Dunes In Meridiani Planum On Mars

HiRISE monitors dune fields across Mars to track how they are changing.
The mobile sand also cleans dust off of the bedrock in inter-dune areas, providing good views of the bedrock structures and colors.

Here we see subtle color differences between layers, and a dense network of fractures. The dunes, in contrast, are uniformly dark and relatively blue in enhanced color (really grey but less red than the bedrock, so they appear blue here).

Larger imagery