This observation shows bright and dark slope streaks in craters in the Arabia Terra region.
Slope streak formation is among the few known processes currently active on Mars. The cause of slope streaks is still debated, and both dry and wet processes have been proposed to explain their formation. They are most commonly believed to form by gravity-driven movement of extremely dry sand or very fine-grained dust in an almost fluid-like manner (analogous to a terrestrial snow avalanche) exposing darker underlying material.
The darkest slope streaks are the youngest and can be seen to cross cut and lie on top of the older and lighter-toned streaks. The lighter-toned streaks are believed to be dark streaks that are brightening with time as new dust is deposited on their surface. Where they occur, dark slope streaks are typically more plentiful than the bright streaks. However in this area, distinct bright slope streaks appear to be more plentiful, especially in the two smaller craters on either side of the larger crater in the center of the image.
Image credit: NASA/JPL/University of Arizona
Note: For more information, see PIA18819: Bright Slope Streaks in Arabia Terra.