This “etch-a-sketch” appearance indicates the presence of dust devil activity in the Thyles Rupes region of Mars.
The dark streaks are a result of a surface-atmosphere interaction that creates thermally-driven vortices. These Martian vortices are giant dust devils that “vacuum” the bright surface dust revealing the darker subsurface rock.
This image was taken in the late southern summer and shows that the dust devil direction of movement changes with the winds as the season progresses from spring to fall. Dust devils are sensitive to changes in low-speed ambient winds and will follow the dominant daytime wind direction.
The direction of the vortex is visible by the scallop pattern that is left behind as a dust devil will erase the front part of the circular track as it follows the wind.
Written by: Circe Verba (19 August 2009)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_013751_1115
Image: NASA/JPL/University of Arizona