Chris Peterson wrote:
I can't think of any mechanism for one body to "siphon off" the atmosphere of the other. No gas molecule on Pluto feels any net gravitational force except downwards, towards the center of Pluto.
Charon certainly encounters gas which has dissipated away from Pluto, in the same way that moons around Jupiter and Saturn encounter and collect material. With the moons, that shows up as differences on the leading and trailing hemispheres. But we don't really have leading and trailing hemispheres in the same way with Pluto and Charon, so any visible effects are likely to be much more subtle.
Can't think of any mechanism for one body to "siphon off" atmosphere? Maybe I can help. How about a double star system with a white dwarf?
Granted I'm picking nits and this would only be in microcosm in comparison. Let's not forget that Pluto and Charon and all their friends have been orbiting along through the same pathway of space as the others. If any one of them (other than Pluto) is capable of retaining gas near their surfaces without the solar wind blowing it off, it only stands to reason it would be the same gases they all encounter just as you said.
While I'm at it, they suggested the naming of the dark patch "Mordor". Then I suggest that the ridge we see to the west and south (assuming true north is up in the photo...) be named Ephel Dúath. Some of the features are not clear enough to me to tell if I'm seeing crater or mountain. However, if one of those bright patches are a mountain peek in that northern dark feature, then it stands to reason the largest be named Orodruin or Mt. Doom...
We hear of space objects being steeped in mystery and mythology. Nice to consider a little pop culture. Tolkien, Popeye, Pluto (woof, woof), Valentines, Bluto. All in all, this whole thing has been well worth it.