Title:[0755] Ground-based telescope photo of Mars by Dr. Robert Leighton Caption: Ground-based telescope photo of Mars by Dr. Robert Leighton Copyright: Credit: National Optical Astronomy Observatories

Title: [0758] Mars from 200,000 miles Caption: A view of Mars obtained by Viking 1 on 18 June 1976 when the spacecraft was about 200,000 miles from the planet. Part of the Valles Marineris canyon can be seen towards the top. The South Pole is in darkness at the lower left. Copyright: Credit: National Optical Astronomy Observatories

Title: [0288] Mars from 31,000 kilometers Caption: From 31,000 kilometers (19,200 miles) above Mars, Viking Orbiter 1 took 15 black-and-white photos through three color filters - violet, green and red - to form this color mosaic of part of the so-called "Grand Canyon of Mars." The canyon, Valles Marineris, lies just a few degrees south of the equator and parallels it for some 5,000 kilometers (3,100 miles) from East to West. In this picture, the equator cuts across the top left corner. North is to upper left. Area covered is about 1,800 by 2,000 kilometers (1,120 by 1,250 miles) - a little more than twice the area of Alaska. The 15 photos, taken near the highest point in the Orbiter's 32nd revolution of Mars (on July 22nd), were computer-processed and combined at the United States Geological Survey Facility in Flagstaff, Arizona. Copyright: Credit: NASA

Title: [0315] Mars from 560,000 kilometers Caption: A color view of Mars made by combining images taken by Viking 1 through violet, green and red filters on 17 June 1976 when the spacecraft was 560,000 km (348,000 miles) from Mars. The Tharsis volcanoes can be seen towards the top and the bright circular feature towards the bottom is a large impact basin, Argyre. Copyright: Credit: NASA

Title: [2003] Mars from Viking 2 Caption: In this view, clouds of water ice can be seen over the western side of the volcano Olympus Mons (the circular dark feature towards the top), as dawn breaks. Icy deposits around the south pole are visible near the bottom. Copyright: Credit: NASA

Title: [0290] Mars' North Pole in mid-summer Caption: This frosty picture near Mars' North pole shows the region in mid-summer when the seasonal carbon dioxide polar cap clears to reveal water ice and layered terrain beneath. Three black-and-white pictures taken Oct. 26 by Viking Orbiter 2 through red, green and blue filters were computer-processed to produce this color composite, which covers about 60 by 30 kilometers (37 by 18 miles). Contact between ice and ground at top occurs at the brink of a scarp about 500 meters (1,640 feet) high. Steps on the scarp face are about 50 meters (165 feet) thick. Regularity of the layers suggests relationship to periodic changes in Mars' orbit- a relationship that, on Earth, may be at least partly responsible for ice ages. Mars' changing orbit may affect frequency and intensity of global dust storms, and hence the amount of material from which sedimentary layered terrain forms. The scarp is apparently an erosional feature; a variety of arc-shaped cliffs illustrates the complexity of erosion in the north polar region. Here, as elsewhere around the north polar cap, dune-like features, darker areas with rippled textures are common. The material that forms the dunes may come from eroded layered terrain, though scientists aren't sure why there is such marked difference in brightness between the materials of the layered areas the dunes. The ice, of unknown maximum thickness, is thin and patchy just above the scarp. Layering in the scarp face is highlighted in places by the frost. Viking took this pictures from a distance of about 2,200 kilometers (1,370 miles). Copyright: Credit: NASA

Title: [0029] Martian landscape from Viking 1 lander Caption: The rock-strewn surface of Mars as viewed from the Viking 1 lander. Copyright: Credit: NASA

Title: [0314] First color photo taken on Mars Caption: The first color image of the martian landscape taken by the Viking 1 which landed in the Chryse Planitia region on 20 July 1976. Copyright: Credit: NASA

Title: [0780] Frost-rimmed crater Caption: Clouds can be seem forming over a frost-rimmed crater near Mare Acidalium in this Mariner 9 image of Mars. Copyright: Credit: National Optical Astronomy Observatories

Title: [0910] Martian impact craters Caption: Cratered terrain on Mars in the Meridiani Sinus region, 15 degrees south of the equator, imaged by Mariner 6. Copyright: Credit: National Optical Astronomy Observatories

Title:[0914] Mariner 9, Nix Olympica Caption:Mariner 9, Nix Olympica Copyright: Credit:National Optical Astronomy Observatories

