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Water on Mars. Computer artwork of river channels (brown, from upper right) creating the Shalbatana Vallis (Shalbatana Valley) on Mars 3.5 billion years ago. North is at bottom right. The water has eroded the ice-covered highlands (white) of Xanthe Terra, flowing into the broad flood plain of the Simud Vallis (across upper left). The Simud is fed by floods from the Marineris Valles (not seen) to the south-west. The water was ultimately lost due to the thin Martian atmosphere and weak gravity. Mars is now a desert world. This image is hundreds of kilometres wide. 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)