The first detailed photographs of the Martian surface were returned from the flybys of Mariner 4 (1965) and Mariners 6 and 7 (1969). These photographs showed a heavily cratered surface much like that of the Moon. However, the Mariner 9 (1971) photographs revealed that the earlier spacecraft had photographed only a single type of terrain on a planet of great geological diversity. There are volcanoes that dwarf the largest volcanic structures on Earth, a huge canyon complex comparable to the East African rift system, meandering channels with multiple braided features and stream-lined islands, sand dunes, and polar caps. The richness and variety of Martian geologic forms was not fully realized prior to the pictures returned by the Viking 1 and 2 orbiters and landers (1976) and the Mars orbiter cameraaboard the Mars Global Surveyor (1999). The surface of Mars is characterized by a wide variety of volcanic and tectonic landforms. However, there is no evidence of the global system of ridges and trenches that are characteristic of active plate tectonics. Thus, it is concluded that Mars does not have active plate tectonics.
The most striking global feature of the Martian surface is its hemisphericasymmetry. Much of the southern hemisphere of Mars is covered by densely cratered highlands, whereas most of the northern hemisphere is made up of lightly cratered plains. The heavily cratered terrain in the southern hemi100 Plate Tectonics sphere is probably the remnant of the postaccretionary surface of the planet, and the younger northern plains are probably volcanic in origin. The southern highlands cover more than 60% of the surface of Mars. Most of the highland terrain consists of ancient densely cratered rock (largelyimpact breccias) formed early in the planet’s history when impact rates werehigh. Extensive lava flows have covered large areas within the highlands. The large, roughly circular basins of Argyre and Hellas (Figure 1–65) are located in the southern hemisphere and are generally believed to be impact basins similar to the mare basins on the Moon. The Hellas basin has a rim diameter of ∼2300 km and is one of the largest impact structures in the solar system. It is the dominant surface feature of the Martian southern highlands. The Argyre basin has a diameter in excess of 1500 km. Volcanic plains cover much of the northern lowlands of Mars. These volcanic plains are similar to the volcanic plains that dominate other planetary surfaces, e.g., Venus, and they are much less cratered than the southern highlands. The approximately hemispherical dichotomy is generally held to be an ancient, first-order feature of the Martian crust. The dichotomy has been ascribed variously to a very long-wavelength mantle convective planform, to subcrustal erosion due to mantle convection, to post-accretional core formation, to one large impact, and to several impact events. Sleep (1994) has proposed that the lowland crust was formed in an episode of seafloor spreading on Mars. He hypothesized a hemispheric subduction event that destroyed the original primitive crust in the northern hemisphere, and proposed a well-defined sequence of seafloor-spreading events that created the northern volcanic plains. One of the major volcanotectonic features on Mars is the Tharsis region, which is a large elevated region composed of relatively young volcanics. The horizontal scale is some 3000 km with the elevation rising about 10 km above the mean surface elevation. The region exhibits a complex history of episodic tectonism, closely associated with volcanism on local and regional scales. The entire Tharsis uplift appears to be the result of extensive volcanism. Three immense volcanic shields (Arsia, Pavonis, and Ascraeus Montes) form the Tharsis Montes, a linear chain of volcanoes extending northeastward across the Tharsis rise (Figures 1–65 and 1–66). These three shields have gentle slopes of a few degrees (the upper slopes are commonly steeperthan the lower slopes), wide calderas, and flank vents. The shields appear to be the result of basaltic flows and are similar to the intraplate shield volcanoes of the Hawaiian Islands. The Martian shield volcanoes rise 10 to 18 km above the Tharsis rise and attain elevations of 18 to 26 km.
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