The Nili Fossae region contains some of the best exposures of ancient bedrock on Mars.
Ancient bedrock can be tilted, folded, and generally complicated and difficult to understand, but the center of this image shows a stack of nearly horizontal layers. These layers might record how the environment on ancient Mars changed over time, and would be a good site for future exploration by a rover.
The differing colors indicate different rock types of alteration. The darkest patches of ground probably consist of volcanic sand that is trapped in relative low areas. The reddest patches may be covered by dust.
Photo credit: NASA/JPL/University of Arizona