Why, for example, does the Martian sky sometimes appear blue? The above image, taken from afar by Hubble, is consistent with what was first seen from the surface by Mars Pathfinder, and shows a sky that can be covered in water-ice clouds where very small particles are responsible for these clouds. They're perhaps one-tenth the size of typical Martian surface dust, or just about one-thousandth the thickness of a human hair. These particles, because they're smaller than the wavelength of visible light, appear bright in the blue portion of the spectrum, but are practically invisible in red light. This teaches us an important scientific lesson as well: the ice particles in Mars' clouds are very small.