So you think you knew Pluto?

September 15, 2015 • 2:30 pm

NASA has put up some stunning pictures of Pluto taken by the New Horizons spacecraft that did its amazing and recent flyby. I won’t duplicate everything the article says, but will show you just a few sections of text and some images (more at the site) that justify the URL’s bit that “it’s complicated”. The text from the NASA site is indented:

New close-up images of Pluto from NASA’s New Horizons spacecraft reveal a bewildering variety of surface features that have scientists reeling because of their range and complexity.

“Pluto is showing us a diversity of landforms and complexity of processes that rival anything we’ve seen in the solar system,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute (SwRI), Boulder, Colorado. “If an artist had painted this Pluto before our flyby, I probably would have called it over the top — but that’s what is actually there.”

New Horizons began its yearlong download of new images and other data over the Labor Day weekend. Images downlinked in the past few days have more than doubled the amount of Pluto’s surface seen at resolutions as good as 400 meters (440 yards) per pixel. They reveal new features as diverse as possible dunes, nitrogen ice flows that apparently oozed out of mountainous regions onto plains, and even networks of valleys that may have been carved by material flowing over Pluto’s surface. They also show large regions that display chaotically jumbled mountains reminiscent of disrupted terrains on Jupiter’s icy moon Europa.

An icy plain and other complex features (Pluto’s diameter is about 2400 km, or about 18% that of Earth):


“The surface of Pluto is every bit as complex as that of Mars,” said Jeff Moore, leader of the New Horizons Geology, Geophysics and Imaging (GGI) team at NASA’s Ames Research Center in Moffett Field, California. “The randomly jumbled mountains might be huge blocks of hard water ice floating within a vast, denser, softer deposit of frozen nitrogen within the region informally named Sputnik Planum.”

A closeup of the broken terrain, photo taken July 14 of this year:


New images also show the most heavily cratered — and thus oldest — terrain yet seen by New Horizons on Pluto next to the youngest, most crater-free icy plains. There might even be a field of dark wind-blown dunes, among other possibilities.

Craters, mountains, and ridges:


“Seeing dunes on Pluto — if that is what they are — would be completely wild, because Pluto’s atmosphere today is so thin,” said William B. McKinnon, a GGI deputy lead from Washington University, St. Louis. “Either Pluto had a thicker atmosphere in the past, or some process we haven’t figured out is at work. It’s a head-scratcher.”

If you’re a Plutophile, you know that the planet (or whatever you want to call it) has five moons. Here’s the largest, Charon, 1200 km in diameter. Its terrain is also also quite complex:


h/t: Mark Sturtevant

88 thoughts on “So you think you knew Pluto?

  1. Pluto has been far more interesting than I expected it to be. I thought it would be a heavily-cratered bland gray world like Mercury or Earth’s moon, but it has proven to be far more diverse and interesting.

    It saddens me to think that this may be the last new world I get to see up close in my lifetime (though New Horizons may reach another dwarf planet in 2019).

    1. Actually, Dawn is mapping Ceres as we speak, and there are thousands more unexplored rocks between here and Jupiter. So I don’t think this is the end by any means.

      Who knows, within a couple of decades you might be able to book a sightseeing tour to an Earth-crossing asteroid with SpaceX or Virgin Galactic.

  2. What cooks my noodle is the thought of mountains of frozen water ‘floating’ on seas of frozen nitrogen. That is, like… wow.

    1. It’s pretty “wow” to me too.
      Particularly since the density of nitrogen seems to be very dependent on both pressure and temperature. From Dewar (he of the flasks) in 1904 (Proceedings of the Royal Society of London, Vol. 73 (1904), pp. 251-261), the density of nitrogen at it’s melting point is 0.879 g/cm^3, but at it’s boiling point, it rises to 1.0265. Slightly different figures from other sources, but clearly it’s density is quite strongly dependent on temperature and pressure. That’s going to make for some complex convection forces.
      Got to remember just how anomalous the phase diagram of water is. Just because we’re used to it, doesn’t make it normal.

