. This composite image demonstrates the relative size, similarities and main difference of Earth (left) and Mars (right). Mars is about half the size of the Earth. Both planets exhibit white clouds and polar caps. Bluish-white clouds of water ice hang above volcanoes on Mars (right – center left) and large dark areas extend across its red surface (right - top right). The residual north polar cap on Mars (right top) is made of water ice, and is circled by dark dunes of sand and dust. The Earth also has clouds and polar caps composed of water ice. However, about 75 percent of the Earth is covered with oceans, while liquid water cannot now exist for long times on Mars. The Earth image was taken from the Galileo spacecraft in December 1990, and the Mars image was acquired by the Mars Global Surveyor in April 1999. (Courtesy of NASA and JPL (left and right) and Malin Space Science Systems (right).)
. This perspective of Mars was obtained from the Hubble Space Telescope (HST) on 10 March 1997 – the last day of spring in the Martian northern hemisphere. The red planet was near its closest approach to Earth and a single picture element of the HST spanned 22 kilometers on the Martian surface. The image shows bright and dark markings observed by astronomers for more than a century. The large dark feature seen just below the center of the disk is Syrtis Major Planitia, first seen telescopically by Christiaan Huygens in the 17th century. To the south of Syrtis Major is the large circular impact basin Hellas (center bottom) filled with surface frost and shrouded in bright clouds of water ice. The seasonal north polar cap (top center) is rapidly sublimating, or evaporating from solid dry ice to carbon dioxide gas, revealing the smaller residual water ice cap with its collar of dark sand dunes. (Courtesy of David Crisp, NASA, JPL and the Space Telescope Science Institute.)
. The English amateur astronomer Warren De La Rue (1815-1889) made this drawing of Mars on 20 April 1856, using a 0.33-meter (13-inch) reflector telescope. It shows bright polar caps and a dark, triangular feature now known as Syrtis Major Planitia (Gulf of Sirte Plains). The Dutch astronomer Christiaan Huygens (1629-1695) first sketched this feature in 1659. From his observations of Syrtis Major, Huygens concluded that the rotation period of Mars is about 24 hours. This drawing is reproduced from Camille Flammarion’s 1892 book La Planete Mars et ses Conditions d’Habitabilité.
. During the opposition of 1877, the Italian astronomer Giovanni Schiaparelli (1835-1910) mapped features he thought he saw on Mars, including a vast network of long, thin, straight lines criss-crossing the planet’s surface. Some of the canals have apparently doubled, or divided in two, in this Mercator projection drawn by Schiaparelli during the opposition of 1881 using an 8.6-inch (22-centimeter) refractor. At this time, the apparent diameter of Mars was only 16 seconds of arc. Schiaparelli named these features canali, and they were likened to man-made water canals by subsequent observers, including Camille Flammarion (1842-1925) and Percival Lowell (1855-1916). Nevertheless, most astronomers failed to see the canals, and spacecraft have not detected them.
. Atmospheric water vapor freezes onto the surface of Mars, producing a thin coating of water ice on rocks and soil. These white patches of frost were photographed from the Viking 2 lander at its Utopia Planitia landing site on 18 May 1979; the frost remained on the surface for about 100 days. (Courtesy of NASA and JPL.)
