Pluto doubles as graffiti artist, paints Charon moon red

Pluto's unique moon Charon gets its blushing red pole thanks to an artistic touch from the dwarf planet.

Amanda Kooser
Freelance writer Amanda C. Kooser covers gadgets and tech news with a twist for CNET. When not wallowing in weird gear and iPad apps for cats, she can be found tinkering with her 1956 DeSoto.
Amanda Kooser
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This enhanced-color view of Charon highlights the reddish color of the pole.


Pluto is the Banksy of our solar system. The dwarf planet has a penchant for painting its largest moon in a lovely shade of red using a palette of methane gas.

NASA sent the New Horizons spacecraft all the way to Pluto, where it conducted a close flyby in mid-2015. One of the first major features it noticed along the way was the reddish polar area of the moon Charon. According to NASA, this unusual feature is unique (as far as we know) in the solar system. On Wednesday, the space agency revealed the answer to the mystery of the painted moon.

Charon's tinged polar region originates with Pluto itself. Methane gas escapes from the dwarf planet's atmosphere, gets sucked in by Charon's gravity and sticks to the pole by freezing in place.

"This is followed by chemical processing by ultraviolet light from the sun that transforms the methane into heavier hydrocarbons and eventually into reddish organic materials called tholins," NASA says.

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Scientists published their findings this week in the journal Nature under the title "The formation of Charon's red poles from seasonally cold-trapped volatiles." NASA describes Charon as a Texas-size moon with a diameter of 753 miles (1,212 kilometers). New Horizons co-investigator Will Grundy notes the reddish stain covers an area the size of the state of New Mexico.

The moon's mystery was solved through image analysis and computer modeling that examined how ice changes on Charon over time. The moon's poles feature some spectacularly unfriendly weather with temperatures diving to -430 Fahrenheit (-257 Celsius) during its lengthy dark winter seasons. That's plenty cold enough to freeze methane gas. The data suggests the methane turns back into gas in the moon's warmer springtime, but the red look remains in place.

Charon's redheaded nature developed over millions of years. NASA notes the same activity is happening on Charon's other pole, which was in darkness at the time of New Horizons' visit. Charon may not be alone after all. Scientists will be keeping an eye out for other moons with the same phenomenon as New Horizons continues its exploration of the Kuiper Belt.