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Water on Mars. Computer artwork looking south-west over water in and around the Pettit Crater on Mars 3.5 billion years ago. The crater is around 100 kilometres across, and is situated in the Amazonis Planitia (Amazon Plain) region of Mars. 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)