On Mars the seasonal polar caps are composed of dry ice (carbon dioxide). In the springtime as the sun shines on the ice, it turns from solid to gas and causes erosion of the surface. Dry ice goes directly from solid to vapor, unlike water ice which melts into liquid when it gets warm.
This image shows eroded channels filled with bright ice, in contrast to the muted red of the underlying ground. In the summer the ice will disappear into the atmosphere, leaving just the channels of ghostly spiders carved in the surface.
This is truly Martian terrain," writes planetary scientist Candy Hansen. "This type of erosion does not take place anywhere naturally on Earth because our climate is too warm."
This image was originally published on the Lunar and Planetary Institute's HiRISE instrument page.