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National Geographic : 1973 Feb
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Dwarfing the Grand Canyon, Mars' great rift cleaves the planet's equatorial zone for 2,300 miles (preceding pages). Plunging to 20,000 feet-nearly four times the depth of the Arizona chasm-the rift measures as much as 150 miles from edge to edge. Here the artist portrays a narrow section only two miles deep. Fierce winds and land slides formed the fluting along the rim. Extraordinary mosaic of Mariner 9 photographs re veals about a seventh of the planet. The complex terrain, its features color coded on the map at far right, startled scientists; flyby photographs made by earlier Mariners had suggested a relatively simple surface dominated by cratered plains. Many geologists now suspect that Mars represents an inter mediate stage in planetary development, between the primitive moon and the rest less earth. Slashing across this pano rama, the planet's awesome rift system would more than span the United States. Prob ably caused by subsidence between surface fractures, the huge rift proves that the planet's interior is geologi cally alive. The system's western end thruststoward a broad volcanic zone crowned by Nix Olympica at far up per left. Surprisingly, the colossal canyon escaped detection until the probe of Mariner 9. THIS PAGE FOLDS OUT SMOOTH PLAINS LIGHTLY CRATERED PLAINS MODERATELY CRATEREDTERRAIN CHANNEL AND CANYON DEPOSITS VOLCANIC CRATERS VOLCANIC DOMES VOLCANIC SHIELDS OLD IMPACT CRATERS CHAOTIC TERRAIN GROOVED TERRAIN FRACTURED PLAINS GRABENS (DEPRESSED AREAS BETWEEN FAULT LINES)
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