#PPOD: 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. As winds blow from right to left, flowing sand on and around the hills leaves picturesque streaks. The dark arc-shaped droplets of fine sand are called barchans and are the interplanetary cousins of similar Earth-based sand forms. Credit: HiRISE, MRO, LPL (U. Arizona), NASA (via APOD)
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