Martian sand dune. Coloured satellite image of a crater-floor sand dune. The blue-grey swirling patterns are trails created by small tornadoes, known as 'dust devils', which lifts the lighter coloured martian dust from the surface to reveal the dark-coloured basaltic sand beneath. Ripples on the surface of the dune were created by the wind. Straight dark-coloured lines (lower middle and upper left) are probably caused by landslides. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 24th August 2009. The width of this image is roughly 1.1 kilometres across.