THEMIS Image of the Day, December 28, 2018. Located in the center of this VIS image is a group of sand dunes. With enough wind and sand, sand dunes are formed.
Dune morphology typically has a shallow slope on the side the wind is blowing from and a steep face on the other side. The darker part of the dunes in this image are the steep slopes. Wind blows sand particles up the shallow slope and then the particles fall off the crest of the dune down the steep side. With time, the constant wind will move the crest of the dune forward.
Depending on the amount of available sand, dunes can grow to large heights and sizes. In the case of this image, the dunes are moving toward the top of the image, which means up the surface slope. In cases such as this, the dunes climb up hills.