Mars may look red from here on Earth, but up close, the surface of the "Red Planet" is actually a variety of colors. In fact, many of them aren't very red-looking at all.
Case in point: the dunes in Mars' northernmost latitudes, like the ones pictured here. But wait a minute, you may be thinking. Those dunes look pretty red to me, or at least a little pinkish. And you'd be right — if what you were describing was in fact the Martian surface. But it isn't. [Hi-res image available here]
What are these black splotches on the surface of Mars? Those black splotches are the surface of Mars. The dunes only appear pink because they are covered in frozen carbon dioxide. NASA HiRISE scientist Cindy Hansen explains:
In the winter, Martian dunes north of 70 degrees latitude are covered by a seasonal layer of carbon dioxide ice (dry ice). In the spring as the ice sublimates (goes directly from solid to gas) numerous seasonal phenomena are observed.
Mars' sand dunes are dark, but appear pinkish because they are still covered by ice. Where the ice has cracked, dark sand is visible. On the steep sides of the dunes sand slides down in thin rivulets.
According to Hansen — who uses images taken by NASA's HiRISE camera to study seasonal changes that take place on Mars' surface — scientists think the biggest, darkest patches (the ones lining the crests of the dunes) may be places where sublimating gas was released in pops; "similar to champagne," she says, "carrying sand out in multiple directions."