Once in a Blue Dune

Sand dunes often accumulate in the floors of craters. In this region of Lyot Crater NASA's Mars Reconnaissance Orbiter (MRO) shows a field of classic barchan dunes.

Just to the south of the group of barchan dunes is one large dune with a more complex structure. This particular dune, appearing like turquoise blue in enhanced color, is made of finer material and/or has a different composition than the surrounding.

https://photojournal.jpl.nasa.gov/catalog/PIA22512