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The character of the Martian terrain has been well established from spacecraft photography and altimetry. Almost the entire planet has been photographed from orbit at a resolution of 20 meters (66 feet) and selected areas at resolutions as high as 20 cm (8 inches). In addition, the laser altimeter on Mars Global Surveyor measured surface elevations for the entire planet, averaged over a circle 300 meters (1,000 feet) across to a vertical accuracy of 1 meter (3.3 feet).
Many maps have been made to illustrate topography, geology, temperature, mineral distributions, and a variety of other data. After Mariner 9 the prime meridian on Mars—the equivalent of the Greenwich meridian on Earth—was defined as passing through a small crater named Airy-0 within the larger crater Airy. Longitude was measured in degrees that increase to the west of this meridian completely around the planet. Later some scientists expressed a preference for a coordinate system with longitude that increases to the east of the prime meridian. Consequently, maps of Mars were published with either or both of these systems.
Despite its small size, Mars has significantly more relief than Earth. The lowest point on the planet, within the Hellas impact basin, is 8 km (5 miles) below the reference level. The highest point, at the summit of the volcano Olympus Mons, is 21 km (13 miles) above the reference level. The elevation range is thus 29 km (18 miles), compared with about 20 km (12.4 miles) on Earth—i.e., from the bottom of the Mariana Trench to the top of Mount Everest. Because Mars has no oceans, a reference level for elevations had to be defined in terms other than sea level. In the early 1970s the elevation at which the atmospheric pressure is 6.l millibars (about 0.006 of the sea-level pressure on Earth) was set as the reference. When Mars Global Surveyor acquired more-accurate elevation data, a better reference was needed, and the planet’s mean radius of 3,389.51 km (2,106.14 miles) was chosen.
One of the most striking aspects of the Martian surface is the contrast between the southern and northern hemispheres. Most of the southern hemisphere is high-standing and heavily cratered, resembling the battered highlands of the Moon. Most of the northern hemisphere is low-lying and sparsely cratered. The difference in mean elevation between the two hemispheres is roughly 6 km (3.7 miles). The topographic boundary between the hemispheres is not parallel to the equator but roughly follows a great circle inclined to it by about 30°. In some places the boundary is broad and irregular; in other places there are steep cliffs. Some of the most intensely eroded areas on Mars occur along the boundary. Landforms there include outflow channels, areas of collapse called chaotic terrain, and an enigmatic mix of valleys and ridges known as fretted terrain. Straddling the boundary in the western hemisphere is the Tharsis rise, a vast volcanic pile 4,000 km (2,500 miles) across and 8 km (5 miles) above the reference level at its center. It stands 12 km (7.5 miles) above the northern plains and more than 2 km (1.2 miles) above the surrounding cratered southern highlands. On or near the Tharsis rise are the planet’s largest volcanoes (see below Tharsis and Elysium). Conspicuously absent in either hemisphere are the types of landforms that on Earth result from plate tectonics—for example, long linear mountain chains similar to the Andes, oceanic trenches, or a global system of interconnected ridges.
The hemispheric dichotomy most likely formed when a large asteroid collided with Mars very early in its history. The resulting northern hemisphere impact crater measures roughly 8,500 by 10,700 km (5,300 by 6,600 miles) across; the object that crashed into Mars would have been more than 2,000 km (1,200 miles) across. Gravity data acquired by Mars Global Surveyor suggest that the Martian crust is much thicker under the southern highlands than under the northern plains (see below The interior).