Pluto’s Moon Has a Mysterious Red North Pole, And We Might Finally Know Why

Pluto’s life partner, Charon, has a disarming red ‘cap’. Since New Horizons broke out the moon‘s rusty north pole during the 2015 flight, scientists pondered the planetary processes responsible for leaving behind such a bold landmark.

Scientists initially suspected the iron-colored smear (nicknamed Mordor Macula) was methane captured from Pluto’s surface, the red color was the result of a slow firing in the sun’s ultraviolet light. It was a nice idea, but begged to be tested.

Now, a mix of modeling and lab experiments has shown that these early assumptions weren’t too far off the mark, with a small twist. The research adds surprising new details to our understanding of Pluto and Charon’s intimate involvement, suggesting there’s more to it. the moons colors than appears at first glance.

NASA’s New Horizons interplanetary space probe, launched in 2006, provided researchers with an unprecedented view of the dwarf planet system Pluto and Charon more than 5 billion kilometers (3.1 billion miles) from the sun.

“Prior to New Horizons, the best Hubble images of Pluto revealed just a faint blob of reflected light,” say Randy Gladstone, a planetary scientist at the Southwest Research Institute (SwRI) in the US.

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

Red may not be an uncommon color to see on iron-rich worlds like ours, or Mars for that matter. But in the frozen outskirts 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 brownish-red mess of chemicals is like the residue left in the oven if the oven used UV light to bake brownies made from simple gases like carbon dioxide or ammonia.

On Pluto, methane would be a likely launch site. To grow into a tholin, these small hydrocarbons would simply very specific color of UV light filtered through spinning clouds of hydrogen, called Lyman-alpha.

Pluto’s pink glow has been the subject of study for decades. New Horizons simply revealed the precise patterns of tholins on the surface in glorious high definition. Finding a rusty shade thrown over his companion’s cap, however, was an intriguing surprise.

It was believed that Pluto’s methane discharge could drift into its orbit around the moon. But the precise timing required for the gas to settle and freeze into such a distinctively diffuse smear has always been a sticking point.

Part of the problem is the struggle between Charon’s weak gravity and the cold light from the distant sun that warmed the surface. As weak as it was, the spring sun could be enough to melt the methane frost and drive it back from the surface.

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

The relatively sudden warming of the North Pole would take place over several years — just the twinkling of an eye into the moon’s 248-year orbit around the sun. During this short period of time, a layer of methane ice just tens of microns thick would evaporate at one pole as it began to freeze at the other.

Unfortunately, the modeling showed 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 — methane’s slightly longer hydrocarbon cousin — would be a whole different story.

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

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

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

There was only one problem. Lyman alpha radiation does not turn ethane into a reddish sludge.

That does not exclude the hydrocarbon. Charged particles flowing from the sun over a longer period of time can still generate longer hydrocarbon chains that would give Charon its characteristic red cap.

“We think that ionizing radiation from the solar wind is decomposing the Lyman-alpha-boiled arctic frost to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon,” say Raw.

Further lab testing and modeling could help solidify the hypothesis that Charon’s rouge staining is much more complex than we ever realized.

This research was published in Science and Geophysical Survey Letters

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