In the furore following from Pluto’s demotion after Eris’s discovery, a few people argued that Pluto of all places deserved to be called a planet because it had a moon. In fact it has at least five: Charon, Kerberos, Hydra, Nix and Styx, not in that order. It certainly seems to make sense that if a world is hefty enough to have its own companions, it ought to count as a planet, but in fact that isn’t how it works, and there are actually a couple of reasons why having moons almost makes a world less planet-like, if by “planet” you mean a solid or fluid spheroidal body with a relatively strong gravitational pull.
Only two of the universally accepted major planets have no companions: Mercury and Venus. These are notably the two next to the Sun, so the reason may be that they lack the gravitational “oomph” to maintain them. Matter circling either wouldn’t have to be very far out before it felt the Sun’s pull more strongly than the planets’. That said, both of them have respectable gravitas of their own and are far more than just a bunch of rocks loosely bound together. This last is the point really. A small object is less able to hold itself together and is therefore more likely to be a collection of stones or chunks of other matter, highly porous and riddled with caves and liable to lose some of itself or not accumulate nearby bits of matter in the first place. Therefore, in a way, if a body has a few moons, this could be more a sign of it not being a proper planet rather than the other way round. The other reason is basically the same but proceeding from the other end. Many Kuiper Belt and scattered disc objects are binary, and quite possibly more than binary. The same is true to a lesser extent of the asteroids. Being binary is therefore a characteristic of agglomerations of matter which are too small to hold together, but confusingly, having moons is also a characteristic of large planets able to pull loads of stuff towards them which is either already in clumps or forms into planet-like worlds in their own right. Hence Pluto having five moons, one of which is very large indeed compared to the planet (yes, planet) itself, doesn’t count towards its possible planethood.
All this aside, Charon is so large that if it orbited alone it would definitely count as a planet, at least if Pluto does. Earth is notorious for having an unusually large moon, if moon it be, of an eighty-first of its density. Charon far outdoes this, and in doing so consequently outdoes all the other planets in this respect, whose moons are generally well under a thousandth of their mass. Charon’s mass is a little under an eighth of Pluto’s, which is deceptively small as it should be remembered that the diameter relates to the cube root of this figure. After all, Cynthia is a large disc in our sky because it’s a quarter of Earth’s diameter, not 1/81. If the ratio applied to Cynthia and Earth, the former would be considerably larger than Mars, and it might even be habitable, which raises the question of whether such double habitable planets exist out there somewhere. Charon is 1212 kilometres in diameter. Cynthia, like many moons, always shows the same face to us, and the same is true of Charon and Pluto, but in their case the situation is mutual. Both worlds face each other at all times.
I’ve allowed Charon to be overshadowed by Pluto in my own mind, and know relatively little about it. The story of its discovery and naming is quite remarkable. The mythological figure Charon is of course the entity who ferries the souls of the dead across the River Styx into the Underworld, and Pluto being king of the aforementioned domain, one might fancy that the motivation for calling the moon that was clear. However, this is not in fact so. The man who discovered Charon, James W. Christy, actually named it after his wife Charlene Mary, whom he calls Char, and had no idea that the Ferryman was called that too. This gives me pause for thought, because it doesn’t seem to work like one would expect it to naturalistically. It’s reminiscent of the fact that before Saturn was believed to have rings, saturnine herbs were those which had prominent rings, and it’s almost as if the names of celestial bodies are “out there” waiting to be discovered rather than invented, like the non-existent American states of Jefferson and Superior. I won’t dwell too long on this here, but a similar phenomenon is manifested in western astrology where hypothetical planets have been used which have turned out to be real, particularly Pluto.
On 22nd June 1978, Christy noticed that his image of Pluto was not circular, and also that it changed shape on a regular, predictable basis:
Pluto appeared to have a lump on its side which appeared and disappeared. Since the planet is far too big to be irregular, it was correctly concluded correctly that it has a moon, and that that moon takes almost six and a half days to orbit Pluto, or rather, that the two of them take that long to orbit each other. Of all moons and planets in the system, other than small irregular ones, Pluto and Charon are respectively the first and second largest worlds in their companion’s skies, even larger than the Sun in Mercury’s sky (which actually isn’t that large though). Due to captured rotation, that’s also the day length for both Pluto and Charon, and it makes Pluto the only planet to have captured rotation with its satellite, to the extent that it actually counts as a planet, not because of the IAU but because it’s binary and almost orbits Charon rather than the other way round. Axial inclination can also be guessed at fairly reliably with this because the two are likely to circle over each others’ equators, and it’s 57°, exceeding 45° and leading to different variations in day length and the like for the two. Any tilt over 45° involves a peculiar set of circumstances where the polar circles are closer to the equator than the tropics are, though at such a distance from the Sun it’s questionable whether it makes much difference. One thing which definitely does make a difference on Pluto is the atmosphere snowing onto the surface in the autumn and evaporating again in the spring, bearing in mind that the dates for these are more than a dozen decades apart. Speaking of dates, there are 14 205½ Charonian (or Plutonian) days in their year.
