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Water on Mars. Computer artwork looking south-west over water in the Tiu Vallis (Tiu Valley) on Mars 3.5 billion years ago. The water is 1.5 kilometres deep. The eastern canyons of the Valles Marineris (Mariner Valley) are seen on the horizon. Mist and cloud are also seen on the horizon and ice (white) lies on the elevated highlands, several kilometres above the sea floor. Mars is now a desert world, but erosion features seen on Mars today suggest that water once flowed there. The water was lost due to the thin Martian atmosphere and weak gravity. The topography of this artwork was drawn with Terragen software, using data from the laser altimeter on the Mars Global Surveyor probe. (KEYSTONE/SCIENCE PHOTO LIBRARY/KEES VEENENBOS)