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 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.

Geologic Landforms and Chronostratigraphic History of Charon as Revealed by a Hemispheric Geologic Map

“…Mordor is the largest observed dark deposit on Charon, and Gallifrey the second‐largest on the encounter hemisphere. However, they might have different deposition mechanisms, for Grundy, Cruikshank, et al () found that Mordor could form from seasonally cold‐trapped volatiles; given that Gallifrey is equatorial, the same argument could not be made. Further hindering a straightforward interpretation, in LEISA spectra, deposits coincident with Ripley crater—which are just dark and not red—are similar to both Mordor and Gallifrey.…”

“…Charon, the smaller companion in the binary Pluto‐Charon system, is a 606.0 ± 3.0‐km‐radius body (Nimmo et al, ) that NASA's New Horizons spacecraft revealed to be geologically diverse when it flew by in July 2015. Numerous researchers have summarized some of its diverse landforms (e.g., Grundy, Binzel, et al, ; Moore et al, ; Stern et al, ) including its large, red polar deposit (Grundy, Cruikshank, et al, ); global tectonics system (Beyer et al, ); features of its equatorial plains (Beyer et al, ); crater population (Robbins et al, ); and craters with abrupt terminus ejecta (Robbins, Runyon, et al, ).…”

Charon tectonics

“…Mordor Macula itself appears to have a very different color signature than the rest of Charon ( Grundy et al, 2016b ), but its structural history is difficult to discern. The stereo data show a very distinct topographic ridge that appears to bound the majority of the dark red material of Mordor Macula running from approximately longitude 215° (where the topographic data begin in the west) to longitude 21° where the ridge (and the dark red material) is interrupted by the Dorothy Gale crater.…”

The Global Color of Pluto from New Horizons

“…Most of these observations additionally included Charon and some observed Nix and Hydra as well. Other papers by Howett et al (2017) and Grundy et al (2016) describe the color of Charon and a mechanism to explain its red North Pole. The colors of the small satellites have been discussed in Weaver et al (2016).…”