APOD: Charon (2015 Jul 17)

[geckzilla]

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?

This type of star system includes a very dense, small member (the white dwarf) and a much larger but less dense member that has begun to swell up and only tenuously holds on to its outer atmosphere (a red giant). The situation is... quite different!

[Pianosorplanets]

No such agreement is likely, because none of these terms are well enough defined to eliminate ambiguity. Both are reasonably called planets (in which case they are a binary system), Pluto is reasonably called a planet and Charon a moon. It really depends on context. And frankly, it's not all that important which terms we use.

A cheer for Chris! It's fun and interesting to learn the intricate details of how the universe works. Thanks to Chris, just the other day I learned the true solar system escape velocity at Jupiter's orbit. But picking nits just because you can doesn't get us anywhere and getting nowhere does get dull after a while (anybody who's spent a week in the hospital knows how that feels). It's nice to have a grounding force (kind of like gravity) to keep one's thinking and one's speculation in the realms of that which is either productive, build's friendships or is good for a nice chuckle once and again.

[Pianosorplanets]

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?

This type of star system includes a very dense, small member (the white dwarf) and a much larger but less dense member that has begun to swell up and only tenuously holds on to its outer atmosphere (a red giant). The situation is... quite different!

Yes, of course it is. Sorry, I was being a bit glib. Hence my use of the word "Microcosm". More to the point is what space gases might the system pass through as a group and which bodies can and can't hang onto some of it. It seems more likely that that is the mechanism for a commonality in any atmosphere discovered than the chance of Charon actually drafting gas off of Pluto. Quite the long shot, no?

[Pianosorplanets]

As I understand it, the term "binary planets" is used when two bodies orbit a point outside either of their masses which is what I understand Pluto and Charon do. Students everywhere will no doubt be pleased to learn that it doesn't matter what terms they use. "Everyone gets 100%," said Alison Tudorland.

Meteors, Oort cloud objects, binary stars and space dust no doubt find themselves in a dance with a nearby object. We can't name them all "planets" just because their center of gravity is outside the physical form of either object.

No doubt, the Keiper belt is filled to the brim with multiple objects rotating around each other in a manner similar to Pluto and Charon. Students need not be told it doesn't matter but be told just how grand the scheme of things really is and that we have to draw the line someplace. And, being normal, intelligent humans, we'll never agree as to where to put that line.

[BMAONE23]

Siphoning isn't required to transfer atmospheric gasses between the two bodies. Solar winds strip some atmospheric gasses and blows them back away from Pluto in a type of tail pointing away from the sun. In just 62 years Charon's orbit will be passing between the sun and Pluto so that, out of every 201 hour orbital period, 24 hours will be spent in the atmospheric tail caused by the solar winds
http://cseligman.com/text/planets/plutorot.htm

[Chris Peterson]

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.

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?

Nope, not even close. Material transfers between the components of a close binary star system because the two objects overlap- in particular, the surface of one extends beyond the apex of the Roche lobe, allowing transfer across the L₁ point. This is only possible with fluid bodies. Rigid bodies can't possibly be that close without tidally disrupting each other.

Pluto and Charon are separated by 20,000 km or so, and Pluto has a tenuous atmosphere extending at most a few hundred kilometers, probably much less.

So there's really no mechanism for material to transfer between the two bodies.

[Chris Peterson]

Students need not be told it doesn't matter but be told just how grand the scheme of things really is and that we have to draw the line someplace.

Actually, students need to understand that some lines make no sense. It doesn't matter what we call planets. It doesn't matter if there are 9 of them or 1000 of them. Nobody really cares. The definition isn't very important. Whether something on Earth is a "hill" or a "mountain" doesn't interest geologists. What's interesting is if some bump is the product of uplift, or deposition, or extrusion. "Hill" and "mountain" are more useful as informal designations understood in context. The same is true for "planet", which is, in fact, how most astronomers use the word in practice. We care about the physical nature of these bodies, and classify them based on that (terrestrial, gas giant). We care about their origin and formation history, and classify them based on that. We care about their orbital dynamics, and classify them based on that. Lots of categories. Those are what matter, not the loose designation that we can safely use without concern over technicalities.

[BMAONE23]

There is the possibility of transfer through solar interaction by traveling through the atmospheric wake tail of gasses that have been stripped from the solar winds during Pluto fall and spring

[Chris Peterson]

Siphoning isn't required to transfer atmospheric gasses between the two bodies. Solar winds strip some atmospheric gasses and blows them back away from Pluto in a type of tail pointing away from the sun. In just 62 years Charon's orbit will be passing between the sun and Pluto so that, out of every 201 hour orbital period, 24 hours will be spent in the atmospheric tail caused by the solar winds

Right, as noted previously Charon can intercept material that diffuses from Pluto. But at that distance, I doubt we're going to have much blown off by the Sun. The atmosphere is already close to a vacuum. Charon's gravity isn't going to retain any stray gas molecules that come its way, so we'd be looking at some sort of adhesion process, I'd think, if anything. But Charon itself is icy. I wouldn't be surprised if it also has an extremely tenuous atmosphere- it may be outgasing more than it receives from Pluto.

