  The ever changing transparency of the Martian atmosphere hinders the
determination of absolute surface colour from spacecraft images. While
individual high resolution images from low orbit reveal numerous striking
colour details of the geology, the colour variation between images caused by
scattering off atmospheric dust can easily be of greater magni-tude. The
construction of contiguous large-scale mosaics has thus required a strategy to
suppress the influence of scatter-ing, most often a form of high-pass
filtering, which limits their ability to convey colour variation information
over distanc-es greater than the dimensions of single images. Here we make use
of a dedicated high altitude observation campaign with the Mars Express High
Resolution Stereo Camera1,2 (HRSC), applying a novel iterative method to
construct a globally self-consistent colour model. We demonstrate that the
model can be used to colour-reference a high-altitude mosaic in-corporating
long-range colour variation information, and show that this mosaic, in turn,
can be used to colour-reference high resolution images from low orbit. By using
only the relative colour information internal to individual images, the
in-fluence of absolute colour changes brought about by scattering is minimised,
while the model iteration enables variations across image boundaries to be
self-consistently reconstructed. The resulting mosaic reveals a previously
unseen diversity and detail of colour, closely related to composition, over the
whole of the martian surface.
