APOD: Charon (2015 Jul 17)

[APOD Robot]

Charon

Explanation: Icy world Charon is 1,200 kilometers across. That makes Pluto's largest moon only about 1/10th the size of planet Earth but a whopping 1/2 the diameter of Pluto itself. Charon is seen in unprecedented detail in this image from New Horizons. The image was captured late July 13 during the spacecraft's flight through the Plutonian system from a range of less than 500,000 kilometers. For reference, the distance separating Earth and Moon is less than 400,000 kilometers. Charonian terrain, described as surprising, youthful, and varied, includes a 1,000 kilometer swath of cliffs and troughs stretching below center, a 7 to 9 kilometer deep canyon cutting the curve of the upper right edge, and an enigmatic dark north polar region unofficially dubbed Mordor.

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[Nitpicker]

I wonder how much brighter a nearly full Moon appears to us on Earth, compared with how Charon appeared to New Horizons for this APOD? Are any technical details available for this image, such as exposure time(s)?

The image resolution reminds me a little of the first time I attached a DSLR to my 6 inch telescope and took a single snapshot of our Moon. LORRI is only slightly larger than my telescope, but I had a lot more light. Totally amazed, both times.

Edit: I suppose I also had atmosphere to shoot through at the Moon, and Charon has a higher albedo, but I only had to bumble into the backyard.

[Ann]

That canyon on Charon is slightly similar to Valles Marineris on Mars. Overall, Charon looks more rugged to me than Pluto. Of course we don't have a really good image of "the dark side of Pluto" (well, the Charon-facing side of it, which is "dark" only because New Horizons wasn't able to resolve it very well), so if we had good global maps of both Pluto and Charon, their respective terrains might not seem so different. But for what it's worth, I get the impression that Charon is more rugged.

I realize that this isn't a very good color image - certainly not an RGB image - but clearly the color of Charon is different from the color of Pluto. Interestingly though, there really is some reddish terrain on Charon near and partly inside its dark polar region.

But as for the much redder hue of the overall color of Pluto, could it possibly have something to do with the atmosphere of Pluto? Neptune's large moon Triton, which is partly reddish, also has an atmosphere. Deep orange Titan has a whopper of an atmosphere. Could it be that the sunlight that reaches the bodies in the outer solar system more easily interacts with gases than with solids to produce reddish compounds? Alternatively, could it be that the reddish compounds originating in atmospheres are more light-colored compared to the reddish compounds of solids, which may be very dark?

Ann

[emc]

Next mission to Pluto will be carrying Popeye’s nemesis Bluto. Yes, Popeye finally did Bluto in and only fittingly stuffed him in a spinach can. The mission plans to eject the can containing Bluto onto Pluto during its close flyby.

Olive Oil was quoted as saying, “Oh dear, with Bluto on Pluto, I hope Popeye won’t take me for granted!”... Olive says she plans to start wearing more green.

The mission planners for the next Pluto encounter are struggling with Popeye’s Pappy’s stubborn request to have the spacecraft emit a “spit-tooey” sound as the can of Bluto is ejected. It is not known at this time if this requested technology will make the final mission cut. Planners are struggling for two reasons... one is the fact that sound waves cannot travel without a medium and the vacuum of outer space is severely lacking in sound wave medium. The other reason is that, unless mission plans change, there would be no one alive within earshot for “billions and billions” of miles. Which brings to mind an ancient philosophical question, “If a tree falls in the forest and no one is around... does it make any noise?” I personally have often mused at this question. This silly question strongly illustrates just how self centered and twisted we humans can be. Of course it makes noise, but many of us are so preoccupied with the inner workings of our own minds that even if we were standing in the forest we still wouldn’t hear the tree fall. At best there would only be a delayed “what’s that?”... But if no one else is standing next to us, did we truly make a sound!? It is ironic that such a typically socially dependent creature such as ourselves can be so individualistic, especially when you consider that most of us would not survive without the company of others... For example, I give thanks every day that I have the ability to pursue and engage several varieties of hot and juicy hamburgers provided 24/7 in many areas of our planet by others not unlike me. Well, at least fundamentally not unlike me.

Apologies to anyone looking for and/or expecting constant fluid texting motion in SA* threads...

[Craine]

But as for the much redder hue of the overall color of Pluto, could it possibly have something to do with the atmosphere of Pluto? Neptune's large moon Triton, which is partly reddish, also has an atmosphere. Deep orange Titan has a whopper of an atmosphere. Could it be that the sunlight that reaches the bodies in the outer solar system more easily interacts with gases than with solids to produce reddish compounds? Alternatively, could it be that the reddish compounds originating in atmospheres are more light-colored compared to the reddish compounds of solids, which may be very dark?

Ann

The color of Pluto and Charon is a curious issue. I realize the atmosphere of Pluto is exceedingly thing. Nevertheless it seems likely to be the main cause of the fairly light color of Pluto's surface; deposits of ices of the various gases.

