Yes, I know I’m supposed to be alternating.
I’m not sure how much to make of the idiosyncratic naming scheme for the moons of the seventh planet from the Sun. As a fan of language and word play, they appeal more to me than they perhaps should if I’m just going to be talking about them in a scientific way, but the fact is, there’s the Universe and there’s the person observing the Universe, and you can’t entirely step outside yourself. The rest of the Universe is, in a sense, your mind reaching out to it and placing it within your own private world. It’s part of you. That said, science tries hard to be objective. However, it’s significant to many of us that twelve Americans walked on Cynthia and that people do romantic things “by the light of the silvery Moon”. Cynthia is culturally significant to us.
With regard to the twenty-seven known moons of the planet I’ve been calling Hamlet, it might be a little hard to imagine how such a small system so far away from us could have any consequences for us Earthians. They don’t figure prominently even in the realms of science fiction and astronomy. If we had sent more than one probe to the system, maybe it would be more significant to us all. If it turned out to be the only other abode of life in the system, it would be considered hugely important. There is in fact at least one aspect to the planet which makes it relevant to life here. There is only a weak internal heat source and the Sun makes little contribution to its temperature, leading to computer models of the atmosphere being dominated by the Coriolis Effect. Due to the abstraction of the model from observed conditions, which of course confirm its accuracy, this constitutes yet another refutation of the hypothesis that Earth is flat, because of how the effect operates in our own atmosphere and attempts by flat Earthers to explain this in terms of solar heating (and perhaps lunar cooling!). Even this, though, is something of a niche explanation.
The moons concerned, taken together, don’t add up to much, which is why I’m dealing with them all in one go. Their total mass is less than half that of Titan, and also of Neptune’s giant moon Triton, but this isn’t the same as saying they’re small for two reasons. Firstly, Titan itself is 96% of the mass of everything orbiting Saturn including the rings, so the seventh planet’s moons are actually bigger en masse than all of Saturn’s except for Titan. Secondly, volume, surface area and diameter are counter-intuitive. Our own moon has only 1/81 Earth’s mass but has a diameter a quarter of our planet’s. By the time you get this far out from the Sun, even many compounds gaseous on Earth are frozen solid. Umbriel is probably the warmest moon, because it’s dark and absorbs more light, and has a maximum temperature of -188°C, barely warmer than the boiling point of air. One consequence of this is that the densities of the moons are very low, which means they’re larger than their masses suggest. It’s also interesting to compare the situation here with that in Neptune’s vicinity.
I’m going to reiterate this yet again in case you’re coming across this post without having read any of the others: the moons of the seventh planet don’t take their names from any mythological tradition, but from works of literature, mainly Shakespeare’s plays. I find this refreshing but there is an element of cultural imperialism to this. Then again, the same is true of the dominant Greco-Roman tradition for the other planets, moons and asteroids in the system, but what’s done is done I suppose. There were two widely separated phases of discovery, which is also true to an extent of the other gas giants but in the cases of Jupiter and Saturn the rate of discovery is rather different. Jupiter’s Galilean moons were all discovered in 1610 CE, then nine moons were found between 1892 and 1975, followed by three via the Voyager probes and a spate of discoveries from 2000 on. Saturn’s show a more regular distribution between the seventeenth and nineteenth centuries, a rush associated with the Voyager missions and a further sequence of discoveries from 2000s on as with Jupiter’s. My experience of Hamlet’s moons is that five were known when I was a child, and because one’s childhood experience is just how things are, and one hasn’t yet gotten used to change, that was just how things were. I wasn’t aware of the peculiar naming scheme because at the time they seemed just to be kind of Latinate, for instance Ariel and Miranda, although one is much more likely to come across a human Miranda in everyday life than, say, a Phœbe, and way more likely than meeting someone called Ganymede. The first four were discovered in pairs in the eighteenth and nineteenth centuries, then Miranda in 1948, then we had to wait until Voyager for any more discoveries. After that, Caliban and Sycorax were found in the ’90s, Perdita was discovered using old Voyager data and the rest come from between 1999 and 2003. Since then, no more discoveries have been made but this might be because Hamlet is a neglected planet compared to the others, so maybe nobody’s looking. It is also very dim and distant, so it might be that.
