Using images captured at a variety of wavelengths by the European Space Agency's Mars Express Orbiter, Riding with Robots creator Bill Dunford has crafted a composite image of the Red Planet's south polar cap that'll make you stop and stare.
Over at Bad Astronomy, Phil Plait describes some of what it is we're looking at in this multicolored, ethereal, almost creamy-looking view of Mars Australis:
Where you see white is a vast region of permanently frozen water ice, many kilometers thick, covered in winter by a few-meter-deep veneer of frozen carbon dioxide, commonly called dry ice. In the Martian summer, the temperature at the pole gets high enough to turn the dry ice into a gas, but the water ice stays frozen. Not all the dry ice disappears, but even in winter the underlying water ice cap is far thicker than the dry ice above it.
And that frozen carbon dioxide (which is rather mysterious in and of itself) gives rise to some interesting seasonal geophysical phenomena as it warms, for example: sublimating from solid to gas in a way that can leave explosive splotch-marks on the planet's surface.