Pluto’s moon has a mysterious red north pole, and we can finally find out why

Pluto’s life partner Charon has a disarming red “cap”. Since New Horizons broke the moon’s oxide-stained north pole in its 2015 flyby, scientists have been reflecting on the planetary processes responsible for setting such a bold milestone.

Scientists initially suspected that the iron-colored smear (called Mordor Macula) was methane captured from Pluto’s surface, its red color the result of slow cooking in the sun’s ultraviolet light. It was a good idea just to ask if they could try it.

Now, a mix of modeling and lab experiments has found that these early hypotheses were not too far from the mark, with a slight twist. The research adds surprising new details to our understanding of the intimate engagement of Pluto and Charon, suggesting that the color of the moon has more things than it seems at first glance.

Launched in 2006, NASA’s New Horizons interplanetary space probe provided researchers with an unprecedented view of the dwarf planetary system Pluto and Charon at a distance of more than 5,000 million kilometers (3.1 billion miles) from the Sun.

“Before New Horizons, the best images of Pluto’s Hubble only revealed a diffuse spot of reflected light,” says Randy Gladstone, a planetary scientist at the Southwest Research Institute (SwRI) in the United States.

“In addition to all the fascinating features discovered on Pluto’s surface, the flyover revealed an unusual feature to Charon: a striking red cap centered at its north pole.”

Red may not be an unusual color to see in iron-rich worlds like ours, or Mars, therefore. But along the frozen suburbs of the Solar System, red is much more likely to indicate the presence of a diverse group of tar-like compounds called tolins.

If it helps, replace the word tholin with “gunk”. The mess of the red-brown chemicals is like the residue left in the oven, if the oven uses UV light to bake brownies made of simple gases such as carbon dioxide or ammonia.

On Pluto, methane would be a likely starting point. To grow in a toline, these tiny hydrocarbons would simply need to absorb a very specific color of UV light filtered through orbiting hydrogen clouds, called Lyman-alpha.

Pluto’s pink glow has been studied for decades. New Horizons simply revealed the exact pattern of tholins on its surface in glorious high definition. However, finding a rusty dye on his partner’s lid was an intriguing surprise.

Pluto’s spilled methane was supposed to drift into its orbiting moon. But the precise moment needed for the gas to settle and freeze in such a distinctively diffuse smear was always a point of contention.

Part of the problem is the competition between Charon’s faint gravity and the cold light of the distant Sun that warmed its surface. As weak as it may be, spring dawn could be enough to melt the methane frost and expel it back from the surface.

To determine what would actually happen, SwRI researchers modeled the saw motion of the largely inclined planetary system. They found that the secret of the rubbish could be the explosive nature of the arrival of spring.

The relatively sudden warming of the North Pole would occur for several years: a simple flicker in the orbit of the Sun’s 248-year-old moon. During this brief period, a layer of methane ice only tens of microns thick would evaporate at one pole as it began to freeze at the other.

Unfortunately, the modeling found that this rapid movement would be too rapid for much of the frozen methane to absorb sufficient amounts of Lyman-alpha to become a tolin.

But ethane, the body of hydrocarbons a little longer than methane, would be another story.

“Ethane is less volatile than methane and stays frozen on the surface of Charon long after the spring sun rises,” says planetary scientist Ujjwal Raut, lead author of a second study that modeled changes in the densities of methane evaporation and freezing.

“Exposure to solar wind can turn ethane into persistent reddish surface deposits that contribute to the red layer of Charon.”

Together with the results of laboratory experiments, the study by Raut and his team showed a feasible way to convert methane to ethane in the poles.

There was only one problem. Lyman-alpha radiation will not turn ethane into a reddish mud.

This does not rule out hydrocarbons. Charged particles flowing from the Sun for a longer period of time could still generate longer and longer hydrocarbon chains that would give Charon its characteristic red cap.

“We believe that ionizing radiation from the solar wind breaks down polar ice cooked with Lyman-alpha to synthesize increasingly complex, red materials responsible for the unique albedo on this enigmatic moon,” says Raut.

More laboratory testing and modeling could help solidify the hypothesis that Charon’s red spot is much more complex than we ever realized.

This research was published in Science and Geophysical Research Letters.

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