Traci Watson

Special for USA TODAY

On the outskirts of the solar system lies an icy little moon called Charon with a mysterious blemish: a great dark blotch that in some photos is the color of a dried bloodstain.

First observed last year, the blotch earned the nickname Mordor, after J.R.R. Tolkien’s forbidding “dark land” in The Lord of the Rings, and launched much speculation. Now scientists think they know why it’s there.

Researchers say in this week’s Nature that Mordor was born of material purloined from the nearby dwarf planet Pluto. As Pluto’s atmosphere drifts into space, some of the escaping molecules are captured by Charon and eventually transformed into dark-red chemicals — a phenomenon seen nowhere else in the solar system so far, says Laurence Trafton, an astronomer at the University of Texas, Austin who was not involved with the study.

The mystery of Mordor began last year, when the New Horizons spacecraft became humanity’s first ambassador to Pluto. Before New Horizon’s flyby, telescopes had only captured the fuzziest images of the dwarf planet and its moons.

As the spacecraft approached its target, scientists saw “this persistent dark spot at the top of Charon … (that) never went away,” recalls Will Grundy, study co-author and a planetary scientist at Arizona’s Lowell Observatory. “We were all scratching our heads.”

Perhaps the dark blotch stemmed from the same geological events that carved out Charon’s ridges and canyons. Or perhaps Mordor formed from chemicals from Pluto that became trapped on Charon’s icy surface.

To help settle the matter, Grundy and his colleagues calculated the temperature on Charon during the moon’s long, dark winter. They found Charon’s north pole stays “ridiculously cold” for decades, Grundy says — some -415 degrees Fahrenheit or below from the mid-1800s to the late 1980s.

That’s cold enough, and for long enough, for molecules that drift away from Pluto’s atmosphere to freeze onto Charon’s north pole. Further calculations show such brutal cold doesn’t strike Charon’s midsection, where no dark spot is seen.

The study makes a compelling case that the molecules at the poles are transformed by radiation into forms that don’t waft away, says planetary scientist Orenthal Tucker of the University of Michigan, who was not associated with the study. Gradually, over time, continued radiation blasts those molecules into reddish chemicals.

If the scientists’ calculations are correct, Charon’s south pole ought to have a blotch, too. New Horizons snapped black-and-white images of the south pole showing a dark region similar to Mordor, further confirming the scientists’ thinking about how Mordor formed.

“If there's another one like it at Charon's other pole, I guess that's going to need a name too,” Grundy says. “Any great ideas?”