Several surface features on Mars are notable for their distinctive topography. Others have been studied in depth by probes from Earth. The subsections that follow describe some of these features in greater detail.
Chryse Planitia is a flat lowland region in the northern hemisphere that was chosen for the landing sites of the U.S. Viking 1 and Mars Pathfinder planetary probes. The Viking 1 lander, which touched down at 22.48° N, 47.97° W, on July 20, 1976, revealed that Chryse Planitia is a rolling, boulder-strewn plain with scattered dusty dunes and outcrops of bedrock. Mars Pathfinder confronted a similar scene when it landed at 19.33° N, 33.22° W, on July 4, 1997.
The surface rocks of Chryse Planitia are believed to be eroded remnants of basaltic lavas carried to the site by large floods during Mars's early history. Analysis of the dusty soil by Viking and Pathfinder lander instruments showed the principal constituent materials (in oxide forms by weight) to be silicon (SiO2; 46 percent), iron (Fe2O3; 18 percent), aluminum (Al2O3; 8 percent), magnesium (MgO; 7 percent), calcium (CaO; 6 percent), sulfur (SO3; 5.4 percent), sodium (Na2O; 2 percent), and potassium (K2O; 0.3 percent). This composition is consistent with igneous rocks formed from magmas that interacted with subsurface ice. The rocks were later affected by weathering and leaching processes that stained their surfaces with reddish iron oxide minerals and concentrated certain sulfates (and possibly carbonates) in the surface soil.
The enormous impact basin Hellas in the southern hemisphere of Mars is the planet's largest recognizable impact feature. Centred at roughly 40° S, 290° W, Hellas measures about 7,000 km (4,400 miles) across, including the broad elevated ring surrounding the depression, and 8 km (5 miles) deep. Its floor, covered with partly eroded sediments, is the lowest place on Mars. The basin was probably created by collision with an asteroid very early in Mars's history, not long after the planet formed.
Nirgal Vallis is a sinuous, branching valley located north of the Argyre impact basin, at about 28° S, 42° W. It is about 400 km (250 miles) long and about 5 km (3 miles) wide. Its name derives from the Babylonian word for Mars. First seen in Mariner 9 spacecraft images, the valley has numerous tributaries and appears to have been cut by slow erosion of running water. The source of the water, whether from rainfall, snowfall, or groundwater seepage, is controversial. Also contentious is the origin of gullies on the valley's steep walls that were photographed by the Mars Global Surveyor spacecraft. Some scientists have proposed that they are the result of recent groundwater seepage; others have suggested that they were created by flows of dry or gas-lubricated debris.
Syrtis Major is a distinctive dark marking centred near 290° W and 10° N, which extends some 1,500 km (930 miles) north from the planet's equator and spans 1,000 km (620 miles) from west to east. It was noticed as early as 1659, for it appears in a drawing of Mars of that date by Christiaan Huygens. It is an extensive regional slope elongated north to south that drops 4 km (2.5 miles) from its western boundary (Aeria) to its eastern edge (Isidis). Assiduously observed for more than a century because of its seasonal and long-term variability, especially near its eastern boundary, Syrtis Major was first considered a shallow sea. Later its variability was attributed to vegetation. Close-up photographs and data returned by the U.S. Mariner and Viking planetary probes during the 1960s and '70s enabled investigators to determine that the changes are caused by wind blowing sand and dust across the surface. In the early 1980s detailed topographical maps, prepared from Earth-based spectroscopic and radar observations as well as from the spaceprobe photographs, indicated that Syrtis Major includes a high-altitude bulge rising to 6 km (3.7 miles) at 310° W.
The northern lava plain Utopia Planitia was selected as the landing site of the U.S. Viking 2 planetary probe. Photographs transmitted from the Viking 2 lander, which touched down at 47.97° N, 225.74° W, on Sept. 3, 1976, depicted a boulder-strewn plain that superficially resembles the Viking 1 landing site in Chryse Planitia. Soil-sample analyses conducted by the landers show that the soils at the two sites are nearly identical in composition, which is probably the result of a mixing of windblown dust from wide regions of the planet. The Utopia plain differs from the Chryse area in that it has a system of shallow troughs, which may be associated with ice-wedge activity as a result of permafrost. Vesicular boulders— i.e., those with small gas-formed pits, indicative of a volcanic origin—observed by the lander at the Utopia site may be either local lavas or rocks ejected from the nearby impact crater Mie.
Images from the Viking 2 lander showed the persistence of a thin layer of white ground frost, composed of water ice, for about 100 days during each of the two Martian winters observed. The frost is probably first precipitated together with carbon dioxide, which then sublimes in the sunlight, leaving only the water portion.
Vastitas Borealis is a nearly level lowland plain that surrounds the north pole of the planet Mars and extends southward to about latitude 50°. The plain lies 4-5 km (2.5-3 miles) below the planet's mean radius. In some places it is characterized by numerous low hills of roughly equal size that may be remnants of an ancient cratered surface now almost completely buried by younger material. Elsewhere it has a polygonal fracture pattern reminiscent of those seen in permafrost regions on Earth. The northern part of the plain is covered with a vast dune field that almost completely encircles the pole.
The origin of Vastitas Borealis is controversial. Spacecraft images have made it clear that enormous flood channels, called outflow channels, once transported large volumes of water into this region from the higher elevations to the south. Some scientists have suggested that the smooth, almost level surface of the plain is the result of the deposition of sediments from an ocean formed by the floodwater and covering most of the northern plains; they point to linear features (interpreted as shorelines) and terraces around low hills as supporting evidence. Others question that the level surface and linear features are the result of marine processes; they suggest that the floodwater occupied only a small fraction of the northern plains.
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