Deimos is the outer and smaller of Mars's two moons. It is an irregular rocky object having a cratered surface covered with a thick layer of fine debris.
Roughly ellipsoidal in shape, Deimos measures about 15 km (9 miles) in its longest dimension. It revolves once around Mars every 30 hours 18 minutes at a mean distance of 23,459 km (14,577 miles) in a circular orbit that lies within 2° of Mars's equatorial plane. The satellite's long axis is always directed toward Mars; as with Earth's Moon, it has a rotational period equal to its orbital period and so keeps the same face to the planet. Tidal forces are causing Deimos to recede from the planet.
In spite of its tiny gravity, only about a thousandth that of Earth, Deimos has retained considerable amounts of fine regolith on its surface. It thus appears smooth because its craters lie partially
Deimos, the outer and smaller of the two known moons of Mars, photographed by the Viking 2 orbiter in October 1977 from a distance of about 1,400 km (870 miles). Although scarred with impact craters, Deimos appears smoother than its companion moon, Phobos, because it is covered with a thick layer of fine rocky debris (regolith). NASA/Goddard Space Flight Center buried under this loose material. The largest crater, located near the satellite's south pole, is about 2.5 km (1.6 miles) wide. The surface of Deimos is gray and very dark; its reflectance is only 7 percent—about half that of the Moon's surface. This fact and the satellite's low mean density (less than 2 g/cm3[1.16 oz/in3]) indicate a carbonaceous composition and suggest that Deimos may be a captured asteroidlike object.
Since the beginning of the space age, Mars has been a focus of planetary exploration for three main reasons: (1) it is the most Earthlike of the planets; (2) other than Earth, it is the planet most likely to have developed indigenous life; and (3) it will probably be the first extraterrestrial planet to be visited by humans. Between 1960 and 1980 the exploration of Mars was a major objective of both the U.S. and Soviet space programs. U.S. spacecraft successfully flew by Mars (Mariners 4, 6, and 7), orbited the planet (Mariner 9 and Vikings 1 and 2), and placed lander modules on its surface (Vikings l and 2). Three Soviet probes (Mars 2,3, and 5) also investigated Mars, two of them reaching its surface. Mars 3 was the first spacecraft to soft-land an instrumented capsule on the planet, on Dec. 2, 1971; it landed during a planetwide dust storm and returned data for about 20 sec.
Mariner 9, the first spacecraft to orbit another planet, was placed around Mars in November 1971 and operated until October 1972. It returned a wide variety of spectroscopic, radio-propagation, and photographic data. Some 7,330 pictures covering 80 percent of the surface demonstrated a history of widespread volcanism, ancient erosion by water, and reshaping of extensive areas of the surface by internal forces.
The central theme of the Viking missions was the search for extraterrestrial life. No unequivocal evidence of biological activity was found, but the various
A portion of rock outcropping within a small crater in the Meridiani Planum region of Mars, shown in an image made by the Mars Exploration Rover Opportunity in late January 2004. The outcropping varies in height from 30 to 45 cm (12 to 18 inches) along the crater. As interpreted by rover mission scientists, the rock layers apparently were laid down as deposits at the bottom of a body of flowing saltwater, probably on the shoreline of an ancient salty sea. NASA/JPL/Cornell University instruments on the two orbiters and two landers returned detailed information concerning Martian geology, meteorology, and the physics and chemistry of the upper atmosphere. Vikings 1 and 2 were placed into orbit during June and August 1976, respectively. Lander modules descended to the surface from the orbiters after suitable sites were found. Viking 1 landed in the region of Chryse Planitia on July 20, 1976, and Viking 2 landed 6,500 km (4,000 miles) away in Utopia Planitia on Sept. 3, 1976.
In 1988 Soviet scientists launched a pair of spacecraft, Phobos 1 and 2, to orbit Mars and make slow flyby observations of its two satellites. Phobos 1 failed during the yearlong flight, but Phobos 2 reached Mars in early 1989 and returned several days of observations of both the planet and Phobos before malfunctioning.
Amid failures of several U.S. spacecraft missions to Mars in the 1990s, Mars Pathfinder successfully set down in Chryse Planitia on July 4, 1997, and deployed a robotic wheeled rover called Sojourner on the surface. This was followed by Mars Global Surveyor, which reached Mars in September 1997 and systematically mapped various properties of the planet from orbit for several years beginning in March 1999. These included Mars's gravity and magnetic fields, surface topography, and surface mineralogy. The spacecraft also carried cameras for making both wide-angle and detailed images of the surface at resolutions down to 1.5 metres (5 feet). Mars Odyssey safely entered Mars orbit in October 2001 and started mapping other properties, including the chemical composition of the surface, the distribution of near-surface ice, and the physical properties of near-surface materials.
A wave of spacecraft converged on Mars in late 2003 and early 2004 with mixed outcomes. Nozomi, launched by Japan in 1998 on a leisurely trajectory, was the first to reach the vicinity of the planet, but malfunctions prevented it from being put into Mars orbit. In mid-2003 the European Space Agency's Mars Express was launched on a half-year journey to the Red Planet. Carrying instruments to study the atmosphere, surface, and subsurface, it entered Mars orbit on December 25; however, its lander, named Beagle 2, which was to examine the rocks and soil for signs of past or present life, failed to establish radio contact after presumably having descended to the Martian surface the same day. Within weeks of its arrival, the Mars Express orbiter detected vast fields of water ice as well as carbon dioxide ice at the south pole and confirmed that the southern summer remnant cap, like the northern one, contains permanently frozen water. It also detected large sulfur-rich deposits, mainly in Valles Marineris, and clay minerals in the heavily cratered terrains.
Also launched in mid-2003 was the U.S. Mars Exploration Rover mission, which comprised twin robotic landers, Spirit and Opportunity. Spirit touched down in Gusev Crater (15° S, 175° E) on Jan. 3, 2004. Three weeks later, on Jan. 24, Opportunity landed in Meridiani Planum (2° S, 6° W), on the opposite side of the planet. The six-wheeled rovers, each equipped with cameras and a suite of instruments that included a microscopic imager and a rock-grinding tool, analyzed the rocks, soil, and dust around their landing sites, which had been chosen because they appeared to have been affected by water in Mars's past. Both rovers found evidence of past water; perhaps the most dramatic was the discovery by Opportunity of rocks that appeared to have been laid down at the shoreline of an ancient body of salty water.
In 2005 the U.S. launched the Mars Reconnaissance Orbiter, carrying an imaging system with a resolution of 20 cm (8 inches), a spectrometer to determine surface composition, and a ground-penetrating radar. The spectrometer revealed that clay minerals and other alteration products indicative of a warm distant past are common in the ancient cratered terrains, while the radar determined the thickness of the ice at the poles and detected glaciers elsewhere.
In 2008 the U.S. probe Phoenix landed in the north polar region of Mars. Phoenix carried a small chemical laboratory to study the arctic soil. It found water ice underneath the surface of Mars.
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