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

(ORDO NEWS) — Pluto’s life partner, Charon, has a disarming red cap. Ever since New Horizons captured the moon’s north pole with a rusty hue during a flyby in 2015, scientists have speculated about the planetary processes responsible for leaving such a bold mark.

Initially, scientists suspected that the iron-colored spot (nicknamed the Mordor Macula) was methane captured from the surface of Pluto, and its red color was the result of slow baking in the sun’s ultraviolet rays. It was a neat idea that just begged to be tested.

Now, a combination of modeling and laboratory experiments has shown that these early assumptions were not too far from the truth, but with slight deviations.

The study adds surprising new details to our understanding of Pluto and Charon’s intimate relationship, suggesting there’s more to the moon’s coloration than meets the eye.

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

Before New Horizons, the best images of Pluto taken by Hubble were just a blur of reflected light,” says Randy Gladstone, a planetary scientist at the Southwestern Research Institute (SwRI) in the US.

“In addition to all the interesting features found on Pluto’s surface, the flyby revealed an unusual feature of Charon: an amazing red cap located at its north pole.”

Red is not unusual on iron-rich worlds like ours or Mars. But out there, on the frozen fringes 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 “garbage”. The brown-red mess of chemicals is similar to leftovers left in an oven when cakes were baked in the oven using simple gases such as carbon dioxide or ammonia using ultraviolet light.

On Pluto, methane would most likely be the starting point. To grow into tholin, these tiny hydrocarbons simply have to absorb a very specific color of ultraviolet light filtered by orbiting hydrogen clouds called Lyman alpha.

Pluto’s pink glow has been the subject of study for decades. New Horizons just showed the exact location of the tholins on its surface in gorgeous high resolution. However, the discovery of a rusty hue on his companion’s hat came as an intriguing surprise.

It was assumed that the methane emitted by Pluto could fall on its orbiting moon. But the exact time required for the gas to settle and solidify into such a distinct scattered spot has always remained an unresolved issue.

Part of the problem lies in the struggle between Charon’s weak gravity and the cold light of the distant Sun, which warmed its surface. Weak as it was, a spring dawn might have been enough to melt the methane frost and push it off the surface again.

To determine what will actually happen, the SwRI researchers simulated the oscillatory motion of a planetary system that has a significant tilt. The secret of the smear, they discovered, may lie in the explosive nature of the arrival of spring.

The relatively sudden warming of the north pole will occur within a few years – just a moment in the 248-year orbit of the Moon around the Sun. During this short period, tens of microns thick frost will evaporate at one pole and begin to freeze at the other.

Unfortunately, simulations have shown that such a rapid movement would be too fast for most of the frozen methane to absorb enough Lyman-alpha to become tholin.

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

“Ethane is less volatile than methane and remains frozen on Charon’s surface long after spring sunrise,” says planetary scientist Ujwal Raut, lead author of the second study, which simulated changes in the density of evaporating and freezing methane.

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

Together with the results of laboratory experiments, Raut and his team’s study demonstrated a real way to convert methane to ethane at the poles.

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

But this does not exclude the possibility of obtaining a hydrocarbon. Charged particles flying from the Sun over a long period of time can still generate ever longer chains of hydrocarbons that will give Charon its characteristic red cap.

“We think the ionizing radiation from the solar wind is breaking down the Lyman-alpha polar frost to synthesize ever more complex, redder materials responsible for this mysterious moon’s unique albedo,” says Raut.

Further laboratory studies and modeling may help confirm the hypothesis that Charon’s blush spot is much more complex than we thought.”

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