A unique feature of Plutos large satellite Charon is its dark red northern polar cap. Similar colours on Plutos surface have been attributed to organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location of this material on Charon implicates the temperature extremes that result from Charons high obliquity and long seasons. The escape of Plutos atmosphere provides a potential feed stock for production of complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charons winter pole was proposed as an explanation on the basis of an image of Charons northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase showing the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon, the supply and temporary cold-trapping of material escaping from Pluto, and, while cold-trapped, its photolytic processing into more complex and less volatile molecules. The model results are consistent with the proposed mechanism producing the observed colour pattern on Charon.
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