SURFACE FEATURESThe color of the Martian surface ranges from orange to brownish black. The darker materials are weathered
basaltic rock, and the lighter are iron oxides. As seen from Earth, streaked areas of contrasting brightness commonly form within or around topographic features. Many of them change seasonally in form and size, indicating that most of the surface is covered by thin deposits of dust and sand that are easily transported by winds. Photographs of the Martian surface provided by the U.S. Viking landers confirm the presence of windblown deposits and also show pebbles and cobbles strewn across the surface. These observations may be typical of most of Mars, as demonstrated by various measurements by both Earth-based and spacecraft instruments.Because Mars has no oceans and hence no sea level, elevations on the planet are referenced to an artificial datum, or average surface level. Using this datum, the topography of Mars can be broadly divided into
southern cratered
highlands that mostly range from 1 to 5 km (0.6 to 3 mi) above the datum, and relatively smooth northern lowlandsÑcovering nearly 40% of the surfaceÑthat range 0 to 3 km (1.9 mi) below it. Superposed on the highlands is the Tharsis rise, more than 3,000 km (1,864 mi) across, which reaches 10 km (6 mi) in elevation and supports several huge volcanic shields. The smaller Elysium rise is as much as 5 km (3 mi) higher than the surrounding lowland plains.The canyon system of Valles Marineris, the largest and deepest known in the solar system, extends for more than 4,000 km (2,500 mi) and has 5 to 10 km (3 to 6 mi) of relief between its floors and the tops of the surrounding plateaus. Two immense circular basins within the southern highlands, Hellas and Argyre, are 1,500 and 800 km (about 900 and 500 mi) across and 7 and 2 km (4.3 and 1.2 mi) deep, respectively.Each Martian pole is covered by layered deposits, forming a plateau 1,000 to 1,500 km (about 600 to 900 mi) across and mostly 2 to 4 km (1.2 to 2.5 mi) thick. The plateaus are partly capped by thin ice sheets. The northern ice cap covers about two-thirds of its plateau, whereas the southern cap covers only about one-fifteenth of its plateau.