The two share many characteristics. Some of these are also shared with Triton, which is closer to Pluto in size and mass than Charon is, but the conditions on the two are even more similar because of their gravitational influence on each other and being the same distance from the Sun, having the same axial tilt and day length and so forth. It’s actually slightly awkward to talk about Charon separately from Pluto, but I’ve written quite a bit about the latter already and don’t want to go over it again. New Horizons managed to take photos of the two together, like this:
This picture is a bit misleading, as it’s effectively taken through a telephoto lens. It wouldn’t be possible to see this similarity near either world because the two are almost 20 000 kilometres apart and Charon is considerably smaller than Pluto even though they are closer in size than any other planet-moon combination. Even so, Charon is notably duller and has a reddish cap over its north polar region, whereas Pluto’s is closer to its equator. This red substance is, however, the same, and seems to have been shed from Pluto and deposited on Charon. Unsurprisingly, it consists of tholins, which are as I’ve said before an organic mixture of dark red tarry stuff which reminds me of the deposits made by herbal tinctures, partly because they actually are quite similar. Tannins in particular spring to mind. To repeat myself from elsewhere on this blog, tholins are the alternative route taken in the Universe by organic chemistry to organic life. The question of how often organic chemistry becomes biochemistry is another question, but there are clearly countless examples of tholins in the Universe judging by how many there are orbiting the Sun. Methane is also deposited on the surface from Pluto. Before any of the stuff gets there, though, it’s been part of Pluto’s atmosphere, and is therefore deposited faster near perihelion. Also, we finally get an answer to why trailing hemispheres are more heavily coated than leading ones: it’s because of gravity. Trailing hemispheres simply bear the brunt of falling material because the material has fallen further by then. The north cap is called Mordor Macula, “macula” meaning “spot”, as in “immaculate” – “spotless”.
Unlike Pluto, whose surface is largely solid nitrogen, Charon’s surface away from the tholin cap is mainly water ice but there are also patches of ammonia hydrates. Also unlike Pluto, there is effectively no atmosphere, so the snowing and sublimating processes on that planet don’t occur here. The south pole is also rather dark, but the north is darker. Although Charon doesn’t have a persistent atmosphere, substances on its surface do sublimate, becoming gas. It’s just that its gravity isn’t strong enough to hold on to any of them. The southern polar region was actually imaged with the help of “plutoshine”, as it was night time there when New Horizons visited, so image processing involved removing the tint of Pluto’s light to restore it to how it would’ve looked if sunlit.
Charon does actually seem to be geologically active, with geysers similar to those on Triton, shedding water ice and ammonia nitrate. This must’ve happened last less than thirty millennia ago, probably a lot less, because the ice deposits are still crystalline and haven’t changed to the glassy form expected after such a long period of time. The different composition of the geyser plumes also means that the moon is different beneath the surface and has geological layers, which was previously controversial as it is quite small. It’s likely that the moon is geologically active due to Pluto raising tides within it, a possibly mutual process, which raises the question of whether there’s substantial heating and an internal water ocean, which it’s becoming apparent is very common in the Universe. Scientists believe that in the distant geological past, it did indeed have an ocean within it but that this froze and expanded, leading to the formation of the enormous canyons visible on its surface in the image at the top of this blog post. This is one way in which water, as a geologically significant compound, behaves differently and leads to different land forms than other substances which melt and freeze. On Earth, water is currently not often a geologically significant “rock”, except at high altitudes and within the polar circles. Beyond the frost line of the Solar System, it often is, and unlike the other liquids, which are often gaseous at Earth-like temperatures, it expands on freezing, leading to geology very unlike ours. Although there are some other substances which expand on freezing, such as bismuth and gallium, they don’t generally occur in bulk. In the case of Charon, water ice is a major and significant mineral which contributes to the landscape and interior in a way something like silicate or carbonate rock does on or in Earth.
