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Pluto’s atmosphere is similar to Earth’s in that it is predominantly composed of nitrogen (N). But Pluto’s atmosphere is ~98% N, while Earth’s is only ~78% N. Pluto’s atmosphere is also considerably thinner than Earth’s with ~10,000 times lower pressure at the surface.
The nitrogen in Pluto’s atmosphere (in the form of N2 gas) is actually flowing away and escaping the planet at an estimated rate of hundreds of tons per hour. We also see what looks like flowing ice on Pluto’s surface in high resolution images made by New Horizons. The water ice (H2O) that we are familiar with on Earth would be completely rigid and stiff at Pluto’s surface temperatures, but ice made out of N2 would be able to flow like a glacier. So where does all of this nitrogen come from?
They have rejected comets as a source, and have predicted that geologic activity on Pluto itself could dredge the nitrogen up from the planet’s interior. (This prediction by the way was made before the New Horizons’ flyby, which has proved it likely.)
If their theory ends up the answer, then they will also prove that Pluto is losing mass, albeit slowly. Nitrogen from within is being processed out of the interior, into the atmosphere as gas, and then into space because Pluto’s gravity is too small to hold it.