How Pluto ‘spray-painted’ its biggest moon

Image: Charon
NASA’s New Horizons spacecraft captured this high-resolution, enhanced color view of Pluto’s largest moon, Charon, just before closest approach on July 14, 2015. Scientists have learned that reddish material in the north (top) polar region – informally named Mordor Macula – is chemically processed methane that escaped from Pluto’s atmosphere onto Charon. (Credit: NASA / JHUAPL / SwRI)

When NASA’s New Horizons probe sent back pictures of Pluto’s largest moon, Charon, researchers were surprised to see a big red spot on its north pole. More than a year later, they’ve published their best explanation for its origin.

Mission scientists guessed at the basic outlines of the answer a year ago, but in a paper published today by the journal Nature, they lay out the computer modeling to back up their guess.

The process begins when molecules of methane escape from Pluto’s thin atmosphere. Those molecules are drawn to Charon, a mere 12,200 miles away, by the moon’s gravitational pull. The rarefied methane gas freezes out and settles onto the surface as ice.

Methane ice piles up when it’s winter in the north, but when the season turns toward spring, the northern polar region is exposed to sunlight. The sun’s ultraviolet rays cook the methane into a mix of hydrocarbons.

As the ice warms up, any methane that remains thaws back into gas. But the heavier hydrocarbons stick around on the surface, and get cooked into reddish organic compounds known as tholins.

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By Alan Boyle

Mastermind of Cosmic Log, contributing editor at GeekWire, author of "The Case for Pluto: How a Little Planet Made a Big Difference," president of the Council for the Advancement of Science Writing. Check out "About Alan Boyle" for more fun facts.

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