The team had also previously suggested that methane from Pluto’s atmosphere gets trapped in the moon’s north pole and slowly gets converted into tholin.
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Surface temperatures during these long winters dip to minus 430 degrees Fahrenheit (minus 257 degrees Celsius), cold enough to freeze methane gas into a solid.
Although polar ice caps have been seen on other planets, making a polar cap by trapping gas from another body seems to be a new phenomenon, Grundy added.
“This study solves one of the greatest mysteries we found on Charon, Pluto’s giant moon”, said Alan Stern, New Horizons principal investigator from the Southwest Research Institute, and a study co-author. That gas accumulates and freezes into the moon’s north pole during its hundred year-long winters.
Mosiac of New Horizons MVIC color observations of Charon obtained during the final 6.4 day rotation on approach to the system during July 7-14, shown in polar orthographic projection. A reddish spot was also spied on Nix, another moon orbiting Pluto, so it’s possible that the same process plays out there.
Grundy is part of NASA’s New Horizons mission which continues to send data to Earth after its historic flyby of Pluto in July 2015.
“It bounces around on the surface of Charon, and if it finds a place that’s cold enough it’ll actually stick”, Grundy says.
New Horizon’s observations of the moon’s other pole which is now in winter darkness and can only be seen through the reflecting light from Pluto.
They say this area of Charon spends long periods in shadow, meaning temperatures drop to extremely low enough to trap the methane as ice.
Previous theories claimed that Charon’s atmosphere turned methane gas from Pluto into the red material, but there was no reliable evidence whatsoever.
For fans of Pluto and Charon, these results will come as no surprise. “Prior to Pluto, the most distant solar system body with detected X-ray emission was Saturn’s rings and disk”.
Writing in Nature they said: ‘When the winter pole re-emerges into sunlight, methane and nitrogen sublimate away rapidly, but heavier less volatile poroducts remain behind.
As ultraviolet radiation from the Sun hit the north pole, it reacted with methane on the surface, knocking out the hydrogen, and turning it into a heavier molecule called a methyl radical that more or less stayed put.
“And it opens up the possibility that other small planets in the Kuiper Belt with moons may create similar, or even more extensive “atmospheric transfer” features on their moons”.
While NASA’s New Horizons spacecraft was speeding toward and beyond Pluto, Chandra was aimed several times on the dwarf planet and its moons, gathering data on Pluto that the missions could compare after the flyby.
Writing in the journal Nature, Dr Will Grundy, an expert on the outer solar system at the Lowell Observatory in Flagstaff, Arizona, and his colleagues said it could explain similar reddish spots found on Pluto’s other moons such as Nix.
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Those molecules, known as tholins, are the reason that photos of Charon tend to show the moon wearing a red cap.
