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Water on Mars. Computer artwork looking west over the Amazonis Planitia (Amazon Plain) at dusk on Mars 3.5 billion years ago. The 9-kilometre-high Elysium Mons volcano is seen on the horizon, with mist and clouds in the sky. 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)