You see, sand here on Earth is mostly made of mineral quartz, and that gives it its telltale, bright shades. Over on Mars however, what is not the result of eroded rock is the remnant of volcanic dark basalt, and this is why we’re faced at times with fearsome-looking black dunes.
We have one right here, in the color-enhanced image of the Wirtz Crater the HiRISE camera snapped from an altitude of 252 km (157 miles) back in 2011. It’s a barchan type of dune, meaning it formed as a result of wind blowing mostly from the same direction.
According to the scientists from NASA and the University of Arizona looking at these photos for a living, if you look close enough you can see ripples and dark streaks that are “tracks left by passing vortices known to us as dust devils,” which lift dust off the dune and bring to light an even darker layer beneath.
As for the Wirtz Crater, it is one of the big ones, measuring 120 km (75 miles) in diameter. It holds a large collection of active dunes, meaning ones that keep changing on account of wind and other forces acting upon them.