NASA's Mars Reconnaissance Orbiter snapped this series of pictures of sand dunes in the north polar region of Mars, showing how dark sand rises to the top as spring progresses (from left to right) and... more 

NASA's Mars Reconnaissance Orbiter snapped this series of pictures of sand dunes in the north polar region of Mars, showing how dark sand rises to the top as spring progresses (from left to right) and a surface layer of carbon dioxide ice crack less