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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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.

Author information

Affiliations

  1. Lowell Observatory, Flagstaff, Arizona, USA

    • W. M. Grundy
  2. NASA Ames Research Center, Moffett Field, California, USA

    • D. P. Cruikshank
    • , K. Ennico
    • , R. A. Beyer
    • , C. M. Dalle Ore
    • , O. M. Umurhan
    •  & J. M. Moore
  3. Southwest Research Institute, San Antonio, Texas, USA

    • G. R. Gladstone
    •  & K. D. Retherford
  4. Southwest Research Institute, Boulder, Colorado, USA

    • C. J. A. Howett
    • , J. R. Spencer
    • , M. W. Buie
    • , J. Wm. Parker
    • , S. B. Porter
    • , K. N. Singer
    • , S. A. Stern
    • , J. C. Cook
    • , C. B. Olkin
    • , A. H. Parker
    • , S. J. Robbins
    • , L. A. Young
    •  & A. M. Zangari
  5. National Optical Astronomy Observatory, Tucson, Arizona, USA

    • T. R. Lauer
  6. George Mason University, Fairfax, Virginia, USA

    • M. E. Summers
  7. Massachussetts Institute of Technology, Cambridge, Massachusetts, USA

    • A. M. Earle
    •  & R. P. Binzel
  8. University of Virginia, Charlotteville, Virginia, USA

    • A. J. Verbiscer
  9. Carl Sagan Center, SETI Institute, Mountain View, California, USA

    • R. A. Beyer
    •  & C. M. Dalle Ore
  10. NASA Jet Propulsion Laboratory, La Cañada Flintridge, California, USA

    • B. J. Buratti
  11. University of Maryland, College Park, Maryland, USA

    • S. Protopapa
  12. Université Grenoble Alpes, CNRS, IPAG, Grenoble, France

    • E. Quirico
    •  & B. Schmitt
  13. Space Telescope Science Institute, Baltimore, Maryland, USA

    • J. A. Stansberry
  14. Johns Hopkins University Applied Physics Laboratory, Columbia, Maryland, USA

    • H. A. Weaver
    • , A. F. Cheng
    •  & R. L. McNutt
  15. Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA

    • V. J. Bray
  16. Washington University in St Louis, St Louis, Missouri, USA

    • W. B. McKinnon
  17. University of California, Santa Cruz, California, USA

    • F. Nimmo
  18. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

    • D. C. Reuter
  19. Lunar and Planetary Institute, Houston, Texas, USA

    • P. M. Schenk
  20. Southwest Research Institute, Boulder, Colorado, USA

    • S. A. Stern
    • , C. B. Olkin
    • , J. R. Spencer
    • , L. A. Young
    • , M. W. Buie
    • , J. C. Cook
    • , C. J. A. Howett
    • , J. A. Kammer
    • , A. H. Parker
    • , J. Wm. Parker
    • , S. B. Porter
    • , H. J. Reitsema
    • , S. J. Robbins
    • , K. N. Singer
    • , A. J. Steffl
    • , C. C. C. Tsang
    • , G. L. Tyler
    • , W. W. Woods
    • , E. F. Young
    •  & A. M. Zangari
  21. University of Colorado, Boulder, Colorado, USA

    • F. Bagenal
    • , M. Horányi
    • , M. Piquette
    • , E. Schindhelm
    • , J. A. Stansberry
    •  & J. R. Szalay
  22. NASA Ames Research Center, Moffett Field, California, USA

    • K. Ennico
    • , J. M. Moore
    • , R. A. Beyer
    • , D. P. Cruikshank
    • , C. M. Dalle Ore
    • , O. M. Umurhan
    •  & O. L. White
  23. Southwest Research Institute, San Antonio, Texas, USA

    • G. R. Gladstone
    • , H. A. Elliott
    • , T. K. Greathouse
    • , D. J. McComas
    • , E. Quirico
    • , B. Schmitt
    • , M. H. Versteeg
    •  & G. E. Weigle II
  24. Lowell Observatory, Flagstaff, Arizona, USA

    • W. M. Grundy
  25. Washington University in St. Louis, St. Louis, Missouri, USA

    • W. B. McKinnon
  26. Johns Hopkins University Applied Physics Laboratory, Columbia, Maryland, USA

    • H. A. Weaver
    • , O. Barnouin
    • , A. F. Cheng
    • , M. E. Hill
    • , P. Kollmann
    • , C. M. Lisse
    • , R. L. McNutt Jr.
    • , J. I. Nunez
    • , J. H. Roberts
    •  & K. Runyon
  27. Universität der Bundeswehr München, Neubiberg 85577, Germany

    • T. Andert
  28. Massachussetts Institute of Technology, Cambridge, Massachusetts, USA

    • R. P. Binzel
    •  & A. M. Earle
  29. University of Bonn, Bonn D-53113, Germany

    • M. Bird
  30. University of Arizona Lunar and Planetary Laboratory, Tucson ARizona, USA

    • V. J. Bray
  31. NASA Jet Propulsion Laboratory, La Cañada Flintridge, California, USA

    • M. Brozović
    • , B. J. Buratti
    •  & J. Hofgartner
  32. Carl Sagan Center at the SETI Institute, Mountain View, California, USA

    • C. M. Dalle Ore
    • , D. P. Hinson
    •  & M. R. Showalter
  33. Rheinisches Institut für Umweltforschung an der Universität zu Köln, Cologne 50931, Germany

    • M. Hahn
    •  & M. Paetzold
  34. University of Maryland, College Park, Maryland, USA

    • D. P. Hamilton
    •  & S. Protopapa
  35. University of Virginia, Charlottesville, Virginia, USA

    • A. D. Howard
    •  & A. J. Verbiscer
  36. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

    • D. E. Jennings
    • , A. W. Lunsford
    •  & D. C. Reuter
  37. National Optical Astronomy Observatory, Tucson, Arizona, USA

    • T. R. Lauer
  38. Université de Reims Champagne-Ardenne, 51687 Reims, France

    • P. Lavvas
  39. Stanford University, Stanford, California, USA

    • I. R. Linscott
  40. Space Telescope Science Institute, Baltimore, Maryland, USA

    • M. Mutchler
  41. University of California, Santa Cruz, California, USA

    • F. Nimmo
  42. Université Grenoble Alpes, CNRS, IPAG, Grenoble, France

    • S. Philippe
  43. Lunar and Planetary Institute, Houston, Texas, USA

    • P. M. Schenk
  44. Johns Hopkins University, Baltimore, Maryland, USA

    • D. F. Strobel
  45. Roane State Community College, Jamestown, Tennessee, USA

    • T. Stryk
  46. George Mason University, Fairfax, Virginia, USA

    • M. E. Summers
  47. Planetary Science Institute, Tucson, Arizona, USA

    • H. B. Throop

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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.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

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

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.

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https://doi.org/10.1038/nature19340

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