[geckzilla]

It doesn't matter, at least, until you get a committee on the job, it seems.
https://en.wikipedia.org/wiki/Parkinson ... triviality

[Chris Peterson]

It doesn't matter, at least, until you get a committee on the job, it seems.
https://en.wikipedia.org/wiki/Parkinson ... triviality

Committees tend to be very good at that.

[Ann]

Siphoning isn't required to transfer atmospheric gasses between the two bodies. Solar winds strip some atmospheric gasses and blows them back away from Pluto in a type of tail pointing away from the sun. In just 62 years Charon's orbit will be passing between the sun and Pluto so that, out of every 201 hour orbital period, 24 hours will be spent in the atmospheric tail caused by the solar winds

Right, as noted previously Charon can intercept material that diffuses from Pluto. But at that distance, I doubt we're going to have much blown off by the Sun. The atmosphere is already close to a vacuum. Charon's gravity isn't going to retain any stray gas molecules that come its way, so we'd be looking at some sort of adhesion process, I'd think, if anything. But Charon itself is icy. I wouldn't be surprised if it also has an extremely tenuous atmosphere- it may be outgasing more than it receives from Pluto.

Well, what do you think of the possibility that an atmosphere might contribute to a reddish color of an outer solar system planet or moon? If Charon itself has an atmosphere, that might just possibly explain this planet/minor planet/moon's slightly reddish hue near its dark pole.

Saturday's APOD shows a Plutonian landscape with cracks, where dark material has gathered in some places. I'm reminded of both Jupiter's Galilean moon Europa and Saturn's small moon Hyperion. We know that on Europa at least, the dark material in and around cracks is reddish.

Ann

[Chris Peterson]

Well, what do you think of the possibility that an atmosphere might contribute to a reddish color of an outer solar system planet or moon? If Charon itself has an atmosphere, that might just possibly explain this planet/minor planet/moon's slightly reddish hue near its dark pole.

I generally avoid speculation in the absence of evidence. We see color and surface structure on cold, icy, airless (or nearly airless) bodies caused by sublimation, by internal outgasing, by space weather, by impacts. If Pluto has some sort of internal activity of unknown origin, Charon may, as well.

I guess we'll just see when more data arrives, especially from some of the other instruments (which are less glamorous than the imagers, but provide essential information that we simply can't get from images alone).

[Bradford]

It's misleading and confusing to say "1/10th the size of planet Earth."
You should say 1/10 the diameter, which is closer to 1/1000 the size.

[Nitpicker]

It's misleading and confusing to say "1/10th the size of planet Earth."
You should say 1/10 the diameter, which is closer to 1/1000 the size.

Both 1/10th the size and 1/1000th the size are somewhat ambiguous, but I wouldn't go so far as to say misleading.

More precise to say that Charon's size is 1/10th the diameter, or 1/100th the surface area, or 1/1000th the volume, of Earth. They are all sizes.

[emc]

So Canton, GA isn’t the center of the universe!? Dang again! Now I feel even less important. If I were any lower I would be inside a black hole...

Not Canton. Austell, GA is.

Well that is not bad news... Austell is only a Hercules's stone's throw from Canton! In cosmic resolution, that's as good as dead on.

[BMAONE23]

So Canton, GA isn’t the center of the universe!? Dang again! Now I feel even less important. If I were any lower I would be inside a black hole...

Not Canton. Austell, GA is.

Well that is not bad news... Austell is only a Hercules's stone's throw from Canton! In cosmic resolution, that's as good as dead on.

Well Ed, I guess that makes you the slightly off center of the universe

[Beyond]

Well Ed, I guess that makes you the slightly off center of the universe

Click to play embedded YouTube video.

[ta152h0]

Now that NH is out there in the vast ocean of nothingness, are the cameras taking images of the nothingness ahead of the spacecract and maybe catch something simply by golly ?

[neufer]

---

Now that NH is out there in the vast ocean of nothingness,
are the cameras taking images of the nothingness ahead of
the spacecract [sic] and maybe catch something simply by golly ?

New Horizons

• What if New Horizons hits my car? — Robin Sheat

[BMAONE23]

A view of a portion of Charon from 100k away

Click to view full size image