![Hellas impact basin on Mars, shown in a topographic map (bottom) and an averaged elevation profile …[Credits : NASA/JPL/Goddard Space Flight Center]](http://media-2.web.britannica.com/eb-media/92/76692-004-5C896F65.jpg)
Hellas impact basin on Mars, shown in a topographic map (bottom) and an averaged elevation profile (top) produced from altimetry data collected by Mars Global Surveyor through early 1999. In the map, north is up and the south pole lies at the bottom centre; the black lines highlight contours of zero elevation. In both images altitude is colour-coded according to the accompanying key.
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