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

Published in 2016: Nature 539, 65-68.

W.M. Grundy1, D.P. Cruikshank2, G.R. Gladstone3, C.J.A. Howett4, T.R. Lauer5, J.R. Spencer4, M.E. Summers6, M.W. Buie4, A.M. Earle7, K. Ennico2, J.Wm. Parker4, S.B. Porter4, K.N. Singer4, S.A. Stern4, A.J. Verbiscer8, R.A. Beyer2,9, R.P. Binzel7, B.J. Buratti10, J.C. Cook4, C.M. Dalle Ore2,9, C.B. Olkin4, A.H. Parker4, S. Protopapa11, E. Quirico12, K.D. Retherford3, S.J. Robbins4, B. Schmitt12, J.A. Stansberry13, O.M. Umurhan2, H.A. Weaver14, L.A. Young4, A.M. Zangari4, V.J. Bray15, A.F. Cheng14, W.B. McKinnon16, R.L. McNutt Jr14, J.M. Moore2, F. Nimmo17, D.C. Reuter18, P.M. Schenk19 & the New Horizons Science Team.

(1) Lowell Observatory, Flagstaff AZ.

(2) NASA Ames Research Center, Moffett Field CA.

(3) Southwest Research Institute, San Antonio TX.

(4) Southwest Research Institute, Boulder CO.

(5) National Optical Astronomy Observatory, Tucson AZ.

(6) George Mason University, Fairfax VA.

(7) Massachussetts Institute of Technology, Cambridge MA.

(8) University of Virginia, Charlotteville VA.

(9) Carl Sagan Center, SETI Institute, Mountain View CA.

(10) NASA Jet Propulsion Laboratory, La Cañada Flintridge CA.

(11) University of Maryland, College Park MD.

(12) Université Grenoble Alpes, CNRS, IPAG, Grenoble France.

(13) Space Telescope Science Institute, Baltimore MD.

(14) Johns Hopkins University Applied Physics Laboratory, Columbia MD.

(15) Lunar and Planetary Laboratory, University of Arizona, Tucson AZ.

(16) Washington University in St Louis, St. Louis MO.

(17) University of California, Santa Cruz, Santa Cruz CA.

(18) NASA Goddard Space Flight Center, Greenbelt MD.

(19) Lunar and Planetary Institute, Houston TX.


A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed 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 chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed 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.

Fig. 1. Charon's red northern pole. a, Polar stereographic projection with Ralph's BLUE, RED and NIR filter images displayed in blue, green and red colour channels, respectively, relative to a Hapke photometric model (see Methods). b, Latitude dependence of the reflectance relative to the photometric model. c, Longitudinal dependence of the NIR/BLUE colour ratio. d, Wavelength dependence at two latitudes (coloured points) compared with spectral models of a laboratory tholin plus a neutral material (grey curves). The vertical bars indicate the standard deviation within each latitude bin; the horizontal bars indicate the filter widths.