. The surface of Mars is dominated by features created and shaped by the wind, such as these sand dunes in Hebes Chasma. They seem to resemble sand dunes in the Sahara desert on Earth, but probably include finer dust. This image was taken from the Mars Global Surveyor on 13 March 1998. It is 2.3 kilometers wide and centered near 0.8 degrees south and 76.3 degrees west. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. The rough, grooved surfaces of these dunes, found in the Herschel Basin, indicates that the sand is cemented into place. Winds had to scour the surface to remove material. The Herschel basin is named after the British astronomers, William H. Hershel (1738-1822) and his son John F. Hershel (1792-1871). This image was taken from the Mars Global Surveyor on 5 May 1999. It is about 1.2 kilometers wide and centered near 15 degrees south and 228 degrees west. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Wind has been steadily transporting dark sand across the Nili Patera region of Syrtis Major. This image is 2.1 kilometers wide, and it was taken from the Mars Global Surveyor in March 1999. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. In this picture, wind has caused the dark and somewhat crescent-shaped dunes to move toward the lower left, across sets of smaller, bright ridges that also formed by wind action. All of these dunes are located on the floor of an impact crater in western Arabia Terra. This image was taken from the Mars Global Surveyor in February 2000. It is about 2.1 kilometers wide and centered near 10.7 degrees north and 351.0 degrees west. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Strange, beautiful dark spots were created when frost was evaporating from the south polar dunes on Mars. The spots are areas where dark sand has been exposed from beneath bright frost as the south polar cap begins to sublimate and retreat. This image was taken from the Mars Global Surveyor on 1 July 1999. It is about 1.2 kilometers wide, and located at 61.5 degrees south and 18.9 degrees west. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. A giant trough in the north polar cap shows dark sand emerging from beneath a veneer of bright frost leftover from the northern winter. This image of dunes in the Chasma Boreale was taken from the Mars Global Surveyor in September 1998. It is about 1.8 kilometers wide. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Spinning columns of warm air, called dust devils, rise above the Sun-heated surface of Mars. Each tornado-like vortex picks up light-colored dust, exposing the darker surface underneath. Dust devils have created this wild pattern of criss-crossing dark streaks in the rippled flats of Argyre Planitia, covering an area 3 kilometers by 5 kilometers at a latitude of 51 degrees south. This image was taken from the Mars Global Surveyor in March 2000. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. The wind is always blowing the lighter particles around, leaving light and dark streaks across the Martian surface. With changing seasons, the winds alter direction, blowing dust away from some regions to reveal darker rocks beneath and covering other dark rocky regions with the brighter dust. This image shows wind streaks formed in the lee of craters and hills in Daedalia Planum, a broad, wind-swept volcanic plain southwest of the Arsia Mons volcano. It covers an area 7.6 thousand by 9.3 thousand meters, and was taken from the Mars Global Surveyor in March 1999. Similar wind streaks have been recorded at mid-latitudes of Mars using cameras aboard the Mariner 9 and Viking 1 and 2 orbiting spacecraft. As seen from Earth, these steaks can combine to form large bright or dark patches on Mars. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Dust-laden clouds swirl above the north polar region of Mars at the end of local summer. Clouds that appear white consist mainly of water ice, while the orange-brown clouds contain dust. These images were taken at two-hour intervals from the Mars Global Surveyor on 30 June 1999. Storms similar to those shown here continued throughout the month of July and into August. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Nothing on our world matches the global dust storms on Mars, dramatically displayed in this pair of natural-color Hubble Space Telescope images. Surface features that were crisp and clear when the first picture was taken (left) were rapidly covered with blinding dust by the time of the second picture (right). (Courtesy of James Bell, Michael Wolf, the Hubble Heritage Team, NASA, JPL and the Space Telescope Science Institute.)
. In winter the southern polar cap is covered with extensive deposits of solid carbon-dioxide frost. In the southern summer, the cap shrinks to its minimum size shown here, about 400 kilometers across. Even though it is summer, the south polar cap remains cold enough that the residual polar frost consists of frozen carbon dioxide, or dry ice. The polar deposits lie on top of sediments that have been carved by wind into a spiral shape. This image was taken from the Mars Global Surveyor on 17 April 2000. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. The portion of the north polar cap that remains in summer is a towering mountain of water ice, about 1200 kilometers across. The summit, which nearly corresponds with the planet’s spin axis, stands about 3 kilometers above the flat surrounding plains. The north residual cap is surrounded by a nearly circular band of dark sand dunes formed and shaped by wind. This image, which was acquired by the Viking orbiters during the northern summer of 1994, strongly resembles that taken by the Mars Global Surveyor in the northern summer of 1999. Both the north and south residual caps contain deep valleys that curl outward in a swirled pattern that has been cut and eroded into the icy deposits, like a giant pinwheel. But the residual ice cover in the south (Fig. 8.16) is made of frozen carbon dioxide and is about the third the size of the one in the north. (Courtesy of NASA, JPL and the U. S. Geological Survey.)
