Dark Sand Cascades on Mars . ... Groups of dark brown streaks have been photographed by the Mars Reconnaissance Orbiter on melting pinkish sand dunes covered with light frost. The above image was taken in 2008 April near the North Pole of Mars. ... When occurring near the top of a dune, dark sand may cascade down the dune leaving dark surface streaks -- streaks that might appear at first to be tree s standing in front of the lighter regions, but cast no shadows. ... NASA Official: Jay Norris. ...