Pluto’s Moon Has A Mysterious Red North Pole, And We May Finally Know Why

Pluto’s life partner Charon has a disarming red ‘cap’. Since New Horizons popped the moonrust-colored north pole in their 2015 flyby, scientists pondered the planetary processes responsible for leaving such a bold landmark.

Scientists initially suspected that the iron-colored smear (dubbed the 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 cool idea just begging to be tested.

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

Launched in 2006, NASA’s New Horizons interplanetary space probe has provided researchers with an unprecedented view of the Pluto and Charon dwarf planetary system 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 diffuse blob of reflected light,” says 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 startling red cap centered on its north pole.”

Red might not be an uncommon color to see on iron-rich worlds like ours, or Mars. 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 starting point. To grow into a tholin, these tiny hydrocarbons would simply need to absorb a very specific color of UV light filtered by orbiting clouds of hydrogen called Lyman-alpha.

Pluto’s rosy glow has been the subject of study for decades. New Horizons simply revealed the precise pattern of tholines on its surface in glorious high definition. Finding a rusty hue on his mate’s cap, however, was an intriguing surprise.

It is assumed that the methane spilled from Pluto could drift to its orbiting moon. But the precise moment required for the gas to settle and freeze into a distinctly diffuse smear has always been a point of contention.

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 was, the spring dawn could be enough to melt the methane frost, driving it off the surface again.

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

Relatively sudden warming of the north pole would occur over the course of several years — a mere blink in the moon’s 248-year orbit around the Sun. During that brief period, a layer of methane ice only tens of microns thick would evaporate at one pole and begin to freeze at the other.

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

But ethane – the slightly longer hydrocarbon cousin of methane – would be another story.

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

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

Along with the results of laboratory experiments, the study by Raut and his team demonstrated a viable way to turn methane into ethane at the poles.

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

This does not exclude hydrocarbon. Charged particles that flow from the Sun for a longer period can 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 polar frost baked into Lyman-alpha to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon,” says Raut.

More lab tests and modeling could help solidify the hypothesis that Charon’s red spot is much more complex than we thought.

This research was published in Science and Geophysical Survey Cards.

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