This observation shows part of 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 are called joints, and 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 unit of rock, and thus into its geologic history. The fractures appear dark; this may be due to trapping of dark, wind-blown sand in the crack, to precipitation of different minerals along the fracture, or both.
Written by: Colin Dundas (7 July 2010)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_001860_1685
Image: NASA/JPL/University of Arizona