Sometimes the sand on Mars acts like a liquid. Although liquids freeze and evaporate quickly into the thin atmosphere of Mars, persistent winds may make large sand dunes appear to flow and even drip like a liquid. On the right side the picture which was taken as the season was changing from Spring to Summer in the Southern Hemisphere are two flat top mesas. A light dome topped hill can be seen in the far left of the image. As winds blow from right to left, flowing sand on and around the hills leaves those interesting streaks. The dark arc-shaped droplets of fine sand are called barchans, and are the Martian cousins of similar sand forms here on Earth. Barchans can move intact a downwind and can even appear to pass through each other.
Image Credit: NASA