On September 21, 1997, the Mars Global Surveyor took pictures of a network of canyons; the one shown here is called the Nigral Vallis. Scientists have two explanations for canyon formation on Mars. First, erosion from surface water cuts downward through sediments. Another method for creating canyons is through erosion associated with groundwater process that causes overlying rock to collapse. All five images show the same area from various perspectives.

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