Color coding in this image of Mars represents differences in elevation, measured by NASA's Mars Global Surveyor. While surface liquid water is rare and ephermal on modern Mars.

Color coding in this image of Mars represents differences in elevation, measured by the Mars Orbiter Laser Altimeter on NASA's Mars Global Surveyor. While surface liquid water is rare and ephermal on modern Mars, the topography of Mars reveals large, ancient valley networks and outflow channels. These are evidence that liquid water was more common and played a much more important role in Mars' past.

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