These dark brown streaks across frosted pink sand dunes on the red planet have the appearance of trees, but do not cast shadows.
Taken by the Mars Reconnaissance Orbiter as part of the the High Resolution Imaging Science Experiment (HiRISE), the images show features among a vast dune region near the martian North Pole.
Over winter, carbon dioxide in the atmosphere freezes, forming an icy layer over the dunes, and evaporates in the spring when warmed by the sun. During this process, sand falls down the dunes to form dark streaks.
This dark color may be due to dust settling on the ice after several such events. The image below also shows polygonal cracks in the ice that disappear when it thaws, as well as plumes of dust created by falling material.
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