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Time for some more sightseeing on Mars. A recent news release from Mars Reconnaissance Orbiter noted the discovery of seasonal avalanches on Mars. This new image of the northern martian sand dunes illustrate again how the surface of Mars changes seasonally. The white patches on the ridges of the sand dunes is frozen carbon dioxide, dry ice that condenses on the crowns of the dunes every winter. When spring comes, the dry ice evaporates, and as it does so it disturbs the underlying sand, which then tumbles down the sides of the dunes, producing the dark streaks.
The process is less dramatic, however, than the avalanches seen in the previous news release, as suggested by this image below, showing the same dunes in summer without the dry ice. The dark streaks in the second image are not significantly different from the first, indicating that the process that forms them is slow and subtle.