Title:[0794] Martian rills Caption:Straight rills in Mare Sirenum on Mars, imaged by Mariner 9. Copyright: Credit:National Optical Astronomy Observatories

Title:[0774] Martian sunset Caption:Darkness falls over the Viking 1 landing site on Mars as the Sun sets. Copyright: Credit:National Optical Astronomy Observatories

Title:[0785] Lava flows on Olympus Mons Caption:A close-up view (on the left) taken by Mariner 9 of lava flows on the flanks of the volcano Olympus Mons in the region shown by the white box in the right-hand image. Copyright: Credit:National Optical Astronomy Observatories

Title:[0311] Noctis Labyrinthus Caption:Noctis Labyrinthus is a complex of canyons at the western end of Valles Marineris. In this image taken from a Viking Orbiter at sunrise, the canyons are filled with bright mist, almost certainly made of water ice. Copyright: Credit:NASA

Title:[0759] Clouds over Olympus Mons Caption:This Viking Orbiter image of Olympus Mons taken in the early morning in summer shows diffuse haze and convective cloud encircling the volcano. Copyright: Credit:National Optical Astronomy Observatories

Title:[0772] Sand dunes at Viking I landing site Caption:Sand dunes as well as angular rocks are visible in this view from the Viking 1 landing site in Chryse Planitia. Copyright: Credit:National Optical Astronomy Observatories

Title:[0783] Shadow of moon Phobos on Mars' surface Caption: Shadow of moon Phobos on Mars' surface Copyright: Credit: National Optical Astronomy Observatories

Title:[0791] Sinuous valley Caption:This Mariner 9 image of a valley over 400 km long suggests that water flowed on the surface of Mars sometime in the past. Copyright: Credit:National Optical Astronomy Observatories

Title:[0769] Martian landscape from Viking 2 Caption:Viking 2 landed in Utopia Planitia on 3 September 1976. The area is a vast plain of sediments littered with boulders. Copyright: Credit:National Optical Astronomy Observatories

Title:[0777] Valles Marineris from Mariner 9 Caption:Part of the Valles Marineris imaged by Mariner 9. This huge rift valley extends for more than 5,000 km (3,100 miles) in all, across the equatorial region of Mars. Copyright: Credit:National Optical Astronomy Observatories

Title:[0768] Valles Marineris from the Viking I orbiter Caption:Part of the Valles Marineris imaged by the Viking 1 orbiter. This huge rift valley extends for more than 5,000 km (3,100 miles) in all across the equatorial region of Mars. Copyright: Credit:National Optical Astronomy Observatories

Title:[0763] Viking 1's first photo of the surface of Mars Caption:This first image of the martian surface taken from the ground shows part of the Viking 1 lander and rocks typical of the huge numbers visible in panoramic views taken subsequently. Copyright: Credit:National Optical Astronomy Observatories

Title:[0888] Chryse Planitia Caption:A wide-angle mosaic of the landscape around the Viking 1 landing site in Chryse Planitia. Copyright: Credit:National Optical Astronomy Observatories

Title:[0756] Viking lander Caption:Viking lander Copyright: Credit:National Optical Astronomy Observatories

Title:[0886] Mangala Valles Caption:A Viking orbiter mosaic showing branching channels, which were probably created by water flowing on the surface of Mars in the remote past. Copyright: Credit:National Optical Astronomy Observatories

Title:[0887] Conical volcanoes and fossae on the Tharsis Ridge Caption:Urania Tholus (top center) and Ceraunius Tholus (bottom center), with Ceraunius Fossae to the left. The fossae are a series of parallel shallow depressions in the surface. Copyright: Credit:National Optical Astronomy Observatories

Title:[0889] Viking 1 with "Big Joe" rock and trench Caption:Part of the Viking 1 craft in a view that shows the 2-meter boulder nicknamed "Big Joe" and a trench the lander had dug several days earlier. The boom contains the meteorological instruments for measuring temperature, pressure, and wind speed and direction. Copyright: Credit:National Optical Astronomy Observatories

Title:[0782] Olympus Mons from Mariner 9 Caption:This historic mosaic revealed the nature of the spot previously know as "Nix Olympica" as the giant shield volcano, now called Olympus Mons, topped by a collapsed caldera (volcanic crater). Copyright: Credit:National Optical Astronomy Observatories

