Formation of Charon's Red Poles From Seasonally Cold-Trapped Volatiles

TL;DR

This paper investigates how seasonal cold-trapped volatiles, originating from Pluto's atmosphere, undergo photolytic processing on Charon's surface, leading to the formation of its distinctive red polar caps.

Contribution

It provides a quantitative model linking Pluto's atmospheric escape, volatile cold-trapping, and chemical processing to Charon's red polar coloration, which was previously only qualitatively suggested.

Findings

Model supports the formation of red poles via cold-trapped, processed volatiles.

Images of Charon's southern hemisphere and approach phase validate the model.

Photolytic processing of Pluto-derived gases explains the observed color pattern.

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 organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location of this material on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons. The escape of Pluto's atmosphere provides a potential feed stock for production of complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation 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 showing the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon, the supply and temporary cold-trapping of material escaping from Pluto, and, while cold-trapped, its photolytic processing into more complex and less volatile molecules. The model results are consistent with the proposed mechanism producing the observed colour pattern on Charon.