Mars (planet)

I

Introduction

Mars (planet), planet named after the Roman god of war, the fourth from the Sun and the third in order of increasing mass. Mars has two small, heavily cratered satellites, or moons, Phobos and Deimos, which some astronomers consider to be asteroid-like objects captured by the planet very early in its history. Phobos is about 21 km (13 mi) across; Deimos, only about 12 km (7y mi).

II

Appearance From Earth

When viewed without a telescope, Mars is a reddish object whose brightness depends on its distance from the Earth. At its closest (56 million km/35 million mi), Mars is, after Venus, the brightest object in the night sky. Mars is best observed when it is both at opposition (directly opposite the Sun in the sky) and near perihelion (its closest approach to the Sun). Such favourable circumstances repeat every 15 or 17 years.

Through a telescope Mars is seen to have bright orange regions and darker, less red areas, the outlines and tones of which change with Martian seasons. (Because of the 25° tilt of its axis and the eccentricity of its orbit, Mars has short, relatively warm southern summers and long, relatively cold southern winters.) The reddish colour of the planet results from its heavily oxidized, or rusted, surface. The dark areas are thought to consist of rocks similar to terrestrial basalts, the surfaces of which have been weathered and oxidized. The brighter areas seem to consist of similar but even more weathered and oxidized material and apparently contain more fine, dust-sized particles than do the dark regions. The mineral scapolite, relatively rare on Earth, seems widespread; it may serve as a store for carbon dioxide (CO₂) from the atmosphere.

Conspicuous bright caps, composed of frozen water and CO₂, mark the planet’s polar regions. Their seasonal cycle has been followed for more than two centuries. Each Martian autumn, bright clouds form over the appropriate pole. Below this so-called polar hood, a thin cap of carbon dioxide frost is deposited during the autumn and winter. By late winter, the cap may extend down to latitudes of 45°. By the spring, and the end of the long polar night, the polar hood dissipates, revealing the winter frost cap; the cap’s boundary then gradually recedes poleward as sunlight evaporates the accumulated frost. By midsummer the steady recession of the annual cap stops, and a bright deposit of frost and ice survives until the following autumn. These remnant polar caps consist mostly of frozen water. They are about 300 km (185 mi) wide at the south pole and 1,000 km (620 mi) wide in the north. Although their true thickness is not known, they must contain ice and frozen gases to a thickness of possibly 2 km (1€ mi).

In addition to the polar hoods—thought to consist largely of frozen CO₂—other clouds are common on the planet. High-altitude hazes and localized water-ice clouds are observed. The latter result from the cooling associated with air masses rising over elevated obstacles. Extensive yellow clouds, consisting of dust lifted by Martian winds, are especially prominent during southern summers.

III

Observation by Spacecraft

The first spacecraft views of the planet were obtained in 1965 when Mariner 4 flew past Mars and revealed the presence of craters on its surface, and further information was gained in 1969 from the fly-by missions of Mariners 6 and 7. Then, in 1971, Mariner 9 went into orbit around Mars. It studied the planet for almost a year, giving scientists their first comprehensive global view of Mars and the first detailed images of its satellites, Phobos and Deimos. In 1976 two Viking landers touched down successfully on Mars and carried out the first direct investigations of the atmosphere and surface. The Viking mission also included two orbiters that studied the planet for almost two full Martian years (from 1976 to 1980). In 1988 the Soviet Union sent two probes to land on Phobos; both missions failed, although one relayed back some data and photographs before radio contact was lost.

In the mid-1990s a new exploratory effort began, with pairs of spacecraft being sent to the planet every two years (to coincide with each Mars opposition). The US Mars Observer probe, launched in 1994, failed during 1995 just as it was entering Mars orbit, but on July 4, 1997, Mars Pathfinder, comprising a 895 kg (1,973 lb) lander and a 10 kg (22 lb) rover (called Sojourner), successfully put down in Ares Vallis, a site carefully selected at the mouth of a major outflow channel system in Chryse Planitia. Multi-spectral cameras identified several different rock types and variable degrees of weathering. An alpha-proton spectrometer aboard Sojourner obtained chemical analyses of selected boulders, as a result of which andesitic lavas were recorded for the first time. Sedimentary rocks containing pebbles were also found. Some drifted material was finer than talcum powder, and several boulders had become coated by this strongly oxidized, windblown dust that appears to be derived from the breakdown of basaltic bedrock. No organic or meteoritic matter was detected.

