Source of Charon’s red north pole is probably Pluto

The ruddy north pole on Charon, the largest moon of Pluto, is probably a stain from Pluto itself, researchers report online September 14 in Nature. Methane gas wafting from Pluto’s surface sticks to the frigid pole during the moon’s decades-long winter; ultraviolet light from the sun then transforms the methane into reddish organic goop known as tholins.

Pluto’s methane has been a suspect ever since researchers first saw Charon’s rust-colored polar cap — not seen anywhere else in the solar system — during the 2015 flyby of the New Horizons spacecraft. Will Grundy, a planetary scientist at Lowell Observatory in Flagstaff, Ariz., and colleagues used images of Charon and calculations to demonstrate that methane from Pluto is a reasonable culprit in forming Charon’s red pole.