed uniformly over the surface of Mars, it would produce a layer 70–110 ft (22–33 m) deep. A far greater amount of water may exist as subsurface ice trapped elsewhere in areas of heavily cratered and fractured terrain.

Even though at present liquid water cannot exist on the Martian surface, the planet's numerous large flood channels suggest that vast amounts of water flowed on the surface early in Martian history. There is strong circumstantial evidence that much of the low-lying northern hemisphere was covered with an ocean roughly 4 × 109 years ago. Although unproven, this hypothesis suggests that 107 mi2 (27 × 106 km2) of the northern plains were also inundated.

Possibility of life

The ample evidence that early Mars had an abundance of liquid water, a thicker atmosphere, and a more clement environment has raised speculation that the planet may have once been, or may still be, an abode for primitive life forms. In 1976, the twin Viking landers conducted experiments on the Martian surface to test for the existence of life. One of the biological experiments gave a positive result—the release of oxygen from a soil sample when humidified—but there is no consensus as to the source of this reaction.

Twenty years later, a scientific team claimed that it had found evidence of fossilized microbes within a known Martian meteorite (ALH 84001). After years of intensive study, scientists have reached no consensus. The mineral associations within ALH 84001 and other Martian meteorites neither confirm the presence of once-living organisms nor exclude it.

Satellites

Mars has two small satellites, Phobos and Deimos. Neither satellite is massive enough to be gravitationally contracted to a spherical shape. Both satellites are saturated with impact craters. Both satellites have the same low albedo, approximately 0.05. This low reflectivity suggests that the Martian satellites may have originated in the asteroid belt. See also Asteroid.