Figure 6-57The Topography of Mars The color coding on this map of Mars shows elevations above (positive numbers) or below (negative numbers) the planet’s average radius. To produce this map, an instrument on board Mars Global Surveyor fired pulses of a light at the planet’s surface, then measured how long it took each reflected pulse to return to the spacecraft. The Viking 1 Lander (VL1), Viking 2 Lander (VL2), Mars Pathfinder (MP), Opportunity, and Spirit landing sites are each marked with an X. The volcanoes depicted in white are higher than the scale on the right can show.