Surface temperatures during Charon's long winters dip to -430 Fahrenheit, cold enough to freeze methane gas into a solid.
"The methane molecules bounce around on Charon's surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring," Grundy said.
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At point, the methane ice quickly vaporizes, leaving heavier hydrocarbons that were created from it on the surface.
Sunlight further irradiates the hydrocarbons and turns them red.
"New Horizons' observations of Charon's other pole, currently in winter darkness -- and seen by New Horizons only by light reflecting from Pluto -- confirmed that the same activity was occurring at both poles," Johns Hopkins University Applied Physics Lab wrote in a press release about the study.
"The distribution of dark, reddish material around Charon's northern pole is notable for its generally symmetric distribution across longitudes and its gradual increase with latitude, although there are local irregularities associated with craters, topographic features and perhaps subsurface variations in thermal properties," Grundy and colleagues write in this week's Nature.
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"These characteristics ... are consistent with our hypothesis that the combination of Pluto's escaping atmosphere and Charon's long, cold winters enables methane to be seasonally cold-trapped at high latitudes, where some is photolytically processed into heavier molecules that are subsequently converted to reddish tholin-like materials," the study shows.
That left the team wondering if the process could be happening elsewhere. Nix, one of Pluto's four small moons, has a reddish spot, but it orbits farther away form Pluto and is much smaller, which would make the process less efficient, the scientists note.
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