Title:[3026] Mars at opposition in February 1995 (HST) Caption:Three views of Mars from the Hubble Space Telescope on 25 February 1995, giving global coverage of the planet, which was 103 million km (65 million miles) away. It appeared cloudier than in previous years. Details as small as 50 km are resolved. In the Tharsis region view, the crescent shaped cloud right of center lies over Olympus Mons. The Valles Marineris is visible in the central image at the lower left. In the right-hand view, the dark area protruding northwards left of center is Syrtis Major Planum. There are clouds over the volcanoes of the Elysium region on the east side (right). Copyright: Credit:Philip James, Steven Lee and NASA

Title:[3027] Mars at opposition in December 1990 (HST) Caption:Mars was 85 million km (53 million miles) from Earth when this Hubble Space Telescope image was taken on 13 December 1990. It shows the dark protruding area known as Syrtis Major Planum. To the southeast of Syrtis Major, the large circular impact basin, Hellas Planitia, is visible. Copyright: Credit:Philip James and NASA

Title:[3028] Enhanced mosaic of Viking images of Mars Caption:104 images taken by the Viking 1 orbiter have been combined and reprocessed to produce this view. The Valles Marineris (lower center) and the Tharsis volcanoes (left) are prominent. Copyright: Credit:NASA/USGS

Title:[3029] Enhanced view of Mars from Viking images Caption:Viking orbiter images from 1976 have been reprocessed to produce this enhanced view of one hemisphere of Mars, showing the south polar cap. Copyright: Credit:NASA/USGS

Title:[4032] Martian meteorite ALH 84001 Caption:If NASA researchers are right, this 1.9-kilogram meteorite contains vestiges of primitive life that thrived on Mars billions of years ago. Meteorite ALH 84001 was discovered in 1984 in Antarctica. ALH 84001 is thought to have been ejected from Mars only 16 million years ago by impact of a large meteorite. Copyright: Credit:NASA-Johnson Space Center

Title:[4033] Martian bacteria-shaped structures Caption:Images obtained with a scanning electron microscope show clusters of elongated shapes no more than 100 nanometers long (4 millionths of an inch). These features are all in (and near) the carbonate mineral globules, which researchers believe to have formed on Mars from liquid water. Copyright: Credit:NASA-Johnson Space Center

Title:[4034] Shapes like bacteria Caption:The features bear a striking resemblance to the earliest microfossils on Earth, which formed 3.45 billion years ago, though their terrestrial analogs were 100 times larger.. Copyright: Credit:NASA-Johnson Space Center

Title:[4035] Terrain at Landing Site of Mars Pathfinder Caption:Portions of Mars Pathfinder's deflated airbags (seen in the foreground), a large rock in mid-field, and a hill in the background were taken by the Imager for Mars Pathfinder aboard Mars Pathfinder during the spacecraft's first day on the Red Planet. Pathfinder successfully landed on Mars at 10:07a.m. PDT July 4, 1997. Copyright: Credit:NASA-JPL

Title:[4031] Sojourner at Mermaid Dune Caption:This color image of the Sojourner rover was taken at the end of day. The rover is perched atop Mermaid Dune, a dark material distinct from the surrounding bright surface. Dark red rover tracks extend from the foreground to the base of the rover's wheels. Copyright: Credit:NASA-JPL

Title:[4028] Sojourner does a wheelie on Yogi Caption:Sojourner has made contact with the rock Yogi in this image, taken with the Imager for Mars Pathfinder at 8:45 p.m. PDT on July 9, 1997. The rover's left rear wheel has driven up onto the Yogi's surface in an attempt to get as close as possible to the rock's surface. Sojourner will later use its Alpha Proton X-Ray Spectrometer (APXS) instrument to conduct a study of Yogi's chemical composition. Copyright: Credit:NASA-JPL

Title:[4030] Clouds over the eastern Martian horizon Caption:Pink stratus clouds are coming from the northeast at about 15 miles per hour (6.7 meters/second) at an approximate height of ten miles (16 kilometers) above the surface. The clouds consist of water ice condensed on reddish dust particles suspended in the atmosphere. The image was taken by the Imager for Mars Pathfinder about forty minutes before sunrise. Copyright: Credit:NASA-JPL

Title:[4029] Sunset over Twin Peaks Caption:This image was taken by the Imager for Mars Pathfinder about one minute after sunset on Mars. The prominent hills dubbed "Twin Peaks" form a dark silhouette at the horizon, while the setting Sun casts a pink glow over the darkening sky. Scientists found that the sky stays bright for up to two hours after sunset, indicating that Martian dust extends very high into the atmosphere. Copyright: Credit:NASA-JPL