Dark slope streak, micro-edition (lower left). With large-scale dark slope streaks, scientists believe they are avalanches of light-tone dust that uncover the underlying dark surface. However, this small-scale streak appears to be a finger of dark material lying on top of the dusty light-toned slope. A couple other small streaks also appear to come from below the rock outcrops, but are lighter in tone, suggesting they are older, and dust has partly erased them.
NASA description: This image was taken by Mastcam: Right (MAST_RIGHT) onboard NASA’s Mars rover Curiosity on Sol 712 (2014-08-07 14:38:40 UTC).