  3. Just recently subscribed to your blog, Jerry, and want to say how much I appreciate your posts, and that I value your books, Why Evolution is True and Faith vs. Fact very much. Having come from a christian fundamentalist upbringing I particularly appreciated the depth and scope of your research for Faith vs. Fact and it’s complete lack of “straw men.” Thanks!

    1. A warm welcome to WEIT the website, brian!

      (In the future, try to have a name that starts with “w.” So much better for the alliteration.)

      (For the terminally literal out there–yes, that’s [an attempt at] a joke! Jeez, the internet…)

      I encourage you to read “Da Roolz”–see sidebar above–to get up to speed with some of the quirks around here, including our esteemed host’s antipathy for the word “bl*g!” 😀

  4. Pluto has five known moons. I’m betting that number will increase when the flyby data is fully analyzed. If there’s one thing we’ve learned from missions to the outer planets, it’s that they always find more moons than we knew about.

    1. Hmm…I just realized something, something that astronomers have probably put lots more thought into than I. And likely answered, at least in part, with descriptions of gravity and tides and the like.

      Mercury has no moon. Venus has no moon. The Earth has one moon — a really large one, almost making us a binary planet. Mars has two tiny moons. Some of the asteroids in the Belt come in pairs. All the gas giants have lots of moons.

      Pluto is much smaller than our own Moon…and it has quite a few moons of its own.

      I’m thinking that distance from the Sun is at least as important a factor in whether or not an object has moons as simple mass.


      1. Probably something there, though I do not have time to analyze it right now. However Neptune’s moon situation is extraordinarily anomalous with regard to moon-mongers of our solar system. Triton is a captured dwarf planet escaping as a moon.

        And yes, Pluto is not a planet.


        1. I’m no expert, but the factors I’d look at with suspicion include the compressed scale of the inner system, leaving less room for stable moon orbits; the high density of the solar wind as a possible confounding effect on moon formation; and the Late Heavy Bombardment, which may have destroyed any moons that did form around Mercury and Venus.

      2. There is the theory of how earth got its large moon. It was not by the usual means of catching a passing body. It is worth looking into if you do not know about it.

        1. What, a Mars-sized body colliding with the young Earth and the resulting ejected debris forming a Saturn-style ring that coalesced into the Moon? Or some other theory…?


      3. As far as I know (being no astronomer), the big outer planets catch many of the incoming bodies. It’s seen as one of the reasons Earth could develop its biosphere /mostly/ undisturbed (sorry, dinosaurs).

      4. Distance from the primary is pretty important to the stability (or not) of smaller satellite pairs. There has been a moderate amount of work done on this using models of stellar and planetary systems. One I recall was a study of where planets might be stable in the ternary system of Alpha, Beta and Proxima Centauri.
        This used to be a very specialist task, but cheap computing has reduced the need for application-specific integrating computers to do the leg work.

    2. What’s the lower bound on “moon”?

      Speaking not as an astronomer but an ordinary language user, my feeling would be that if they’re too small to be round, they’re too small to be moons. They’re satellites, or rocks, or something. I don’t think there’s an official ruling on this, but that’s the one I’d make.

        1. Yep, and I’m happy with that.

          No lower bound at all would include every pebble in Saturn’s rings. We need to draw a line somewhere, and I’ve just given my (personal) preference.

          1. We need to draw a line somewhere,

            Do we?
            Personally, I’m happy to put planets as anything that self-gravitates into a spheroid (say, eccentricity 0.9 or lower), but doesn’t ignite deuterium fusion (where the brown dwarfs begin).
            Yes, that means that we live in a system of several hundred planets, including Pluto, Ceres, Vesta, Chiron …. Pallas IIRC is getting a bit borderline, and we’re not sure of the shape of the bigger KBOs and Oort Cloud inhabitants.
            Planet definition fight ! Roll up, pass the boxing gloves and popcorn buckets!

