Pluto's Moon Has a Mysterious Red North Pole

Charon, Pluto's companion in life, wears a menacing crimson "cap." Scientists have pondered the planetary mechanisms responsible for leaving such a striking monument ever since New Horizons' 2015 flyby of the moon captured images of its rust-colored north pole.

The iron-colored smudge, also known as the Mordor Macula, was once thought to be Pluto's surface methane, with its red hue coming from a long baking process in the Sun's UV rays. It was a clever concept that begged to be tried out.

These early hypotheses weren't entirely off base, but with a little twist, according to a combination of modeling and laboratory tests. The study suggests there may be more to the moon's hue than meets the eye and adds startling new elements to our knowledge of Pluto and Charon's close relationship.

NASA's interplanetary spacecraft New Horizons, which was launched back in 2006, gave scientists a previously unattainable look at the minor planets Pluto and Charon, which are located more than 5 billion kilometers (3.1 billion miles) from the Sun.

"Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light," according to planetary scientist Randy Gladstone of 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."

On worlds like ours with abundant iron, or perhaps Mars, red could not be that unusual of a hue. Red is far more likely to represent the presence of a wide range of tar-like substances known as tholins, however, all the way out in the freezing suburbs of the Solar System. 

If it helps, just replace the word tholin with 'gunk'. If the oven had been used to bake basic gas brownies, such carbon dioxide or ammonia, it would have left behind a brownish-red residue.

Methane would be a plausible location to start on Pluto. These microscopic hydrocarbons would just need to absorb Lyman-alpha, a highly particular UV light hue filtered by circling hydrogen clouds, to develop into a tholin.

The pink light of Pluto has been studied for many years. Simply said, New Horizons showed the exquisite high resolution patterning of tholins on its surface. But it was an interesting surprise to discover a rusty tinge thrown across the crown of its partner.

Methane released by Pluto was thought to have a chance of reaching its moon in orbit. But it was never easy to determine just when the gas needed to settle and freeze into such a distinctly dispersed smear.

The struggle between Charon's feeble gravity and the distant Sun's chilly light, which warmed its surface, contributes to the issue. Even if it was dim, the spring light could be enough to melt the methane frost and remove it from the surface once more.

SwRI researchers studied the see-sawing motion of the mainly inclined planet system to ascertain what would really occur. They discovered that the explosive nature of spring's impending arrival could hold the key to the stain.

In the 248-year orbit of the moon around the Sun, the comparatively abrupt warming of the north pole would occur over a period of few years. A cloud of methane frost only tens of microns thick would melt at one pole during this brief period while starting to ice over at the other.

Sadly, the modeling revealed that this speedy movement would happen much too quickly for most of the frozen methane to absorb enough Lyman-alpha to transform into a tholin.

But ethane, a somewhat longer hydrocarbon relative of methane, would be a very other situation.

"Ethane is less volatile than methane and stays frozen to Charon's surface long after spring sunrise," according to the author of a second research that simulated variations in the densities of methane evaporating and freezing, planetary scientist Ujjwal Raut.

"Exposure to the solar wind may convert ethane into persistent reddish surface deposits contributing to Charon's red cap." 

Raut and his team's work showed a practical manner methane may be converted into ethane at the poles, along with the outcomes of laboratory trials.

There was just one issue. Ethane won't become a crimson sludge when exposed to Lyman-alpha radiation.

The hydrocarbon is still possible despite this. The ever longer chains of hydrocarbons that give Charon its distinctive crimson hue might potentially be produced by charged particles pouring from the Sun over a longer time period.

"We think ionizing radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon," explains Raut.

The idea that Charon's rouge stain is far more sophisticated than we previously thought might be confirmed with more laboratory research and modeling.

This research was published in Science and Geophysical Research Letters.