Where did Pluto's moon get that red hat?

Scientists have discovered that Pluto is spraying its largest moon, Charon, with methane gas that has been turned red by the sun over many decades.

To determine this, researchers analyzed images collected by NASA’s New Horizons spacecraft in July 2015 – which showed a mysterious red spot covering Charon’s polar region – and combined those observations with computer models showing how ice evolves on Charon’s poles.

Reporting their finding in the journal Nature on Wednesday, the research team explained that when methane gas leaks from Pluto’s atmosphere, Charon’s gravity pulls it to its poles, where it freezes on the surface. "It's almost like Pluto is a graffiti artist, spray-painting Charon's poles with its escaping atmosphere, leaving planet-scale colored spots," planetary scientist Will Grundy of Lowell Observatory in Flagstaff, Ariz. writes in an email to The Christian Science Monitor.

Then, ultraviolet light from the sun converts the methane into the reddish materials called tholins.

“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,” Dr. Grundy added in an announcement. In warmer weather, the methane ice sublimates away, leaving behind the heavier and more colorful hydrocarbons.

Pluto, a dwarf planet in the outer reaches of our solar system, and its Texas-sized moon Charon, one of five, orbit the sun every 248 years. This makes for dreadful weather, with a century of total sunlight alternating with another century of total darkness, say scientists. Because surface temperatures in winter can fall to -430 Fahrenheit, methane gas is able to freeze into a solid.

According to the study, Pluto's other moon, Nix, also has a reddish spot. But Nix orbits farther from Pluto and is very small, so the methane artwork isn’t as striking as it is on Charon.

Another study of Pluto on Wednesday reported that the dwarf planet is emitting X-rays, based on NASA's Chandra X-ray Observatory’s detection last year. Only Saturn has been known to emit X-rays until now. The source of Pluto's X-rays is still unknown.

Material from the Associated Press was used in this report.