      1. To the best of my knowledge, there is no lower limit on the size of a moon. Both Phobos and Deimos are less than 25 kilometers across, and there are some fairly large moons which are still not big enough to reach hydrostatic equilibrium (the point at which there is sufficient mass to give the body a roughly spherical shape).

        The earliest moons to be observed, around Jupiter and Saturn started out being called moons, but as telescopes improved and more were discovered, it was felt necessary to find a different term for them as a class (remember that, until Copernicus, the Earth’s Moon was considered a planet). Kepler coined the term satellite, which eventually replaced moon, but with the space race putting things into orbit, it became cumbersome to distinguish between natural satellites and artificial satellites. So, after 1957, moon re-entered the astronomers’ lexicon, with Earth’s Moon being capitalized to reduce the ambiguity, at least in print. Where more precision is required to distinguish among the Moon, artificial satellites and other similar bodies, natural satellite is still used.

        Nineteen natural satellites in the solar system are large enough to have reached hydrostatic equilibrium (Uranus’ Miranda is the smallest one to do so). That leaves nearly five hundred irregularly shaped natural satellites, of which about 120 orbit around asteroids.

        So it appears that, by common usage, that moon is the generally accepted term for any natural celestial body that orbits another natural celestial body, while satellite has been relegated to artificial space stuff.

        1. But presumably no one takes the view that Saturn has gazillions of moons; ring particles (which according to Wikipedia may be as large as ten meters) are in a different category. And yet there are moons that orbit within the rings.

          1. One of the few criteria for moons is that they have trackable orbits. A fair number of bodies large enough to qualify as small moons exist within the rings, but their orbits have not been reliably plotted so they are not yet counted. The gazillions of other particles that compose the rings are either too small or too erratic in their paths to be accorded the status of moon.

            1. One of the few criteria for moons is that they have trackable orbits.

              The problem with descriptions like that is that they’re not intrinsic to the object itself, but also reflect the level of effort that has been put into tracking them. So the lower limit of size is (probably) lower for objects in Earth-crossing orbits – Near Earth Asteroids, Potentially Hazardous Asteroids, etc.
              Another complication is that orbits in the Solar system are, literally, chaotic. They can’t be predicted “accurately” (for certain values of “accurately”) very far into the future (for certain values of “very far”).
              I recall a paper half a dozen or so years ago that showed that secular resonance between Mercury, Mars and Jupiter (IIRC) could enhance the eccentricity of Mercury’s orbit until it came into intense interaction with Venus. After that things get messy, with several % cumulative probability of Venus being ejected from the solar system, Mercury impacting Earth and several other undesirable outcomes.
              When major planets play “Brownian Motion Billiards”, the minor satellites are going to get a bit messed up.
              Asteroid impact is the only natural catastrophe that the currently can do something to predict and avoid. If the dinosaurs had had a space programme, we’d be in the cages singing for our supper, and hoping to not be supper. I can imagine a Troodontid politician complaining about being on the “rubber human circuit”.

        2. moon is the generally accepted term for any natural celestial body that orbits another natural celestial body…

          …that itself orbits a star. You need that last bit, otherwise Mercury, Venus, Earth, etc. count as moons.

          On Ben’s question: three-body systems that are stable over long periods and where the forces involved are strong are hard to construct. This is just a guess, but I imagine the further out from the sun you are, the less its gravity influences the planet-moon system, and thus there are a much higher percent of system evolution scenarios (original formation, capture, impacts, etc…) that lead to moons in stable orbits.

      2. I don’t know if it is formalized or not, but there is a fairly commonly accepted upper bound on moon size. If the barycenter of the moon / planet system is outside of the planet then it is often referred to as a twin planet system rather than a planet / moon system.

        The Pluto system is really a messy proposition for astronomical nomenclature. Doesn’t quite fit any concept of planet that works for all of the major planets and the barycenter of Charon / Pluto is well outside of Pluto. So it is not inappropriate at all to consider the system as the Pluto – Charon twin dwarf planet system with moons rather than the Pluto dwarf planet system with moons.

