Recent laboratory work indicates that the color results when ultraviolet light and the solar wind react with hydrocarbons and transform them into a soup of organic compounds called tholins.
Until NASA’s New Horizons spacecraft drifted past Pluto in 2015, very little was understood about Charon, which is Pluto’s largest moon, with a diameter of approximately 750 miles. However, New Horizons’ close-up images indicated a frozen world with a ruddy-colored polar cap in the moon’s northern hemisphere.
At the time, scientists believed that the red cap was produced when ultraviolet light from the sun broke down molecules of methane that had exited Pluto and been captured by Charon. That response resulted in “tholins,” a term Carl Sagan meant to refer to a soup of organic compounds. Yet, the latest research described in two papers by scientists at the Southwest Research Institute (SwRI) in Texas indicates that the process that forms the tholins is a bit more convoluted.
“Our findings indicate that drastic seasonal surges in Charon’s thin atmosphere, as well as light breaking down the condensing methane frost, are key to understanding the origins of Charon’s red polar zone,” SwRI’s Ujjwal Raut, a planetary scientist and lead author of one of the latest papers illustrating the new findings, announced during a statement.