This observation from NASA's Mars Reconnaissance Orbiter shows the floor of a large impact crater in the southern highlands, north of the giant Hellas impact basin. Most of the crater floor is dark, with abundant small ripples of wind-blown material. However, a pit in the floor of the crater has exposed light-toned, fractured rock.
The light-toned material appears fractured at several different scales. These fractures, called joints, result from stresses on the rock after its formation. Joints are similar to faults, but have undergone virtually no displacement. With careful analysis, joints can provide insight into the forces that have affected a rock, and thus yielding clues into its geologic history. The fractures appear dark, which may be due to dark, wind-blown sand, precipitation of different minerals along the fracture, or both.
Image Credit: NASA/JPL-Caltech/University of Arizona