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Eroded Martian landscape. Image 1 of 2. Computer artwork looking east along Mariner Valley (Valles Marineris) on Mars, where water would have flowed 3.5 billion years ago. Mars is now a desert world, as its thin atmosphere and weak gravity could not keep the water. Melas Chasma canyon (centre to upper right) and Candor Chasma canyon (upper left) form part of the massive Mariner Valley crack in the Martian crust near the equator. The image is hundreds of kilometres wide and the canyon walls are several kilometres high, The topography was drawn with Terragen software, using data from the laser altimeter on the Mars Global Surveyor probe. For water in this area, see image R350/170. (KEYSTONE/SCIENCE PHOTO LIBRARY/KEES VEENENBOS)