Mystery of Charon's Red Cap Solved

Mosiac of New Horizons images of Charon.

When NASA's New Horizons spacecraft flew through the Pluto system last year, scientists were surprised to find that Charon, Pluto's largest moon, has a dark red polar cap.

A new study may have figured out the reason why: trapped gas.

Lowell Observatory astronomer Will Grundy and colleagues combined analysis of New Horizons imagery with computer models to show that Charon's north pole grew cold enough during its century-long winter to trap methane escaping from Pluto. As sunlight returned to the pole, the methane was then converted into red-colored chemicals, known as tholins, the study shows.

"Who would have thought that Pluto is a graffiti artist, spray-painting its companion with a reddish stain that covers an area the size of New Mexico?" Grundy said in a press release . "Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes."

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.

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.

Read more at Discovery News