Mars has a vast sea of sand dunes in the high latitude region encircling its north polar cap, known as the North Polar erg. These dunes are made up of basalt and gypsum sand grains.
In some regions of the North Polar erg where the sand supply is limited they take on an elongated crescent shape. The icy ground that the dunes are on top of has irregular polygonal patterns. In other areas with an abundant supply of sand the dunes are continuous.

Image is less than 1 km (0.67 mi) top to bottom and is 320 km (199 mi) above the surface. North is to the right. For full images including scale bars, visit the source link.