Titania is the largest. This is quite possibly the poorest decision ever in naming a moon. Titan was already known by the time it was discovered and there are different ways of pronouncing it. And how do you refer to something to do with Titania without people thinking you’re talking about Titan? However, we can talk about the place. It’s the largest and most massive of the moons in a system which isn’t particularly large or massive. Here it is:
That slight blurring is probable due to the impossibility of correcting entirely for Voyager 2’s motion blur. About forty percent of its surface has been seen. Like the other moons, Titania doesn’t orbit near the plane of the Solar System due to its planet rotating on its side, meaning that that illuminated surface in the picture remains in daylight for decades at a time, just as the other side stays in night. This means that one pole is somewhere near the middle of the lit portion of that image, in this case the south, because like all such images of the moons, this was captured in 1986. All the large moons are about half rock and half ice, so they’re actually denser than many of Saturn’s, and Titania is both the largest and densest of all of them. All the moons also have largely grey surfaces, Umbriel being darker than the others, hence its name. Titania is half Cynthia’s width and has icy and dry ice patches on its surface. It’s considered likely that it’s differentiated into distinct layers with a rocky core and icy outer layers. There may be a little liquid water inside at some level. There could also be a very thin non-collisional atmosphere of carbon dioxide.
Oberon was discovered with Titania and is slightly smaller, orbiting outside Titania’s path. It’s more heavily cratered. Both are at comparable distances from their planet as Cynthia from Earth. For some time after the pair was discovered, it was thought that there were six moons overall but after many years the others came to be considered spurious, although of course there are other moons. A significant difference between it and Titania is that the latter orbits entirely within the magnetosphere whereas Oberon passes in and out of it. Again, only forty percent of the surface has been mapped. It’s also the outermost large moon. Oberon’s features are named as follows:
The surface has a sheen to it and is slightly red except where newer craters have yet to acquire that: those are slightly blue. This reddening is due to space weathering, where electrically charged particles hit the surface. Unlike all the other large moons, the trailing hemisphere has more water ice than the trailing one. It’s almost exactly the same size as Rhea, which makes me wonder if there’s a peak in moon sizes at about this diameter across the Universe as it’s also quite close to Titania in size. There are apparent rift valleys, such as Mommur Chasma. In the distant past, when the moon was young, processes within it had an influence, namely its slight expansion by about half a percent of its diameter. Mommur Chasma is apparently named after the original French version of the tale of Oberon’s home, «Huon de Bordeaux».
Miranda and Umbriel are probably the most distinctive of the large moons. “Miranda” the word is a gerund meaning “worth seeing”, hence the “-anda” names Amanda – “worthy of love” and Miranda. Samuel Johnson once said of the Giant’s Causeway that it was “worth seeing, but not worth going to see”. Well, Miranda seems to fall into the same category. It is indeed worth seeing but given that only one spacecraft has ever been there, possibly not worth going to see. However, it’s still remarkable. Here it is:
As you can see, it looks rather rough. It has a diameter of 370 kilometres and is therefore on the lower edge of worlds whose gravity is able to smooth them into an approximate sphere. At some point in the past, it was hit by something and shattered into small pieces which then all fell back together haphazardly. There are enormous cliffs all over the moon, including the highest cliff in the System, Verona. Twenty kilometres high, if an object falls off Verona cliff it would take ten minutes to fall to its foot. Although it’s tempting to believe that these cliffs are the result of the shattering, they’re more likely to be due to the same kind of expansion as Oberon’s chasms. The number of craters suggests Miranda was only formed during the Mesozoic, or at least that whatever happened to it took place then.
Umbriel is the only major moon not at least ambiguously named after a Shakespeare character. Instead, the name is taken from Alexander Pope’s ‘The Rape Of The Lock’, where it refers to a “dusky melancholy Spright”, also referred to as a gnome. Clearly the name is related to the Italian and Latin “umbra” – shadow. As well as being particularly dark, Umbriel has a crater outlined in bright white material where a pole would’ve been if it orbited normally, but it so happens not to be situated there because of its primary’s odd axial tilt:
The mere fact that the light ring is at the top of this picture shouldn’t be taken to indicate that it’s at any kind of pole, because the moon rolls round as it orbits in a manner typical of such bodies, but its orientation here makes it look like a polar feature. Its name is Wunda and the feature is ten kilometres wide. Its origin is unknown. The surface is generally dark bluish, although that’s a relative way of describing it along the lines of “black” often being tinged with a cast of a particular hue rather than it being pure black. However, it also seems odd to me because most dark objects in the outer system are red-tinged rather than blue, suggesting that it isn’t the usual tholins that are coating the surface. Nothing other than craters are known on the surface unless you count the ring.
