Mars, a planet of dramatic contrasts, displays heavily-cratered, moon-like terrains in the southern highlands, towering volcanoes, the Valles Marineris canyon system (2000 miles long and 12,000 feet deep) that dwarfs the Grand Canyon, deep channels gouged by biblical-sized floods, dune fields rivaling the Sahara, and smooth, sparsely-cratered northern hemisphere plains. Its thin, mostly carbon dioxide atmosphere has a surface pressure less than one percent that of the Earth. Surface temperatures range from near freezing during southern hemisphere summer to -190° F at the poles in winter. Although once much wetter, the red planet is now bone dry and frigid (Table 1 on p.4). Any liquid water either seeps into the ground and freezes, or evaporates. Yet copious volumes of water once carved out dendritic (or branched) river valleys-now dry-scattered across much of the cratered southern highlands.
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