Colour composite of Phobos taken with the ExoMars orbiter’s Colour and Stereo Surface Imaging System (CaSSIS) on 26 November 2016. The observation was made at a distance of 7700 km and yields a resolution of 87 m/pixel.
To create the final colour image, two images were taken through each of the four colour filters of the camera – panchromatic, blue–green, red and infrared – and then stitched together and combined to produce the high-resolution composite.
Two of the colour filters used by CaSSIS lie outside the wavelength response of the human eye, so this is not a ‘true’ colour image. However, showing the data as a colour representation can reveal details of the surface mineralogy. Different colours are clearly seen, with the bluest part in the direction of the large crater Stickney, which is out of view over the limb to the left. Although the exact composition of the material is unknown, the colour differences are thought to be caused by compositional variations on scales of hundreds of metres to several kilometres.