Mystery of Charon's Red Cap Solved

The reddish hue above the north pole of Pluto's largest moon may be caused by trapped gas.

Image: Mosiac of New Horizons images of Charon. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute 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.

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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."

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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.

GALLERY: Dive Into Pluto's High-Resolution Landscape

Newly returned pictures taken by NASA’s New Horizons spacecraft are giving scientists -- and the rest of us -- the most detailed views of Pluto’s stunningly diverse landscape. “We continue to be amazed by what we see,” NASA chief scientist John Grunsfeld said in a statement. The latest images form a strip 50 miles wide and were taken when New Horizons was about 15 minutes away from its closest approach to Pluto on July 14.

PHOTOS: New Pluto Pics Show Beautiful, Complex World

As NASA’s New Horizons spacecraft raced toward a July 14 close encounter with Pluto, the probe’s telescopic long-range camera got to work on a sequence of pictures that revealed features smaller than half of a city block. Pluto’s surface turned out to be unexpectedly diverse, evidence of a complicated and rich geology. The mosaic pictured here starts about 500 miles northwest of Pluto's smooth Sputnik Planum region and covers the rugged al-Idrisi mountains, the shoreline of Sputnik Planum and its icy plains.

This image has been scaled and rotated, for the full, high-resolution view,

check out the mission website


which includes a video


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Scientists aren’t sure yet how some of Pluto’s craters came to contain layers, such as the one picture here in the upper right. “Layers in geology usually mean an important change in composition or event, but at the moment New Horizons team members don’t know if they are seeing local, regional or global layering,” NASA said. New Horizons’ Long Range Reconnaissance Imager (LORRI) took a series of images about 15 minutes before the spacecraft’s July 14 close encounter with Pluto. The dark crater at the center of the image is apparently younger than the others because material thrown out by the impact is still visible. Most of the craters are within a 155-mile wide region known as Burney Basin, the outer rim of which appears as a line of hills at the bottom of this image.

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New Horizons gathered a 50-mile-wide view of Pluto’s rugged northern hemisphere, including a 1.2-mile high cliff, seen here from the left to the upper right, during a series of pictures taken by the spacecraft’s telescopic Long Range Reconnaissance Imager (LORRI) on July 14. The cliff is part of a canyon system that stretches for hundreds of miles across Pluto’s northern hemisphere. Scientists believe the mountains in the middle are comprised of water ice that has been changed by the motion of nitrogen or other exotic ice glaciers over the eons. At the bottom of the image, which was taken when New Horizons was about 10,000 miles from Pluto, the badlands meet the giant icy plains of Sputnik Planum.

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Blocks of ice, upper left, appeared to be jammed together in an area the New Horizon scientists are calling the al-Idrisi mountains. Some of the mountains seem to be coated with a dark material, while others are bright. Scientists think material crushed between the mountains may be from the ice blocks jostling back and forth. The mountains end at the shoreline of a region named Sputnik Planum, which is marked by soft, nitrogen-rich ices that form a nearly flat surface. New Horizon’s Long Range Reconnaissance Imager (LORRI) took a series of images in the span of about one minute at 11:36 Universal Time on July 14, about 15 minutes before the spacecraft’s closest approach.

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