When NASA's New Horizons probe zipped by Pluto in 2015, scientists were blown away by the complexity of the dwarf planet. But they were also pleasantly surprised by Pluto's largest moon, Charon: It appeared to have a relatively young, complex surface, suggesting recent geological activity had smoothed over ancient impact craters. It also featured intriguing variations across its terrain, including a bright red spot sitting atop its polar region like a festive cap. A new study lends evidence to the popular theory that Pluto itself is responsible for coloring its moon.

There were signs of this partnership from the very beginning. Charon's spot, dubbed "Mordor Macula" by the New Horizons team, appears to be the same reddish hue that tints much of Pluto's surface. Because New Horizons had already detected a wispy, lightly held atmosphere on Pluto, scientists speculated that the spot may be the result of some transfer of material from the dwarf planet to its moon.

Pluto gets its rusty-red coloring from tholins — tar- or soot-like particles formed when nitrogen and methane break apart in ultraviolet light and recombine in new, complex macromolecules — created high in the planet's upper atmosphere. Because Pluto is so small and has so little gravity, Pluto loses a lot of its atmosphere to space. Charon, which is nearly half of Pluto's size and stays locked in an orbit with it, could presumably catch some of that atmosphere, and its chilly polar regions might be the only place cold enough to keep the gas from immediately escaping. But researchers were not sure that it would be possible for Charon to turn that gas into tholin macromolecules.

By combining images from New Horizons' flyby with computer modeling, researchers determined that Charon probably experiences winters plenty cold and long enough to keep methane trapped in its poles. And in the spring, the sun's rays help paint the town red.

According to a study published Wednesday in Nature, this atmospheric transfer is indeed a plausible origin story for Charon's cap.

As Pluto and Charon slowly orbit the sun — an orbit that takes 248 years — Charon's poles go through century-long periods of total sunlight and total darkness. During its pitch-black, 100-year winter, Charon's poles settle down to a frosty minus-430 degrees Fahrenheit. At that temperature, methane freezes solid.

“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,” lead study author Will Grundy, a New Horizons co-investigator from Lowell Observatory, said in a statement.

When the methane ice turns back to gas and drifts into space, heavier hydrocarbons remain on the surface. Ultraviolet radiation from the sun helps turn those particles into reddish tholins. Over millions of years, enough of them have accumulated to give parts of Charon a deep-red coat. Although only the north pole has been imaged directly — the southern half of Charon was plunged in darkness when New Horizons flew by — there's evidence that the south pole is similarly bedecked in red.

For fans of Pluto and Charon, these results will come as no surprise. The dwarf planet and its largest moon have an unusually intimate relationship. Charon is the largest moon in relation to the size of its host planet ever discovered. The celestial bodies are also tidally locked, meaning that Charon and Pluto constantly face one another, orbiting around a common point like two ice dancers with their hands clasped between them. In many ways, Pluto and Charon seem more like a binary planet system than a dwarf planet and its 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?" Grundy asked in a statement. "Every time we explore, we find surprises. Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes." 

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