Defrosting Dunes in the North Polar Sand Sea (MULTISPECTRUM; credits: Lunexit)

This image shows defrosting sand dunes near the North Polar Region of Mars.

Around Mars’ North Pole is a vast Region or “sea” of sand dunes that become covered with CO2 frost or ice in the Northern Hemisphere’s Winter. The light areas indicate that parts of the dunes are still covered in frost or ice.
As Mars’ Northern Hemisphere enters into Spring and begins to warm, the CO2 sublimates. The CO2 sublimates in surprising ways, with trapped gas bursting through the ice in jets that leave dark streaks when the wind is blowing

During the Summer, all the frost will have sublimed leaving dark sand dunes. The unfrosted dunes are dark because the sand is derived from dark volcanic rocks.