More precisely, the reason for those canyons is that as the interior of the moon froze, it expanded and fractured the surface, leading to the formation of a number of features referred to as “chasmata” – “chasms”. These include Tardis, Serenity, Nostromo, Caleuche, Mandjet, Argo and Macross. Many of these have a rather obvious naming scheme, which is fun. Caleuche, which is named after a mythical boat which sails the coast of Chile collecting the souls of the dead, is a Y-shaped canyon thirteen kilometres deep, among the deepest chasmata in the system. Mandjet is thirty kilometres wide, four kilometres deep and 385 kilometres long. Serenity is two hundred kilometres long as a chasma but runs an additional two hundred as an unpaired escarpment. All of these chasmata run around the moon’s equator, separating the northern Oz Terra from the southern Vulcan Planum, which is named after Spock’s planet. Oz is a kilometre higher than Vulcan over its whole surface. Both Oz and Vulcan extend across into the portion of the moon which was dark when New Horizons got there, but it seems likely that each occupies an entire hemisphere. Vulcan is less heavily cratered, suggesting that there’s recently (relatively) been geological activity there which has erased them by remodelling the surface. However, there are some craters and also central mountains, including Kirk and Kubrick. Spock, Sulu and Uhura are also represented thus, as well as Clarke (Arthur C Clarke). The entire area seems to have been covered by a large flow of liquid over the entire hemisphere, probably water.
Other craters include Vader, Pirx, Alice, Organa, Dorothy, Nemo, Skywalker, Ripley, Revati, Sadko, Nasredin, Cora and Kaguya-Hime. I do wonder how people whose religion includes some of these figures feel about the avowèdly fictional characters represented here, but perhaps the day will come when the Vulcan and Jedi world views become official religions too, if they haven’t already. There is another macula, Gallifrey, through whose middle Tardis runs. This means, oddly, that the confusion the Bi-Al Medical Foundation receptionist shows in the ‘Doctor Who’ adventure ‘The Invisible Enemy’ could be explained in a fangirlish way by the presence of this feature, which creates an Ontological Paradox similar to the one created by K-9’s motherboard, introduced in the same episode.
That, then, is Charon, which deserves considerable attention as the largest and best-known of Pluto’s moons. However, there are four more to be covered, and this raises a question: how do they orbit? All other known satellite systems with more than two members consist of a relatively large planet and a number of much smaller moons, and although the orbital dynamics can be somewhat peculiar, such as coörbital moons regularly swapping positions, Pluto-Charon is a different matter. There are two relatively similar masses and other moons in the immediate vicinity. It was calculated at one point that there could be stable orbits in such a situation if an object was at least 3.5 times closer to one mass than the other or if it was at least 3.5 times the maximum separation between the pair, and there are also improbable but stable orbits of various kinds between them such as a figure of eight. Ternary star systems usually have two close companions and a third, much more distant one: this is true, for example, of the Centauri system, where Proxima is much further away than A and B are from each other. The Pluto-Charon system is unique as far as is known in the Solar System in this respect.
Where, then, are the other moons?
This is an image taken by the Hubble Space Telescope three years before New Horizons reached Pluto, and was used to plan the mission. It’s notable that Charon and Pluto actually look fainter in this image than Hydra and Nix, or at least smaller. Styx doesn’t seem to be far away enough to maintain its trajectory. This picture shows that the moons are outside the Pluto-Charon region, separated by a small gap but all relatively close to each other, in an arrangement which reminds me slightly of the TRAPPIST-1 system where several planets are within the habitable zone. They don’t seem to be spaced any way like the Titius-Bode Series and although there is a space between the inner two and the rest, the relative distances of the others are not like those of ternary stars. It also raises two questions in my mind: is this similar to how planetary systems might be arranged around binary stars? Also, is this where Earth’s other moons would be if we had any?
There’s a further surprise. At least two of them are merged double moons themselves, namely Hydra and Kerberos. Going off on a tangent for a moment, bearing in mind that scientists now have sufficient reliable information to establish that two of the small moons of Pluto are former double moons, what the heck do flat Earthers and people who believe, and I quote, “space is fake” think is going on here? Why would NASA, other space agencies and the global astronomical community bother to put in that kind of detail about an entirely bogus cosmos? On the other hand, it is also true that esoteric blind alleys have been known to become highly elaborate, so maybe they think it’s along those lines. Also, fictional universes can be very intricate too. It just strikes me as highly implausible that something like this would be made up and makes me wonder about how flat Earthers think.