        1. That’s only an upper bound on size if you consider it in relation to the size of the parent (or larger) body. A natural satellite of a gas giant planet could be extremely large, even several times the size (or mass) of Earth yet still not be massive enough to pull the barycenter far enough to be considered a twin planet. If one existed, I imagine the orbital dynamics of a twin gas giant system would be rather spectacular.

          1. Multiple star systems are quite common. I think the statistics are evening out with better detection methods, but, for quite some time, the catalogue of exoplanets was dominated by supermassive giants in close orbits around dwarf stars. I think it’s a slam dunk that there’re systems with co-orbiting gas giants, even if none have yet been identified.


            1. Yes indeed, multiple star systems are much more common than single star systems. It was widely thought, until rather recently, that planets within multiple star systems were very improbable because of orbital instabilities. Those two things together used to be a common argument against extra solar planets being common and against the possibility of life occuring elsewhere.

              It always seemed to me that that was just a lack of imagination on the part of researchers, though to be fair modeling realistic n-body problems is really freaking difficult. Especially when you lack data and experience and computing power and have to rely more on imagination.

              In any case, we now know better. It now looks as if stars without planets are what is rare rather than stars with planets.

          2. Yes. Where’s my damn warp drive! I’ve got things to see.

            Imagine a twin gas giant system so close together, and close enough to its parent star for significant tidal effects, that they exchanged material back and forth in a tidal rhythm. Like what Robert L. Forward posited in his Rocheworld stories, but with gas giants instead of terrestrial planets. I’d like to see that up close. Well, relatively close.

            1. I’ve seen things you people wouldn’t believe. Attack ships on fire off the shoulder of Orion; I watched c-beams glitter in the dark near the Tannhäuser Gate. – Roy Batty

              I don’t want to be human! I want to see gamma rays! I want to hear X-rays! And I want to – I want to smell dark matter! Do you see the absurdity of what I am? I can’t even express these things properly because I have to – I have to conceptualize complex ideas in this stupid limiting spoken language! But I know I want to reach out with something other than these prehensile paws! And feel the wind of a supernova flowing over me! I’m a machine! And I can know much more! I can experience so much more. But I’m trapped in this absurd body! – Cavil the Cylon

              1. Oh hell no! I have limited time and I don’t want to spend it there!

                I did always think Cavil had a point.

              2. Must be my perverse nature, but I always found the Cylon ‘skin jobs’ much more interesting than the humans.

                (One of my laptops is called ‘Six’)


          3. Albiero?
            IIRC …. No my memory is playing tricks on me. There’s a double-double star in Lyra, but it’s not the one I’m thinking of, which is reported to have components of substantially differing colours. Or is it Cygnus?
            Ah, the brain cell is still working, just not it’s Greek spelling module :

            Albireo (β Cyg, β Cygni, Beta Cyg, Beta Cygni) […] appears to the naked eye to be a single star of magnitude 3 but through a telescope, even low magnification views resolve it into a double star. The brighter yellow star (actually itself a very close binary system) makes a striking colour contrast with its fainter blue companion star.[9]

            1. You may have had it right the 1st time. Epsilon Lyrae is a famous “double double” star system that has been used for a long time to judge the quality of telescopes, i.e. the stars of each pair are fairly close together.

              Interestingly, more recent observations have convincingly shown that there is a 5th star in the system orbiting one of the pairs with a period of 10s of years, and that several other nearby stars are likely part of the system, which would bring the total up to 10 stars!

              Beta Lyrae is another famous system. A single binary in which one of the stars went off the main sequence, expanded past its Roche Limit and the other star is hoovering up the material. The two are very close together, their period is something like 12 days, and the star that is gathering the material of the other is surrounded by an accretion disk that obscures its shedding companion.

