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Eroded Martian landscape. Image 1 of 2. Computer artwork looking east along the edge of the Isidis Planitia (Isidis Plain, left) on Mars. 3.5 billion years ago, a vast sea could have filled the Isidis Plain, a lowland area that is several kilometres lower than the highlands to the south. The thin Martian atmosphere and weak gravity could not keep the water, and Mars became a desert world. The crater mountains of the Libya Montes are seen down right. 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. For water in this area, see image R350/172. (KEYSTONE/SCIENCE PHOTO LIBRARY/KEES VEENENBOS)