The American Mars exploration programme suffered setbacks at the end of 1999 with the loss of two NASA spacecraft—the Mars Climate Orbiter and the Mars Polar Lander. However, the Mars Global Surveyor (MGS), launched in 1996, went into orbit and began a detailed topographical mapping of the planet on April 1, 1999, using a laser altimeter that enables measurements to be made to an accuracy of 2 m (6 ft). A map released by NASA in 1999 revealed that the northern hemisphere is about 5 km (3 mi) lower in altitude than the south, indicating that the northern regions may have held any oceans that existed on Mars in the past. Further support was given to the ocean theory by the profile of the Martian crust produced by the MGS, which revealed 200-km (125-mi) wide subterranean channels that would once have been surface features. The three-dimensional mapping also showed that the distance between the highest and lowest points on Mars is one-and-a-half times as great as that between Mount Everest and the deepest ocean trench on Earth, and that the thickness of the crust is about 80 km (50 mi) beneath the southern highlands and Tharsis ridge, compared to about 35 km (22 mi) beneath the northern lowlands and Arabia Terra. In April 2001 another probe, Mars Odyssey, was launched; it reached the planet’s orbit in October 2001. In May 2002 scientists announced that the probe had detected large quantities of water-ice crystals less than 1 m (3 ft) below the surface over much of the planet.

Three probes were launched to Mars in 2003—the European Space Agency’s Mars Express and NASA’s two Mars Exploration Rovers. Mars Express went into orbit around Mars in December that year for a two-year survey of the planet. As Mars Express approached the planet it released a British-built lander called Beagle 2, which was targeted to land on Isidis Planitia, a lowland plain, but no communications were received from it. Mars Express carried a stereoscopic camera, a spectrometer to map the mineral composition of the surface, other instruments to measure the composition of the atmosphere and plot its circulation, and a radar to penetrate the upper 2-3 km (1€-2 mi) of Martian crust in search of subsurface ice and water. In an early finding, the spectrometer confirmed that the south polar cap was composed of water ice and frozen CO₂, thereby achieving one of its goals, that of identifying water in some form on Mars.

In January 2004 two American landers, called Spirit and Opportunity, touched down on opposite sides of the planet—Spirit in a crater called Gusev that is once thought to have held a lake, and Opportunity in an area called Meridiani Planum, where deposits of grey haematite, a mineral that forms in the presence of water, have been detected. Both rovers were designed to spend at least three months exploring the surface, analysing rock and soil samples, but continued to function long beyond this limit. In August 2005, NASA’s Mars Reconnaissance Orbiter was launched as a follow-up to the European Space Agency’s Mars Express, and in August 2007 NASA’s Phoenix was launched with the aim of landing on the planet in May 2008 to explore its climate and geology and to continue the search for life. Next to leave for Mars will be NASA’s Mars Science Laboratory, due to be launched in 2009. Future missions will continue the exploration of the surface, leading eventually to a return of Mars samples to Earth.

IV

Atmosphere

The Martian atmosphere consists largely of carbon dioxide (95 per cent), with smaller quantities of nitrogen (2.7 per cent), argon (1.6 per cent), and oxygen (0.2 per cent), and trace amounts of water vapour, carbon monoxide, and noble gases. The atmospheric pressure at the surface of Mars fluctuates by about 30 per cent owing to the seasonal freezing and evaporation of CO₂ at the poles; the average is about 0.6 per cent of that on Earth and equal to the pressure at a height of 35 km (22 mi) in the Earth’s atmosphere. Surface temperatures vary greatly with time of day, season, and latitude. Maximum summer temperatures may reach 17° C (63° F), but average daily temperatures at the surface do not exceed -33° C (-27° F). Owing to the thinness of the atmosphere, daily temperature variations of 100° C (180° F) are common. Poleward of about 50° latitude, temperatures remain cold enough (less than -123° C/-189° F) throughout the winter for some of the atmosphere’s CO₂ to freeze out into the white deposits that make up much of the polar caps.

The amount of water vapour present in the Martian atmosphere is extremely small and variable. The concentration is greatest near the edges of the receding polar caps in spring. Mars is like a very cold, high-altitude desert. Surface temperatures and pressures are too low for water to exist in the liquid state in most places on the planet, although it exists in frozen form at the poles and just below the surface.

At certain seasons, some areas on Mars are subject to winds strong enough to move sand on the surface and to suspend dust in the atmosphere. In the southern hemisphere between late spring and early summer, when Mars is near perihelion and the heating of southern near-equatorial latitudes is most intense, dust storms begin to form and may reach global proportions, obscuring the planet’s surface for weeks or even months. The dust entrained in these clouds is very fine and takes a long time to settle.