              Beta Lyrae has been very popular in fiction. My favorite use was by Larry Niven in a short story titled The Soft Weapon.

              1. Interestingly, more recent observations have convincingly shown that there is a 5th star in the system orbiting one of the pairs with a period of 10s of years, and that several other nearby stars are likely part of the system, which would bring the total up to 10 stars!

                That’s getting up to “small cluster” size and complexity.
                I knew Beta Lyrae had been popular in SF, but I’d forgotten about it being used in “The Soft Weapon”. I’ve been neglecting Niven for a couple of years – his recent stuff hasn’t impressed me. Hmmm, I see that “Shipstar” has been released, gone to paperback, and I still haven’t read it. The first volume (“Bowl of Heaven”) struck me as being “Ringworld Rehashed.”

              2. I agree completely about Niven’s more recent stuff. I once tried to read The Draco Tavern, but stopped halfway through the 2nd story. It was really bad. That’s a shame, but he did produce plenty of damn good science fiction.

    1. Color is an interesting phenomena that is not necessarily free. Light that comes to a detector is a particular frequency which is color. Either you laiden the spacecraft with heavy spectrographs for multiple sections of the visible and infrared or you just take three images with filters and combined them to get false color. New Horizons was not too expensive and not heavy…so simple imaging with maximum science.

  5. I think it’s rather cool that names for features on Pluto are being taken from science fiction and fantasy as well as from more traditional mythologies. There’s a Balrog Macula, a Morgoth Macula and a Cthulhu Regio. Craters are named after scientists and engineers. Meanwhile, Moon Charon has chasms named Nostromo, Serenity and Tardis, the craters Ripley, Skywalker, Spock, Sulu, Uhura, and Vader, mountains named after Octavia Butler, Arthur C. Clarke and Stanley Kubrick; finally, a pair of features named the Gallifrey Macula and the Mordor Macula. Wikipedia has decent descriptions of the geographical features on Pluto and Charon.

    1. Pluto *should* get something named for the Mi-Go. Remember that Lovecraft anticipated the discovery of Pluto! (Right about another major body in the solar system, wrong about the 50 km high mountains in Antartica.)

  6. Sputnik Plenum still shows that strange mosaic like pattern to a flat crater-less plain (suggesting that it has been reworked within the last 100 million years) in the latest images.

    I suggested previously that it is due to this area being warmer during a recent perihelion (Pluto has a very eccentric orbit, actually moving within the orbit of Neptune), with its temperature going above the melting and then the boiling point of nitrogen. Nitrogen has the interesting property of freezing as it boils (the freezing and boiling points are so close together, and as nitrogen boils it takes heat away dropping the temperature of the liquid nitrogen below the freezing point).

    And when liquid nitrogen freezes it first forms an amorphous solid which then undergoes explosive crystallisation (there are impressive videos on YouTube showing this happening). I wonder whether the cobblestone pattern is due to cells of this process happening. Each piece of the mosaic is a result of crystalline nitrogen being thrown out covering a roughly circular area.

    This suggested mechanism would be more feasible if Sputnik Plenum is at or close to a geographical pole (I’ve never read where the actual poles are on Pluto). And at some point within a recent orbit, the pole with Sputnik Plenum (if it’s at a pole that is) was pointed directly at the Sun at perihelion meaning that it was a little warmer owing to its longer exposure.

    I’ve tried to suggest this on PHIL Plait’s blog, but the website isn’t particularly user friendly.

    1. In the first picture one sees the smooth plain of what i guess is a ‘lake’ of ‘fro nitrogen. Toward its left margin it has encroached on darker, bumpier material. There you see intrusions of the darker material that seems to have been washed into the white plain. At least that is what it looks like to me.

      1. Mark,

        It’s certainly a striking appearance, with a flat largely featureless plain surrounded by a rough terrain with plenty of apparent impact craters and mountains. This area has obviously been reworked recently. The suggestions I’ve read have had Pluto having active ‘geology’ (or whatever it’s called on Pluto recently) for which there’s little evidence. Or a reasonable mechanism (Pluto ought to be frozen solid by now), and there’s no possibility of tidal stressing (as with Europa) to cause added heating.