Anyway. . .
Hydra and Kerberos are former double moons, and this is evident from their shapes. This is Hydra:
This shape is similar to the comet being studied by the Rosetta probe, and in the comet’s case it’s thought to result from the merging of two bodies. This is that comet, known as 67P:
In the comet’s case, it’s been suggested that the shape results from the heat of the Sun eroding the nucleus. However, each lobe has concentric strata, suggesting that it was originally two bodies which got stuck together. Were it only one, it would have layers indicating a former, more regular form. Hydra is fifty-one kilometres long. Like all the small moons, Hydra is shiny with water ice, and is the outermost moon at a distance of 64 738 kilometres from the barycentre, which is outside Pluto. It’s probably receded from Pluto-Charon due to tidal forces. The name is a bit unusual and sticks out because it isn’t named after a humanoid mythological figure, and this principle also applies to the next moon in.
Which is Kerberos, named after the four-headed (the snake forming the tail has a head) guard dog of the Greek Underworld. Isaac Asimov once suggested that the tenth planet should be called Cerberus so that a mission approaching the Solar System from the great beyond would encounter the system’s guard dog first. To that end, it makes more sense that Hydra be called Kerberos and since the latter was already known to be closer to Pluto than Hydra when it was discovered, its name lacks elegance in a way. There are no good images of the moon:
This image gives the impression that the moon has done something naughty and needs to have its identity protected, but it can again be seen to have two lobes, suggesting again that it’s the result of the collision of two former moons. The two-lobed “dumb bell” appearance is quite common and approached by orbit-swapping moon pairs of moons near other planets. It’s about nineteen kilometres long and averages 57 783 kilometres from the barycentre. This figure combined with Hydra’s gives some indication of how close together the outer moons are, as these are the two outermost and there’s a highly unstable region close to Pluto-Charon, so there isn’t much space between them for moons to exist. Kerberos was named after an online poll and was not the most popular choice, and it’s spelt that way because there’s already an asteroid called Cerberus. The final choice was made by the IAU. Hmmm.
The next moon in, Nix, also has a story behind its name, which has again been re-spelt. Nyx is the Greek goddess of night, but since there was already an asteroid with that name, it became Nix in Pluto’s case, which is the Coptic spelling: “Ⲛⲓⲝ”. There’s actually a pretty good image of Nix from New Horizons:
To me, the brown smudge closest to the camera, which is eighteen kilometres across, looks like tholins, and there are also white bits which I imagine are water ice. Nix is almost exactly fifty kilometres long. Like all the smaller moons, Nix doesn’t have captured rotation but tumbles, so all these four moons have no north or south in the rotational sense.
The innermost small Plutonian moon is Styx, and if you thought Kerberos had a poor image, just look at this:
It can be conjectured to be elongated like Nix and is the dimmest known object in the Solar System at a magnitude of 27. That is, it’s as dim compared to a star like Vega as Vega itself is to the Sun, from Earth of course. I’m a little surprised by this because I would’ve thought Adonis, for example, would be dimmer, since that asteroid is only two hundred metres across, but that’s actually hundreds of times brighter at 18. Styx is a sensible name because crossing its orbit brings one into Pluto’s kingdom, more or less, and it’s also the next moon out from Charon. Styx’s longest dimension is sixteen kilometres, so it’s smaller than the oft-employed Isle Of Wight yardstick. It takes twenty days to orbit the barycentre, 42 656 kilometres away.
All of the outer moons have orbital resonances with each other. Styx is almost in harmony with Pluto-Charon too. This brings up the question of their probable mode of formation. All are grey, unlike Pluto, and are thought to have been formed in a similar manner to Cynthia, with an impact from a large body kicking up débris from the surface which later fell into orbits and coalesced. These orbits would’ve been closer to Pluto than they currently are. Interestingly, three of the moons were named in 1940 in a SF story by Peter Hamilton: Cerberus (sic), Charon and Styx. Their orbits are fairly chaotic and not fixed over millions of years.
Next time I’ll turn to the other largish worlds beyond Neptune. We’re really approaching the end now. Thank you for your patience.
A Box Of Chocolates 