Dwarf planet Pluto may be sharing its atmosphere with its moon Charon, producing the red spot at the north pole of its largest natural satellite.

Researchers conducted a new study based on data gathered by NASA's New Horizons spacecraft, which flew by Pluto and Charon. Findings suggest that the conditions on these two extraterrestrial worlds over the past billion years would make it possible for components of Pluto's traveling atmosphere to freeze out on the frigid environment of Charon.

Methane gas that escapes from Pluto's atmosphere gets trapped by Charon's gravity and freezes on its cold and icy surface. Ultraviolet light from the sun, on the other hand, converts methane into heavier hydrocarbon and eventually into the reddish residue that make up the visually striking red polar region.

Will Grundy, a planetary scientist from the Lowell Observatory in Arizona who is part of the team behind New Horizons mission, modelled conditions on Charon over the past few billion years. The researchers used images of Charon taken by New Horizons as well as computer models on how ice evolves on the moon's poles.

They found extreme weather at the poles of Charon characterized by 100 years of continuous sunlight that alternate with another 100 years of continuous darkness.

During the long winter, the surface temperatures would dip to -430 degrees Fahrenheit, which is cold enough to freeze methane into solid. Methane molecules that land on the cold pole freeze and form a thin coating of methane ice that lasts until sunlight returns.

"The winter pole is absurdly cold, so any methane that hits it is unable to hop around like it would elsewhere on Charon. It just stays there in the cold trap," Grundy said. "Eventually you get quite a bit of it up there."

Although methane ice easily sublimates, the heavier hydrocarbons that were produced from the evaporative process remain on the moon's surface. These leftover hydrocarbons turn into the reddish material called tholins when sunlight further irradiates them.

"We model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped," Grundy and colleagues wrote in their study, which was published in the journal Nature on Sept. 14.

"The model results are consistent with the proposed mechanism for producing the observed colour pattern on Charon."

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