But with Pluto and Charon in such close orbits I figured Charon should be able to siphon off some of that atmosphere. In which case the surface of Charon would also be covered in the same ice. Unless the weight of the different components of the atmosphere makes a difference. Methane gas is lighter then Nitrogen (N2). It may be that Charon is siphoning of the lighter gases, giving it a different colored surface.

All idle speculation of course. I am sure the boffins will come up with something better soon.

[rochelimit]

But as for the much redder hue of the overall color of Pluto, could it possibly have something to do with the atmosphere of Pluto? Neptune's large moon Triton, which is partly reddish, also has an atmosphere. Deep orange Titan has a whopper of an atmosphere. Could it be that the sunlight that reaches the bodies in the outer solar system more easily interacts with gases than with solids to produce reddish compounds? Alternatively, could it be that the reddish compounds originating in atmospheres are more light-colored compared to the reddish compounds of solids, which may be very dark?

Ann

The color of Pluto and Charon is a curious issue. I realize the atmosphere of Pluto is exceedingly thing. Nevertheless it seems likely to be the main cause of the fairly light color of Pluto's surface; deposits of ices of the various gases.

But with Pluto and Charon in such close orbits I figured Charon should be able to siphon off some of that atmosphere. In which case the surface of Charon would also be covered in the same ice. Unless the weight of the different components of the atmosphere makes a difference. Methane gas is lighter then Nitrogen (N2). It may be that Charon is siphoning of the lighter gases, giving it a different colored surface.

All idle speculation of course. I am sure the boffins will come up with something better soon.

I have a theory. I think Charon original color is dark, the same darkness as that dark north polar region. The equatorial part captured part of Pluto's atmosphere, coating the equatorial area with lighter coat of methane and nitrogen. meteoroid strikes at some point of its history, the crater produced darker material oozing out from the base of the crater, and so here you are Charon as we know now.

I don't how to explain those escarpments.

And, does Charon looks a lot like Moons of Uranus? Oberon has dark material at the base of craters, and those grooves and ridges looks a lot like Ariel and Titania.

[Chris Peterson]

But with Pluto and Charon in such close orbits I figured Charon should be able to siphon off some of that atmosphere. In which case the surface of Charon would also be covered in the same ice. Unless the weight of the different components of the atmosphere makes a difference. Methane gas is lighter then Nitrogen (N2). It may be that Charon is siphoning of the lighter gases, giving it a different colored surface.

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.

[Craine]

But with Pluto and Charon in such close orbits I figured Charon should be able to siphon off some of that atmosphere. In which case the surface of Charon would also be covered in the same ice. Unless the weight of the different components of the atmosphere makes a difference. Methane gas is lighter then Nitrogen (N2). It may be that Charon is siphoning of the lighter gases, giving it a different colored surface.

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.

A long time ago I learned that the lighter elements (mostly hydrogen and helium) in Earths atmosphere end up higher and gradually dissipate away. And our moon is a strong contributing factor contributing to that loss. I figured something similar might be going on between Pluto and Charon and that Charon might capture some of that gas.
Though I admit that 'siphoning' was a poor word choice.

Anyway...still just idle speculation.

[Chris Peterson]

A long time ago I learned that the lighter elements (mostly hydrogen and helium) in Earths atmosphere end up higher and gradually dissipate away. And our moon is a strong contributing factor contributing to that loss.

The first statement is correct. The second is not.

[Milky Waster]

Before too many more photos of Pluto and Charon are downloaded from New Horizons, could writers, editors, journalists and correspondents please agree whether Charon is Pluto's largest moon or the two are binary planets. Today's APOD caption was quite a setback for those wishing to see the term "binary planet" explained and absorbed into our understanding of such things.

[Chris Peterson]

Before too many more photos of Pluto and Charon are downloaded from New Horizons, could writers, editors, journalists and correspondents please agree whether Charon is Pluto's largest moon or the two are binary planets. Today's APOD caption was quite a setback for those wishing to see the term "binary planet" explained and absorbed into our understanding of such things. :?

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.

[Milky Waster]

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.

[Chris Peterson]

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.

That's only accurate if the bodies are "planets", and that word isn't used very consistently.

The thing is, Pluto and Charon aren't really in orbit around each other. Dynamically, they are better seen as being in independent orbits around the Sun. That further confuses the question of "binary planet system" beyond just the definition of "planet".

[Milky Waster]

Could you then please clarify for me whether Charon orbits Pluto. If it does not, then surely it cannot be described as a moon.

[Chris Peterson]

Could you then please clarify for me whether Charon orbits Pluto. If it does not, then surely it cannot be described as a moon.

And some scientists prefer to view the pair as planets, or maybe "binary dwarf planets". Whether we say Charon orbits Pluto depends on frame of reference. It is certainly possible to base our reference frame on the barycenter of the pair and understand them as in orbit around each other. But in terms of the Solar System, they are better understood as independently orbiting the Sun. If Pluto suddenly vanished, Charon would continue orbiting the Sun, with its orbit virtually unchanged. BTW, this is also true of the Earth-Moon system. The Moon is actually in orbit around the Sun, not simply around the Earth.

Again, the terminology isn't particularly important here. Some definitions are most useful when they are very broad, and this arguably includes both "planet" and "moon".

