Steep-sloped dunes in Mars’ north polar region are etched with dark streaks and circular bull’s-eye blotches in this image from the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter, part of the recent PDS release of over 400 new images acquired in 2011. These are known as barchan dunes; wind-driven piles of soil that move across the landscape in the direction of their steepest slope. They are seen in many areas of Mars, as well as on Earth.
During the frigid Martian winter, carbon dioxide collects and freezes in underground pockets. But as springtime brings more sunlight and relatively milder temperatures to the northern hemisphere, these pockets of dry ice can sublimate and burst through the surface, scattering darker soil onto the surrounding ground.
If the area is a slope, the dark material slides downhill. If the land is flat, the material falls back down in a surrounding ring. If there happens to be a stiff Martian breeze the material will be cast in the direction of the wind, creating a fan.