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Preliminary images sent back by the orbiting Mars Global Surveyor show a desert-like terrain pock-marked with craters. The mapped region (above) is called Nirgal Vallis, which is one of a number of canyons known as valley networks.
The origin of these valleys is the subject of debate. Some believe they were formed by water flowing across the surface, but other suggest they were formed by the collapse and erosion associated with groundwater processes.
A closer look at the terrain is likely to help resolve the controversy.
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Surveyor maps desert valleys of Mars. Nature 389, 530 (1997). https://doi.org/10.1038/39127
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DOI: https://doi.org/10.1038/39127