[neufer]

If Pluto suddenly vanished, Charon would continue orbiting the Sun, with its orbit virtually unchanged.

BTW, this is also true of the Earth-Moon system.

One should note, however, that this is not true for most of the moons of the Giant Planets.

Whenever the fastest orbital speed of a satellite exceeds 0.414 x the average orbital speed of its parent planet
then it is possible for that satellite to escape from the Sun were that planet to suddenly vanish.

This holds true for:

• 1) The Galilean moons of Jupiter.
  2) Saturn's moons interior to Iapetus including Titan.
  3) Uranus's 5 main satllites.
  4) and Neptune's Triton.

[geckzilla]

The Moon is actually in orbit around the Sun, not simply around the Earth.

I think I have learned this before but I still had to go test it out in a simulation and watch it. Sure enough, if Earth disappears, the Moon and a few other things which were preloaded in the simulation (like Chandra, B44E, and the STEREOs) stayed right where they were. Even something in low earth orbit like Hubble, which I put into the simulation for the next test, stays there. I'm not sure why I imagine that they all fly away if Earth disappears.

[emc]

The Moon is actually in orbit around the Sun, not simply around the Earth.

I think I have learned this before but I still had to go test it out in a simulation and watch it. Sure enough, if Earth disappears, the Moon and a few other things which were preloaded in the simulation (like Chandra, B44E, and the STEREOs) stayed right where they were. Even something in low earth orbit like Hubble, which I put into the simulation for the next test, stays there. I'm not sure why I imagine that they all fly away if Earth disappears.

Dang! So the only objects dependent on Earth is us! Dang! 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...

[Boomer12k]

WOW....good shot...
The dark north pole region looks like AMERICA and Canada,....you have the Gulf of Mexico, and a bit of Florida. But you don't quite have Baja....


:---[===] *

[emc]

WOW....good shot...
The dark north pole region looks like AMERICA and Canada,....you have the Gulf of Mexico, and a bit of Florida. But you don't quite have Baja....


:---[===] *

So Charon is “MOCKING” us. And dang again! It is not enough to prompt scientific insight as to our lowly position in the cosmos... we must also endure other worldly geological mocking!

[Craine]

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.

[Donnageddon]

Just to get this straight in my head: If you traced the Moon's (our moon) orbital path around the sun, it would be a wave pattern, getting slightly closer to the sun, and then farther away as it orbits the Earth, then back again, right?

Now if the Earth suddenly disappeared, would the Moon continue this oscillating path? Would it not settle into a more steady elliptical path?

[Chris Peterson]

Just to get this straight in my head: If you traced the Moon's (our moon) orbital path around the sun, it would be a wave pattern, getting slightly closer to the sun, and then farther away as it orbits the Earth, then back again, right?

Now if the Earth suddenly disappeared, would the Moon continue this oscillating path? Would it not settle into a more steady elliptical path?

The wiggle would go away. Think of the Moon's orbit like this: when it is in the part of its path around the Earth where it is traveling in the same direction as its orbit around the Sun, it is essentially moving a little faster than a body at Earth's orbital radius can move, so it moves a little closer to the Sun. When it is in the part of its path around the Earth where it is traveling in the opposite direction as its orbit around the Sun, it has slowed down a bit, and so it moves outward. This monthly motion is what generates the wiggle in its path around the Sun. Without the Earth, its orbital speed would not change (except according to Kepler's Second Law). Depending on where the Moon was in its orbit around the Earth when the Earth vanished, the distance of the Moon from the Sun would vary just a tiny bit. And then it would be on that smooth path forever.

[MarkBour]

But with Pluto and Charon in such close orbits I figured Charon should be able to siphon off some of that atmosphere. In which case the surface of Charon would also be covered in the same ice. Unless the weight of the different components of the atmosphere makes a difference. Methane gas is lighter then Nitrogen (N2). It may be that Charon is siphoning of the lighter gases, giving it a different colored surface.

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.

It's interesting to think about. I guess there are multiple competing theories about the formation of the Pluto-Charon pairing. Perhaps there was a collision that both survived. And they remained gravitationally bound together. Presumably at first they were not yet tidally locked. During that initial period, since Charon is so close and so relatively large compared to Pluto, I assume the tidal forces on the atmospheres would have been quite impressive. Once they became tidally locked, there are no more tides in the atmosphere of Pluto, but it must be noticeably stretched (in density) along the mutual axis of the bodies. I suppose the other smaller moons are at least able to "stir up" the mixture to some extent. I can see the sense of the statement that there is no "siphoning" going on, but with all of that history, it seems there would likely have been a fair amount of sharing, and possibly there is still some noticeable random transfer of gas molecules between the two bodies, just from the random motions of gas.

I often come away from such a thought process thinking I should get some hard numbers and do a careful computer simulation. It feels like mental laziness to stop after a little imprecise musing. But it's also often time to just stop at this point and see if someone else has already done it and will give an educated answer. Heck, 10 years ago, somebody knew enough to put instruments on this baby that will hopefully tell us a great deal about these very questions. I hope the data is good!

[Pianosorplanets]

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.