The depicted region reveals very strong light and dark contrasts. Against the Martian landscape, with its well-known ochre colouring, larger dark areas can be seen in two craters that have been almost levelled by erosion, as well as a pattern of fine dark lines spread over an area of over 2000 square kilometres. The large dark area in the northern crater (on the right-hand side of the image), around 35 kilometres across, is a dune field of dark sands blown by the wind. A small dune field can be seen on the floor of the southern crater (on the left-hand side of the picture), this time consisting of material from the exposed dark layers towards the top of the crater wall. The dark lines in the centre of the image are aeolian phenomena, that is, they are caused by wind. These are the tracks of small whirlwinds, or ‘dust devils’, which are the result of atmospheric turbulence. When a dust devil travels across the Martian surface, it lifts a thin layer of light-coloured sand from the ground, exposing the underlying darker material. The fine dark tracks mark the paths taken by dust devils.