From Earth-based observations, Mars appears as a red planet. It is the iron-oxide that gives the planet its reddish appearance. In other words, Mars is a rusty planet. Like the Moon and Mercury, there are both light-colored and dark-colored areas on Mars as shown in Figure 1.6. After all the flyby and orbital missions of NASA, a complete global coverage of Martian surface has become available. It is now clear that the light-colored areas represent the heavily cratered highlands, whereas the dark areas are the relatively smooth lowlands. One striking feature, however, is that the light and dark areas take on a hemispheric distribution. If the highlands are considered the equivalent of continents on Earth, and the lowlands are the equivalent of ocean floors, one hemisphere of the Martian surface would become a continent and the other hemisphere would be covered with an ocean. This is a very interesting feature, considering that two hundred million years ago, there was also only one super-continent and one giant ocean on Earth.

At present, no good explanation exists to account for the hemispheric distribution of these two distinct landforms. The Mars Global Surveyor was able to complete a global topographic mapping using altimetry measurements in 1999. A detailed surface topography map is given in Figure 1.7. In this map, the areas with the warm colors (i.e., red, orange, and yellow) are the heavily cratered southern highlands and the areas with cool colors (i.e., blue and green) are the lowlands. Both Viking landers landed on the smooth northern hemisphere.