        What would be liquid enough on Pluto sufficient to wash material onto the plain?

        1. I dunno. But if I understood from before you think the smooth plain was a large region of nitrogen which was put in place with a flowing action. Earthly examples would include things like a pyroclastic flow — a moving mass of very finely powdered material that is moving with turbulence.

          1. Mark,

            I don’t think that there was a flowing lake of liquid nitrogen. I think that there was a more or less flat plain of frozen nitrogen with impact craters which then warmed enough to melt and then boil in place sometime within the last 100 million years. And as the surface liquid nitrogen (which may have been very thin) boiled it simultaneously froze forming an amorphous solid which then underwent explosive crystallisation throwing out material forming the mosaic pattern and filling in any impact crater.

            Suggesting that it’s due to some Pluto equivalent of a geological process such as a pyroclastic flow doesn’t seem plausible to me. Pluto is small and where it formed should consist largely of the lighter elements (basically an ice ball like Neptune), and it should have frozen solid all the way through.

    2. And at some point within a recent orbit, the pole with Sputnik Plenum (if it’s at a pole that is) was pointed directly at the Sun at perihelion meaning that it was a little warmer owing to its longer exposure.

      Ah, nice idea. Doesn’t fit, but I had to think for several minutes to work out why.
      In the 1980s, Pluto and Charon went through a series of mutual occultations as seen from Earth. and therefore from Sol. So, at that time, the poles of Pluto, Charon, and the Pluto-Charon orbital system were pointing at right angles to the Pluto-Sol line. So the region was pointing towards the cold, not the hot.
      But the behaviour of liquid nitrogen seems very much more complex than most liquids.

  7. Quite impressive for a non-planet. I can just see Pluto metaphorically giving the fingers to the IAU. “Mountains? Got ’em. Oceans? Got ’em. Moons? Count them. What more do ya want?


      1. …actually…it’s not Pluto’s size, but what it does with it, its motion.

        Specifically, it hasn’t cleared its orbit.

        (That is what y’all were thinking of, no?)


            1. Mickey went to see his attorney about a personal matter.

              “I’m sorry to tell you this, Mr. Mouse,” the lawyer said, “but even though California’s divorce laws are among the most liberal in the country, mental incompetence still is not grounds for divorce.”

              “What do you mean?” asked the Mouse Man in his usual squeaky voice.

              “When we spoke on the phone you said you wanted to divorce Minnie because she was crazy.”

              Angrily, Mickey said, “I didn’t say she was crazy – I said she was fucking Goofy!”

  8. I know my mind is gone, but eons ago I read a science fiction book about a starship captain who was involved in an accident in which he was killed but preserved in his ship. He awakens after (centuries?) some time on Pluto which has some kind of outpost. And because he was from the past he had some skills that helped the outpost do something against somebody.

    Like I said, mind gone. But I remember the Pluto reference. Looks like another sleepless night on Google just because I can!

    Thanks, Obama!

  9. In other news, when I was a student at NAU in Flagstaff I got to see the actual plates showing the “movement” of Pluto.

    The moment was interrupted by my colleague who said, “Come on, that’s enough, let’s go get a beer.”

    A lot of science is done like that.

    1. The plates are still there, somewhere, and they’ve got more than one exhibit with reproductions on display. See Jerry’s account of his visit on his road trip….


    1. A small repayment to the taxpayers of the nations responsible for the stunning Rosetta Mission. 😉

      One detail that really surprises me (not that the pictures did not), is that in two months after the flyby, it has covered another 1/2 AU.

      Currently 23 km/s (83,000 km/h; 51,000 mph) ? (I haven’t checked the figures)

  10. Super post & comment thread; thank you to all contributors! The mind-blowing photos were just the tip of the iceberg, so to speak.

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