Why are these sand dunes on Mars striped? No one is sure. The featured image shows striped dunes in Kunowsky Crater on Mars, photographed recently with the Mars Reconnaissance Orbiter��������s HiRISE Camera. Many Martian dunes are known to be covered unevenly with carbon dioxide (dry ice) frost, creating patterns of light and dark areas. Carbon dioxide doesn��������t melt, but sublimates, turning directly into a gas. Carbon dioxide is also a greenhouse material even as a solid, so it can trap heat under the ice and sublimate from the bottom up, causing geyser-like eruptions. During Martian spring, these eruptions can cause a pattern of dark defrosting spots, where the darker sand is exposed. The featured image, though, was taken during Martian autumn, when the weather is getting colder �������� making these stripes particularly puzzling. One hypothesis is that they are caused by cracks in the ice that form from weaker eruptions or thermal stress as part of the day-night cycle, but research continues. Watching these dunes and others through more Martian seasons may give us more clues to solve this mystery.