It's an enigma no longer. A big red patch on Charon, Pluto's biggest moon, came from a nearby source: Pluto. 

Charon orbits around Pluto about every 6 days, and the rest of the moons in the system orbit around both of those celestial bodies, almost like a binary planet. Now, NASA has announced that the rust-colored spot on Charon came from Pluto— methane gas escaped from Pluto and eventually caused the red color on its biggest moon.

"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?" Will Grundy, a New Horizons co-investigator from Lowell Observatory in Flagstaff, Arizona, and lead author of a new study announcing the finding in the journal Nature, said in a statement.

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At 753 miles in diameter, Charon is frequently compared to the size of Texas, but unlike the Lone Star State, it wouldn’t be a comfortable place to live: at the moon’s poles, 100 years of sunlight gives way to 100 years of darkness. It’s very cold during the dark phase; the temps get down to -430 Fahrenheit.

The methane freezes in that cold environment.

“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 in the statement.

The end result— after sublimation (when a solid becomes a gas) and the sun’s light on what’s left behind— is the red material. The scientists think the same thing happens at the other pole.

NASA’s New Horizon’s spacecraft gets the credit for taking the picture of Charon and its red pole when it flew by Pluto in 2015.

Follow Rob Verger on Twitter: @robverger