New Horizons blew me away with the first images of Pluto. But there was more to the Pluto system than just a stunning world filled with nitrogen glaciers, icy peaks and smooth plains.

Pluto’s largest moon Charon held a few surprises of its own. Deep canyons carve their way across the mostly gray surface. I say mostly gray because Charon’s most stunning feature is an enormous red splotch at the moon’s north pole.

An enhanced color view of Charon makes the red pop from the surrounding terrain.

Charon red north pole

What’s going on here? A new paper in the scientific journal Nature dives into that question. Scientists spent the past year going over images, and other data New Horizons beamed back at an excruciatingly slow 2,000 bits per second. Hell, that’s still better than Comcast sometimes.

Analysis of the data and images shows the red splotch isn’t coming from Charon. It’s coming from Pluto. Methane gas leaks away from Pluto’s atmosphere and is grabbed by Charon’s gravity. It then freezes on the moon’s frigid, icy north pole. But that still doesn’t explain the red color. That’s where the sun comes in. Ultraviolet light from the sun causes the methane to transform into heavier hydrocarbons and eventually into tholins which give the surface its red color.

These same tholins are responsible for turning parts of Pluto’s ice red. The image below is also color enhanced.

Pluto red ice

Here’s how Will Grundy, a New Horizons co-investigator and lead author of the new paper, describes it:

“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? 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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Let’s take a deeper look into how the methane turns into tholins and gives Charon’s north pole the red tint we see today. New Horizons’ team used data gathered by the spacecraft and models to get a better glimpse at the weather at Charon’s north pole.

Charon’s north pole alternates between 100 years of sunlight and 100 years of darkness. During these long, dark winters – temperatures plunge to -430 degrees Fahrenheit. This is important because it’s cold enough for methane gas to freeze into a solid.

According to Grundy, methane molecules bound around the surface of Charon until they either drift back into space or make their way to the north pole. Once at the north pole, they freeze “forming a thin coating of methane ice that lasts until sunlight comes back in the spring,” says Grundy.

As the sun rises on the north pole, the methane ice sublimates away (melts from solid to gas, skipping the liquid phase). While the methane ice turns quickly into a gas, the heavier hydrocarbons formed in this process stay on the surface. Exposure to sunlight turns these hydrocarbons into a reddish material called tholins. This process has repeated for millions of years and gives us the deep, reddish color we see today.

As for the south pole? It’s sitting in a long, dark winter right now – but New Horizons was able to confirm (thanks to light reflecting from Pluto, or Pluto-shine) the same process was happening on the south pole.

Alan Stern, New Horizons principal investigator, says the discovery has implications for other small planets throughout the Kuiper Belt. “It opens up the possibility that other small planets in the Kuiper Belt with moons may create similar, or even more extensive ‘atmospheric transfer’ features on their moons.”

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That possibility may be answered as New Horizons heads for its next target – 2014 MU69. New Horizons will be on the lookout for moons around the small Kuiper Belt object when it flies by on January 1, 2019. And if it finds any, you can bet the New Horizons team will be looking for the same process between Pluto and Charon there.

You can read more about what New Horizons will see during its flyby of 2014 MU69 in my earlier post.

Image credits: NASA

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