. These layers, exposed in the south polar residual cap, consist of bright, frozen carbon dioxide and dark, fine dust deposited over millions of years. Water ice could also be mixed in, but no one knows for sure. The layered terrain in both the north and south residual caps is thought to contain detailed records of the climate history of Mars. This image covers an area of 10 kilometers by 4 kilometers. It was taken from the Mars Global Surveyor in October 1999 at 87 degrees south and 10 degrees west, near the central region of the residual south polar cap. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. This mosaic of Viking images displays the southern hemisphere of Mars. This half of the Martian surface retains the cratered scars of an ancient bombardment dating back to the first 500 million years of the solar system, as well as the marks of a continued bombardment since then. The conspicuous, light-colored, circular depression (lower right) marks the Hellas impact basin. Several large craters are located directly northeast (upper left) of Hellas, including those named after Giovanni Cassini (1625-1712), Christiaan Huygens (1629-1695), and Giovanni Schiaparelli (1835-1910). The residual south polar cap is located near the bottom. An enlarged version of the central part of this image is given in Fig. 8.21. (Courtesy of NASA, JPL and the U.S. Geological Survey.)
. White frost fills the Lowell crater, named after the American Percival Lowell (1855-1916). This image taken from the Mars Global Surveyor during autumn in the southern hemisphere of Mars. The crater is 20 kilometers in diameter, and is located at a latitude of 52.3 degrees south and at 81.3 degrees west longitude. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Heavily cratered highlands dominate the Sinus Sabeus region of Mars, located just south of the equator between 0 and –30 degrees latitude. A large impact crater named after the Italian astronomer Giovanni Schiaparelli (1835-1910) marks the northern part of this mosaic image, taken from the Viking orbiters. A full disk image that shows this feature in lower resolution is shown in Fig. 8.19. (Courtesy of NASA, JPL and the U.S. Geological Survey.)
. The lobate, layered material surrounding this crater may have been ejected when an impacting object melted the permafrost, or frozen ground, on Mars. Multiple layers of successive flows resemble the overlapping petals of a flower. The thin flow partly buries one crater, and is halted and deflected by the rim of another one. The muddy sludge seems to have sloshed across the surface, and was then refrozen. Such ejected features have not been found around craters on the Moon or the other planets. This crater, named Yuty after a town is Paraguay, is 19.9 thousand meters in diameters and located in Chryse Planitia at 22.4 degrees north and 34.2 degrees west. (A Viking image courtesy of NASA.)
. Low-lying volcanic plains, each designated as a planitia, are located throughout the northern hemisphere of Mars. Other relatively smooth regions are found at the top of elevated plateaus, each called a planum; they are located in the southern side of the equator. Small solid dots denote volcanoes, including Olympus Mons that rises out of the Tharsis bulge, or uplift. The small crosses designate the landing sites of Viking 1 and Pathfinder in Chryse Planitia (upper right) and Viking 2 in Utopia Planitia (top center).
. Extensive volcanic plains and towering volcanoes are found on Mars. This image, taken from the Mars Global Surveyor, captures lava frozen into the surface of Daedalia Planum, southwest of the Arsia Mons volcano, probably between 1 and 2 billion years ago. An area of 1.5 thousand meters by 2.0 thousand meters is covered in this image. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. The magnetic fields that are currently on Mars are located mainly in the older, heavily cratered southern hemisphere, below the dichotomy boundary (solid line) that separates the highlands (bottom) from the younger, smoother lowland plains in the north (top). A global magnetic field, which no longer exists, was probably imprinted in the highland rocks when they formed more than 4.0 billion years ago, before the lowland plains existed. Hellas and Argyre are two deep impact craters blasted out of the Martian surface after the global magnetism was no longer present. The blue and red colors represent magnetic fields pointing in opposite directions, in and out of the planet, as measured by an instrument aboard the Mars Global Surveyor. They form a banded pattern shown in greater detail in Fig. 8.26. (Courtesy of NASA, JPL and GSFC.)
