These 'metallic' dunes on Mars look like sci-fi. What are they really?

If you still picture Mars as a monotonous red desert, it may be time for an update.The European Space Agency's Mars Express orbiter has been capturing some of the Red Planet's most surreal landscapes, and its latest images reveal a sprawling field that looks like molten metal frozen across the floor of an ancient Mars crater.The shimmering "waves" aren't metal at all, however. They are dark sand dunes dusted with seasonal frost, much of it carbon dioxide, or "dry ice," that settles on the surfac