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Water on Mars. Computer artwork looking south-west over three large craters flooded by water from the Marineris Valles (Mariner Valley) region of Mars 3.5 billion years ago. The Aram Chaos (centre) and the Galilaei Crater (centre right) are seen. The Marineris Valles is an enormous fault system (not seen) to the west, channelling water to this area. This image is hundreds of kilometres across. Mars is now a desert world, as the water was ultimately lost due to the thin Martian atmosphere and weak gravity. The topography was drawn with Terragen software, using data from the laser altimeter on the Mars Global Surveyor probe. (KEYSTONE/SCIENCE PHOTO LIBRARY/KEES VEENENBOS)