A dark red northern polar cap on Pluto’s largest moon Charon may be caused by trapped gas, according to research published in Nature this week. Analyses of images from the New Horizon mission and modelling of the evolution of the ice cap help to explain how this polar feature was formed. Previous speculation suggested that methane from Pluto’s atmosphere is trapped in Charon’s north pole and is slowly converted into red material, but there has been no modelling to support this speculation.

Data from the New Horizon mission enable Will Grundy and colleagues to determine whether the conditions around Charon’s north pole could allow the capture and processing of methane gas. Models of the surface environment using Pluto and Charon’s known orbit around the Sun demonstrate that Charon’s north pole has experienced long periods of continuous extremely low temperatures, cold enough to trap methane that has escaped from Pluto’s atmosphere. These long, cold winters last for more than 100 years. The models also suggest that the re-emergence of the winter pole into sunlight triggers the conversion of methane products into red-coloured chemicals.