This view was taken by the Viking Orbiter 1 spacecraft. These canyons are some 4000 kilometres long and are over 6 kilometres deep in places. They are thought to have been formed by the Martian crust being stretched apart by the formation of another large landform, the Tharsis Bulge, rather than by erosion by water in the past. Two Viking spacecraft were launched towards Mars by NASA in 1975, each carrying a lander spacecraft and an orbiter. Both succesfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms. No evidence of life was found, but more recent studies of the Martian landscape suggest that in the past surface water may have been present, which could have enabled life to exist.
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