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The formation of Charon’s red poles from seasonally cold-trapped volatiles

Abstract

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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Figure 1: Charon's red northern pole.
Figure 2: Winter pole in Pluto-shine.
Figure 3: Thermal environment.

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Acknowledgements

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.

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Contributions

W.M.G. led the study and wrote the paper, with significant input from G.R.G. and M.E.S. on the escape of Pluto’s atmosphere and its transport to Charon, D.P.C. on radiolytic production of tholins, C.J.A.H. and J.R.S. on thermal models, and T.R.L. on processing of Pluto-shine images. M.W.B., A.J.V., and B.J.B. developed photometric models. A.M.E. and R.P.B. computed insolation as functions of time and location. K.E. processed MVIC approach images. G.R.G., M.E.S., J.Wm.P., and K.D.R. assessed ultraviolet irradiation from various sources. K.N.S., S.B.P., and S.J.R. assessed effects of impacts from dust and larger projectiles. S.A.S. contributed diverse insights and led the overall mission. R.A.B., P.M.S., A.H.P., and A.M.Z. helped with geometric registration and image processing. C.M.D.O., J.C.C., S.Pro, C.B.O., E.Q., B.S., and S.Phi. interpreted infrared spectral data. J.A.S. and O.M.U. contributed insights on cold trapping of volatiles. R.L.M. contributed insights on the charged particle environment. L.A.Y. led development of the observation sequences that produced the data used here, and also checked the thermal models. H.A.W. and A.F.C. played key roles in LORRI instrument development and data processing, and D.C.R., C.B.O., A.H.P., and C.J.A.H. did the same for Ralph. W.B.M., J.M.M., K.N.S., F.N., P.M.S., and V.J.B. contributed insights on geophysics and geology of Charon.

Corresponding authors

Correspondence to W. M. Grundy or W. M. Grundy.

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Competing interests

The authors declare no competing financial interests.

Additional information

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.

Extended Data Figure 2 Additional panchromatic observations of Charon’s poles.

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.

Extended Data Figure 3 Thermal models for cases of low and high thermal inertia.

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

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