A piece of Mars: What is this big swoosh? It’s a big dark dune. The dark/light striping across it is found in all of the dunes in this area, but what is it? We’re probably seeing the inside of the dune: the wind may so strong here that it erodes the highest point of the dune, showing off the interior structure. (HiRISE ESP_034909_1744, NASA/JPL/Univ. of Arizona)