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Charon, Pluto's biggest moon
NASA’s New Horizons spacecraft captured this high-resolution, enhanced color view of Pluto’s largest moon, Charon, just before closest approach on July 14, 2015. Scientists have learned that reddish material in the north (top) polar region – informally named Mordor Macula – is chemically processed methane that escaped from Pluto’s atmosphere onto Charon. (Credit: NASA / JHUAPL / SwRI)

When NASA’s New Horizons probe sent back pictures of Pluto’s largest moon, Charon, researchers were surprised to see a big red spot on its north pole. More than a year later, they’ve published their best explanation for its origin.

Mission scientists guessed at the basic outlines of the answer a year ago, but in a paper published today by the journal Nature, they lay out the computer modeling to back up their guess.

The process begins when molecules of methane escape from Pluto’s thin atmosphere. Those molecules are drawn to Charon, a mere 12,200 miles away, by the moon’s gravitational pull. The rarefied methane gas freezes out and settles onto the surface as ice.

Methane ice piles up when it’s winter in the north, but when the season turns toward spring, the northern polar region is exposed to sunlight. The sun’s ultraviolet rays cook the methane into a mix of hydrocarbons.

As the ice warms up, any methane that remains thaws back into gas. But the heavier hydrocarbons stick around on the surface, and get cooked into reddish organic compounds known as tholins.

The computer models developed by the New Horizons team show that Charon’s poles would experience 100 years of continuous darkness, soon followed by 100 years of continuous sunlight.

During the depths of winter, temperatures dip as low as 430 degrees below zero Fahrenheit – cold enough for methane ice. In summer, the temperature rises to 351 degrees below zero. That’s way colder than it ever gets on Earth, but warm enough on Charon to turn methane back into a gas. The models point to 414 degrees below zero F (25 Kelvin) as the transition temperature.

Over the course of millions of years, reddish tholins should build up at the poles like layers of paint.

“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?” lead study author Will Grundy, a New Horizons co-investigator from Lowell Observatory, said in a NASA news release. “Every time we explore, we find surprises. Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes.”

When New Horizons zoomed past Pluto and Charon on July 14, 2015, the south pole was shrouded in wintry darkness. The spacecraft’s cameras could see it only in the ultra-dim light reflected by Pluto’s surface. Nevertheless, those observations confirm that the same process is at work in the south.


The researchers note that Nix, one of Pluto’s smaller moons, shows a slight reddening as well.

“This study solves one of the greatest mysteries we found on Charon, Pluto’s giant moon,” said Alan Stern, New Horizons principal investigator from the Southwest Research Institute. “And it opens up the possibility that other small planets in the Kuiper Belt with moons may create similar, or even more extensive ‘atmospheric transfer’ features on their moons.”

Grundy and Stern are among more than 80 authors of the Nature paper, titled “The Formation of Charon’s Red Poles From Seasonally Cold-Trapped Volatiles.”

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