From Scientific American:
New research suggests that conditions on the two worlds over the past few billion years would allow Pluto’s traveling atmosphere to freeze out on the frigid moon Charon, while radiation would quickly transform the methane and nitrogen ices to a sticky residue known as tholins.
“Methane is volatile enough that it can only stick to the surface during the long, cold polar winters,” Will Grundy, lead author of the new study, told Space.com by email. Grundy, a planetary scientist at the Lowell Observatory in Arizona, is part of NASA’s New Horizons mission that made a historic flyby of Pluto in July 2015. After modeling conditions on Charon over the past few billion years, he and his colleagues found that, after methane from Pluto froze, radiation stripped away the hydrogen. (Methane is made up of hydrogen and carbon.) The carbon left behind joined with other molecules to make heavier fragments of materials able to stick around even after temperatures warmed [Amazing Pluto Flyby Photos by NASA’s New Horizons]
“As more and more fragments join up, they build progressively bigger and more carbon-rich, complicated molecules until they are so complex that we don’t even try to give them chemical names, just describe them with generic terms like ‘organic molecules’ or ‘tholins,'” Grundy said. “These are what produce the reddish color.”
The research was published online Sept. 14 in the journal Nature.
When New Horizons made its historic flyby of Pluto, it also studied the moons around the dwarf planet. The largest of these moons, Charon, is nearly the size of Pluto itself, leading many scientists to classify the pair as a double-planet system. New Horizons revealed that the massive moon sports a red spot at its northern pole.