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

Pluto’s life partner, Charon, has a disarmingly red “cap.” Ever since New Horizons broke the moonfrom the rust-tinged North Pole during its 2015 flyby, scientists have been looking into the planetary processes responsible for the departure of such a bold landmark.

Scientists first suspected that the iron-colored smear (dubbed Mordor Macula) was methane captured on Pluto’s surface, its red color resulting from slow baking in the Sun’s ultraviolet light. It was a good idea just waiting to be tested.

Now, a mix of modeling and lab experiments has revealed those early assumptions weren’t too far off the mark, with a slight twist. The research adds startling new details to our understanding of Pluto and Charon’s intimate engagement, suggesting there’s more to the moon’s coloring than meets the eye.

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

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

“In addition to all the fascinating features discovered on the surface of Pluto, the flyby revealed an unusual feature on Charon; a startling red cap centered on its north pole.”

Red might not be an unusual color to see on iron-rich worlds like ours, or Mars for that matter. But throughout 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 tholins.

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

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

Pluto’s pink glow has been studied for decades. New Horizons simply revealed the precise patterning of tholins on its surface in glorious high definition. Finding a projected rust hue on its mate’s hood, however, came as an intriguing surprise.

It has been speculated that the methane released by Pluto could drift to its orbiting moon. But the precise moment it takes for the gas to settle and congeal into such a distinctly diffuse smear has always been a sticking point.

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

To determine what would actually happen, the SwRI researchers modeled the rocking motion of the widely tilted planetary system. The secret to the smear, they discovered, could be the explosive nature of spring’s arrival.

The relatively sudden warming of the north pole would take place over several years – a mere blink of an eye in the Sun’s 248-year orbit of the moon. During this brief period, a sheet of methane frost a few tens of microns thick evaporated at one pole as it began to freeze at the other.

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

But ethane – the slightly longer hydrocarbon cousin of methane – would be a whole different story.

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

“Exposure to solar wind can convert ethane into persistent reddish surface deposits contributing to Charon’s red cap.”

Along with results from lab experiments, Raut and his team’s study demonstrated a feasible way to turn methane into ethane at the poles.

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

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

“We believe that ionizing radiation from the solar wind breaks down Lyman-alpha baked polar frost to synthesize ever more complex and redder materials responsible for the unique albedo on this enigmatic moon,” says Raut.

Further lab testing and modeling could help strengthen the hypothesis that Charon’s Red Spot is far more complex than we ever imagined.

This research was published in Science and Geophysical Research Letters.

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