These brightly reflecting fields of snow or frost are on the slopes of a crater rim in the northern hemisphere of Mars. They are 500 meters or so long and have lasted through about eight months of the Red Planet's spring and summer weather.

What causes the unusual white color on some Martian mountains? The answer can be guessed by noticing that the bright areas disappear as springtime takes hold in the south of Mars: dry ice. Unlike water ice, dry carbon dioxide ice sublimates directly to gas from its frozen state.