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Water on Mars. Computer artwork looking south over the large flood plain of the Simud Vallis (Simud Valley) on Mars 3.5 billion years ago. This valley is an eastern outflow of the massive canyon system of the Valles Marineris (stretching away to top centre). The two narrow channels of the Shalbatana Vallis (right) have been eroded by water flowing north from the Orson Welles Crater (upper right), through the ice-covered heights (white) of Xanthe Terra. The water was ultimately lost due to the thin Martian atmosphere and weak gravity. Mars is now a desert world. 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)