... 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. ... 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. ... Publications with keywords: Mars - sand dunes . Publications with words: Mars - sand dunes . ...