NASA's Mars Global Surveyor shows craters in the regions north and northwest of the giant basin, Hellas, with sedimentary rocks with distinct polygonal cracks in them. Such cracks might have formed by desiccation as an ancient lake dried up on Mars.

4 December 2004
Exposures of sedimentary rock are quite common on the surface of Mars. Less common, but found in many craters in the regions north and northwest of the giant basin, Hellas, are sedimentary rocks with distinct polygonal cracks in them. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows an example from the floor of an unnamed crater near 21.0°S, 311.9°W. Such cracks might have formed by desiccation as an ancient lake dried up, or they might be related to ground ice freeze/thaw cycles or some other stresses placed on the original sediment or the rock after it became lithified. The 300 meter scale bar is about 328 yards long. The scene is illuminated by sunlight from the upper left.

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