. Alternating bands of magnetic polarity are most prominent in this part of the southern highlands, near Terra Cimmeria and Terra Sirenium. The magnetic data were obtained from an instrument aboard the Mars Global Surveyor. This map is color coded red for a positive magnetic field pointing out of the planet and blue for a negative one pointing in, with a strength up to 1,500 nanoTesla, or 1.5 x 10-6 Tesla. Stripes of alternating polarity, or direction, extend up to 2000 kilometers across the planet in the east-west direction. They are similar to the magnetic patterns seen in the Earth’s crust at both sides of the mid-oceanic ridges, where the spreading crust has recorded flip-flop reversals in the Earth’s dipolar magnetic field. (Courtesy of NASA, JPL and GSFC.)
. Thousands of Viking images of Mars have been pieced together to provide the first detailed map of the entire globe of another planet. This mosaic, centered on 20 degrees latitude and 60 degrees west longitude, is most like the view seen by a distant observer looking through a telescope. The projection includes the great equatorial canyon system, Valles Marineris (below center), the four huge Tharsis volcanoes (left), and the north polar cap (top). Also note the heavy impact cratering of the highlands (bottom and right), and the younger, less heavily cratered terrains elsewhere. (Courtesy of NASA, JPL, and the U.S. Geological Survey.)
. When Mars was young, it experienced internal adjustments that resulted in a swollen and cracked surface. The most prominent bulge, known as the Tharsis uplift, has four gigantic shield volcanoes on top of it. They are the Olympus, Arsia, Pavonis and Ascraeus Mons. Lava has flowed down the flanks of these volcanoes for perhaps a billion years, but the most recent eruptions could be 300 million years old, or less. Very old, presumably extinct volcanoes are also found on the Tharsis uplift, such as Ceraunius Tholus (Fig. 2.31) with an estimated age of 2.4 billion years.
. The Martian volcanoe Olympus Mons is about 25 kilometers high. The volcano’s flanks have a gentle slope, with a diameter at their base of about 600 kilometers. Thus, Olympus Mons is about 24 times broader than it is high. (Courtesy of NASA, JPL, and Malin Space Science Systems.)
. The summit region of Olympus Mons contains nested caldera pits, radiating lava-flow texture, and broad flow terraces. The crater-like pits have been formed by repeated collapses after eruption, each one marking the point where lava has withdrawn from a chamber within the volcano. The largest collapse center in this Viking image is about 25 kilometers across and nearly 3 kilometers deep. (Courtesy of NASA and JPL.)
. Internal forces split Mars open billions of years ago, creating a vast system of connected canyons (center). Landslides, winds and water have subsequently modified the terrain. Collectively known as Valles Marineris, the canyons extend from the Noctis Labyrinthus in the west (left) to the chaotic terrain in the east (right), spanning more than 4000 kilometers and averaging 8 kilometers in depth. This panorama is a mosaic of images taken from Viking 1. (Courtesy of NASA, JPL and the U.S. Geological Survey.)
. The uplift that created the Tharsis Montes and nearby shield volcanoes (left) seems to have fractured the terrain and opened up an enormous network of chasmata, or canyons, knows as Valles Marineris (center). Catastrophic floods originating in the vicinity of these canyons flowed north (top) into Chryse Planitia, which contains the site of the Viking 1 lander and the Mars Pathfinder lander.
. This valley meanders over 500 kilometers across the heavily cratered terrain on Mars. It is shown in topographic relief with a vertical accuracy of approximately 1 meter provided by the Mars Orbiter Laser Altimeter on the Mars Global Surveyor. Nirgal Vallis is located at 28.4 degrees south and 42.0 degrees west longitude (east longitude shown here). Nirgal is the word for “Mars” in Babylonian. (Courtesy of NASA, JPL and GSFC.)
. The origin of this long, winding valley is enigmatic. Some features, such as the terraces and the small stream in its floor (top), suggest that water flowed continuously across the surface for an extended period of time, deeply eroding the rock layers like rivers do on Earth. Such conditions would suggest that liquid water was once stable on Mars, which would have required a denser atmosphere and a higher ground temperature than exists now. Other features, including the dearth of tributaries, suggest that ground collapse could have also contributed to the valley’s formation. Both continual flow and collapse probably played a role in Nanedi’s formation. This image, taken from the Mars Global Surveyor, covers an area of 9.8 kilometers by 18.5 kilometers; the valley is about 2.5 thousand meters wide, while the small stream near the top is just 200 meters wide. Nanedi Vallis is located at 28.4 degrees south and 42.0 degrees west. Nanedi is the word for “planet” in Sesotho, national language of Lesotho, Africa. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. All of the deep tributaries of Nirgal Vallis have blunt ends and steep walls, with no indication of erosion between them. These characteristics suggest an origin by collapse into cavities that were eroded by underground streams rather than surface rivers. This Viking image is 60 thousand meters across. (Courtesy of Michael Carr and NASA.)
