When dunes are located in a complex topographical area such a canyon, they become ideal candidates for detecting changes to their shapes and sizes over time.
The dunes here in Ganges Chasma--a canyon that's on the eastern end of Valles Marineris--could be strongly influenced by winds, and it's important not to underestimate the erosional power of wind. Because HiRISE has such good resolution, we can track these changes over time to tell us which way a dune is moving, how much, and in what direction. From these observations we can decipher present-day atmospheric processes.
Photo credit: NASA/JPL/University of Arizona