Pluto’s partner Charon has a disarming red “cap”. After the New Horizons probe photographed the rusty North Pole of Pluto’s moon on its 2015 flyby, scientists pondered the planetary processes responsible for such a bold landscape.

Scientists initially suspected that the iron-colored spot – nicknamed the Mordor Macula – was methane captured from Pluto’s surface, its red color the result of slow “baking” in the Sun’s ultraviolet light. It was a great idea, waiting to be tested. Now, a combination of modeling and laboratory experiments have found that these early assumptions held a grain of truth. The research adds surprising new details to our understanding of the “intimate engagement” of Pluto and Charon, suggesting that the natural satellite’s hue hides more than we thought.

Launched in 2006, NASA’s interplanetary spacecraft New Horizons gave researchers an unprecedented view of the dwarf planet system Pluto and Charon, more than 5 billion kilometers from the Sun. “Prior to New Horizons, Hubble’s best images of Pluto revealed only a faint patch of reflected light,” says Randy Gladstone, a planetary scientist at the Southwest Research Institute (SwRI) in the US. indicates Science Alert.

“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 might not be an uncommon color on iron-rich worlds like ours or Mars, for that matter. But to the icy end of the Solar System, the red is much more likely to indicate the presence of a diverse group of tar-like compounds called tholins.

The mysterious reddish north pole of Pluto’s moon

The red-brown mess of chemicals is similar to the residue left in a microwave oven if it used UV light to bake cookies made from simple gases like carbon dioxide or ammonia.

On Pluto, methane would be a likely starting place. To turn into a tholin, these tiny hydrocarbons would simply have to absorb a very specific color of UV light filtered by the orbiting hydrogen clouds, called Lyman-alpha. Pluto’s rosy glow has been the subject of study for decades. New Horizons has simply revealed the precise pattern of tholins on its surface to a whole other level of clarity. Still, finding a tinge of rust on its companion’s surface was an intriguing surprise.

It was assumed that methane spilled from Pluto could drift to its orbiting moon. But the precise timing required for the gas to deposit and freeze in such a diffuse and distinct way has always been a point of debate.

What actually happened?

Part of the problem is the competition between Charon’s weak gravity and the cool 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 back from the surface. To determine what would really happen, SwRI researchers modeled the see-saw motion of the largely tilted planetary system. The secret to the spot, they discovered, could be the explosive nature of spring’s arrival.

The relatively sudden warming of the north pole would occur over many years – a mere blip in the Moon’s 248-year movement around the Sun. In this short period, a frozen layer of methane just tens of microns thick would evaporate at one pole while beginning to freeze at the other. Unfortunately, the modeling found that this rapid motion would be far too fast for much of the frozen methane to absorb sufficient amounts of Lyman-alpha to become a tholin.

But ethane – a slightly longer hydrocarbon of methane – would be a different story altogether. “Ethane is less volatile than methane and remains frozen on Charon’s surface long after spring arrives,” says planetary scientist Ujjwal Raut, lead author of a second study that modeled changes in the densities of evaporating and freezing methane.

Exposure to the solar wind can transform ethane into persistent reddish surface deposits

“Exposure to the solar wind can turn ethane into persistent reddish surface deposits, contributing to Charon’s red cap.” Together with the results of laboratory experiments, Raut and the rest of his team demonstrated a feasible way to convert methane to 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 streaming from the Sun over a longer period could still generate longer and longer chains of hydrocarbons, which would give Charon its characteristic red cap.

“We believe that ionizing radiation from the solar wind breaks down Lyman-alpha polar ice to synthesize increasingly complex and redder materials responsible for the unique color on this enigmatic moon,” says Raut. Further laboratory testing and modeling could help strengthen the hypothesis that Charon’s red spot is much more complex than we’ve ever realized.

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