. Massive floods of water from the highlands into the Chryse basin in the lowlands may have carved these channels, located in the region of Mangala Vallis. The tributaries are rather shallow features, and join their main channels at quite acute angles – evidence of their rapid formation. This image, taken from the Viking 1 orbiter, has a width of 400 thousand meters. (Courtesy of NASA.)
. A raised crater rim acts as a barrier to the catastrophic floods that discharged from the outflow channel Ares Vallis. The water flowed from the southwest (bottom left) with a peak discharge more than 2,000 times that of the Mississippi River. (Courtesy of Michael Carr and NASA.)
. In these images, red denotes elevated regions of roughly 8 kilometers above the mean level, represented by green. Blue corresponds to 10 kilometers below the mean elevation. Flowing water would run downhill and collect in such low-lying regions. They include the Hellas basin (upper image), about 2.8 million meters across and the largest impact basin in the solar system, the Valles Marineris (lower image), shown as a horizontal gash beside the Tharsis volcanoes (pink), and the extensive lowland plains in the north (top of both images). These maps of the global topography of Mars were obtained with the laser altimeter on Mars Global Surveyor – also see Fig. 2.39 of Section 2.4. (Courtesy of NASA, JPL and GSFC.)
. The inside cliff faces of this crater are etched with gullies that liquid water may have cut. Thick sequences of layered rock are also present, attesting to a Martian past of substantial geologic activity. Water has apparently seeped from between layers of rock high on the wall, and flowed downhill in deep, channels that have merged together. The lack of small craters superimposed on the gullies and their deposits indicates a geologically young age. They could have formed hundreds, thousands or millions of years ago, and they might even be contemporary. This crater wall shown in this image, taken from the Mars Global Surveyor, is located in central Noachis Terra at 47 degrees south and 355 degrees west. (Courtesy of NASA, JPL and Malin Space Science Systems).
. The gullies on Mars have three parts – the alcove, the channel and the apron. Water seeps out from high up on the wall of a crater or other type of depression and the alcove (right center) forms by collapsing into the material removed by the seepage. Water and debris run down the slope from the seepage area, cutting channels below the alcove and depositing rock, soil and ice into a fanlike apron (bottom middle). This image, taken from the Mars Global Surveyor, covers an area of 1.3 kilometers wide by 2.0 kilometers long. It is located on the south-facing wall of an impact crater in Noachis Terra near 54.8 degrees south and 342.5 degrees west. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Narrow gullies are eroded into the north wall of a small crater, about 7 kilometers across, that is itself located on the floor of the larger Newton Crater, named after the British scientist Isaac Newton (1643-1727). Flowing water may have caused these gullies and transported debris downhill, creating the lobed and finger-like deposits on the floor and at the base of the crater wall (bottom center). The individual deposits were used to estimate that 2.5 million liters, or 660 thousand gallons, of water flowed down each gully. This image, taken from the Mars Global Surveyor, is about 4 kilometers across, and centered at 41.1 degrees south and 159.8 degrees west. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. This image, taken from the Viking 2 lander, shows angular rocks that have been tossed across the Martian surface, perhaps as the debris of a nearby crater-forming impact. They cast razor-sharp shadows in the cold, thin atmosphere. Most of the rocks have numerous small holes due to the bursting of bubbles of volcanic gas and pitting by small meteorites. Fierce winds have also eroded the rock surfaces, and piled up fine-grained soil in their lee. A trough filled with the fine-grained sediment extends from the upper left to the lower right. Just beyond the trough, in the right half of the picture there is a large boulder about 1 meter across. (Courtesy of NASA and JPL.)
