HiRISE: Two-color dunes in Meridiani Terra

Why are these dunes different colors? Sand on Mars is typically dark in tone, as it commonly derived from volcanic rocks like lava flows. In HiRISE enhanced color, which operates in long infrared wavelengths (beyond what the human eye can detect), surfaces like these dune crests are dark blue to purple. Sand comes in sizes that are ideal for surface transport by the wind, where sand “hops” along forming bedforms, like these dunes after large amounts of time. The lighter toned “tan” or “reddish” surfaces are indeed composed of…. [More at link]