A unique feature of Pluto’s large satellite Charon is its dark red northern polar cap1. Similar colours on Pluto’s surface have been attributed2 to tholin-like organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location on Charon implicates the temperature extremes that result from Charon’s high obliquity and long seasons in the production of this material. The escape of Pluto’s atmosphere provides a potential feedstock for a complex chemistry3,4. Gas from Pluto that is transiently cold-trapped and processed at Charon’s winter pole was proposed1,2 as an explanation for the dark coloration on the basis of an image of Charon’s 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 that show the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped. The model results are consistent with the proposed mechanism for producing the observed colour pattern on Charon.
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This work was supported by NASA’s New Horizons Project. E.Q., B.S. and S.Phi. acknowledge the Centre National d’Etudes Spatiales (CNES) for its financial support through its ‘Système Solaire’ programme.
The authors declare no competing financial interests.
All spacecraft data presented in this paper will be delivered to NASA’s Planetary Data System (https://pds.nasa.gov) in a series of stages in 2016 and 2017 in accordance with the schedule established by NASA and the New Horizons project.
Reviewer Information Nature thanks L. Trafton and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Figure 1 Three MVIC colour images obtained on approach showing Charon’s northern pole as Charon rotates.
a, Observation obtained 2015 July 11 3:35 ut, with MET label 0298891582. b, Observation obtained 2015 July 13 3:38 ut, with MET label 0299064592. c, The same observation as Fig. 1a of the main text, obtained 2015 July 14 at 10:42 ut, with MET label 0299176432. Unlike in Fig. 1a, these images are not re-projected or divided by photometric models. North, as defined by the angular momentum vector, is up.
a, Second stack of 120 images of Charon’s southern hemisphere illuminated by Pluto-shine obtained approximately 2 h after the stack in Fig. 2a. b, Corresponding photometric model. c, Observation/model ratio. d, Sunlit northern hemisphere. e, Corresponding photometric model. f, Similar to Fig. 2d, with the first Pluto-shine stack indicated by blue points and the second stack indicated by red points (offset left and right for clarity). The horizontal bars indicate the widths of the latitude bins and the vertical bars indicate the standard deviation of the mean within each latitude bin.
a, Thermal history for lower limit thermal inertia Γ = 2.5 J m−2 K−1 s−1/2. b, Thermal history for upper limit thermal inertia Γ = 40 J m−2 K−1 s−1/2.
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Grundy, W., Cruikshank, D., Gladstone, G. et al. The formation of Charon’s red poles from seasonally cold-trapped volatiles. Nature 539, 65–68 (2016). https://doi.org/10.1038/nature19340
Nature Astronomy (2017)