. Billions of years ago, when water flowed on Mars, great floods rushed out of the outflow channel, Ares Vallis, and emptied into the Chryse Planitia, or Plains of Gold, region of Mars (color inset). The flowing water carved out streamlined islands around craters (top right). This area was chosen as the Mars Pathfinder landing site for three reasons: it seemed safe, with no steep slopes or rough surfaces; it had a low elevation, which provided enough air density above the surface for a parachute to work, and it appeared to offer a variety of rock types deposited by the floods. The ellipses mark the area targeted for landing of Mars Pathfinder, as refined several times during the final approach to Mars. An X within the smallest ellipse marks the location of the lander at 19.33 degrees north and 33.55 degrees west. The site is about 850 kilometers southeast of the location of Viking 1 lander. (Courtesy of NASA and JPL.)
. On 4 July 1997 Mars Pathfinder landed safely on a windswept plain littered with the debris of catastrophic floods early in the planet’s history, including a diversity of rock sizes and types. Between and partially covering some of the rocks is reddish iron-oxide dust, the result of chemical weathering of exposed rock surfaces here and elsewhere. There are also bare rocks left uncontaminated by the seemingly ubiquitous dust. In the foreground the dust or sand is partly cemented. The two modest-sized hills in the distance have been dubbed the Twin Peaks. They are roughly 30 meters tall and about 1 kilometers from the lander. (Courtesy of NASA and JPL.)
. The tiny rover Sojourner hit the ground with all six wheels running; energized by a solar panel on its top which delivered up to 16 watts of power. The diminutive rover is just over half a meter long, but equipped with three cameras and an instrument that determined the chemical composition of rocks and soil. Sojourner has demonstrated that a small, unmanned rover is an effective vehicle to explore another planet, and that such exploration can be done quickly and relatively cheaply. (Courtesy of NASA and JPL.)
. This image, taken from Mars Pathfinder, shows the rocks named “Shark” and “Half Dome” at upper left and middle, respectively. The rocks in this area are inclined and stacked, as if deposited by rapidly flowing water. The smooth, angular rocks are relatively dust free and apparently uncontaminated, allowing unambiguous measurements of their chemical constituents by the Sojourner Rover. (Courtesy of NASA and JPL.)
. This image, taken with a scanning electron microscope, shows an unusual structure located in a carbonate globule within meteorite ALH 84001 from Mars. The tube-like feature in the center of the image is only about 200 nanometers, or 0.0000002 meters, long, which is about one hundredth the width of a human air. It appears segmented, as if it were a filament composed of separate elements. The minute structure looks like some very small, fossilized bacteria found on Earth. Some scientists interpret it as a possible Martian microfossil of exceedingly small bacteria that may have lived on Mars about 3.6 billion years ago. Other scientists dispute this interpretation. (Courtesy of NASA and JPL.)
. This electron microscope image shows many rod-shaped objects in a carbonate globule within meteorite ALH 84001 from Mars. The minute structures could be microscopic fossils of a colony of primitive, bacteria-like organisms that lived on Mars more than 3.6 billion years ago. Even the largest structures are very small, about 100 nanometers, or 0.0000001 meters, in length; nanobacteria on Earth are the same size. Nevertheless, the tubular structures could also have a non-biological origin that has nothing to do with life on Mars. (Courtesy of NASA and JPL.)
. A close-up image of the Martian moon Phobos, taken from the Mars Global Surveyor. It shows the moon’s highly irregular shape and battered, cratered surface. The largest crater, Stickney (top left), is 10 kilometers in diameter, and about half the size of Phobos. This crater is named after Angeline Stickney (1830-1892), wife of Asaph Hall (1829-1907), who discovered the two Martian moons. Individual boulders can be seen near the rim of the crater, presumably ejected by the impact that formed Stickney. (Courtesy of NASA, JPL and Malin Space Science Systems.)
. Three irregularly shaped, cratered objects, each about the same size, are shown at the same scale and nearly identical lighting conditions. They are Phobos (lower right), the largest moon of Mars, Deimos (lower left), the other Martian moon, and the asteroid 951 Gaspra (top) which is about 17 kilometers across. The Phobos and Deimos images were obtained from one of the Viking orbiter spacecraft in 1977; the Gaspra image was taken from the Galileo spacecraft in 1991 on its way to Jupiter. (Courtesy of NASA and JPL.)
. Summary Diagram.