Ariel is the other major moon with an ambiguous name, as it could be named after either Ariel from Shakespeare or Ariel from Pope. Its mass is about the same as all the water on Earth’s surface. It’s somewhat bigger than Miranda and slightly larger than Ceres. It’s half ice and half rock, and despite its name has no washing powder on its surface. That comment isn’t quite as flippant as it sounds because other bodies in the Solar System do have washing soda in and on them, including Ceres.
What the heck is it about this planet and its system which leads to it having such peculiar names‽
Right, so Ariel is the second closest major moon to its planet. It’s also the brightest per area at around four times as bright as Cynthia, although being twenty times as far from the Sun it only has a four hundredth of the sunlight falling on each square metre in the first place and is well under half the size. Its surface is more varied than the likes of Umbriel, as far as has been seen anyway, with canyons, ridges, craters and plains all present. The chasms are often bowed in the middle rather than flat or tapering, and seem to result from freezing water and ammonia altering the dimensions of the moon. Chasms often become ridges, suggesting that they are a similar response to the freezing of liquids, so the moon’s surface could be seen as a mixture of the wrinkly deflating balloon and the cracks of an expanding soufflé (but without the bubbles). The plains are probably similar to lunar maria, in this case involving the eruption of a thick liquid, possibly a mixture of ammonia and water. There are no large craters, suggesting that the surface is younger than the Late Heavy Bombardment period early in the system’s history. The largest crater is the 78 kilometre-wide Yangoor. Ariel has similarities with Saturn’s Dione.
Those, then, are all the large moons. To summarise that bit of the system, they are in order Miranda, Ariel, Umbriel, Titania and Oberon. Their spacing corresponds to a law similar to the Titius-Bode Series relating to the spacing of the planets, if that is indeed valid. Mary Blagg’s 1913 generalisation of Bode’s Law yielded the formula A(1.7275)n(B+f(α+nβ)), where A for this system was 2.98 and B 0.0805. Hence there seems to be something orbital resonance-related going on here. Some of them were probably warmer in the past due to having less circular orbits and so more vigorous tides.
I want to mention a slight personal peculiarity at this point. As a small child I used to delight in memorising the names of the moons of the outer planets. This led to the oddness of Jupiter’s moons having their names changed to my considerable confusion in the late ’70s. In the case of “Hamlet”, the seventh planet, the planet whose name one dare not speak, the list was rather short and didn’t really stick in my memory, but oddly it had an extra member according to my unreliable recollection: Belinda. I didn’t think much of this because the subject of those moons rarely or never arose until 1986, and even then it wasn’t all that, partly due to the Challenger disaster. Belinda is a small moon orbiting below Miranda which wasn’t discovered until 1986. I had no knowledge of ‘The Rape Of The Lock’ at this time, so I can’t account for the fact that for well over a decade I thought there was a moon called Belinda when it didn’t even get named until after the Voyager 2 mission. This seems to be rather akin to a Mandela Effect, such as the placement of single releases in my memory being several years different than in reality. For what it’s worth, Belinda is an elongated moon 128 kilometres long by sixty-four kilometres wide and extremely dark, and it may collide with other moons in a hundred million years or so, so it could be a future ring. There are thirteen known moons within Miranda’s orbit and many of them are elongated, although I personally wonder if that’s the reality or whether it’s motion blur. Presumably that’s been taken into account though. These cis Mirandan moons are known as the “Portia Group” and are named Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, (the second largest, at 156 kilometres maximum diameter), Rosalind, Cupid, Belinda, Perdita, Puck and Mab. Puck is the largest, with a diameter of 162 kilometres and was the first discovery after the larger moons, in 1985, by Voyager 2 shortly before it began the main part of its mission. It’s heavily cratered, dark and has water ice on its surface. Because it was the first moon to be discovered, there was time to program the probe to get more information on it than the other small moons. Three of its craters are named: Butz, Lob and Bogle, named after impish spirits in European mythologies.
Then there are the nine known outer moons, which are trans Oberonian: Francisco, Caliban, Stephano, Trinculo, Sycorax, Margaret, Prospero, Setebos and Ferdinand. Sycorax is the largest of these at 157 kilometres diameter. It’s more than twenty times further out than Oberon and is light red in colour. It has its own rotation period of seven hours, not locked to the planet and takes three and a half years to orbit. It averages twelve million kilometres from Hamlet. All of the outer moons orbit backwards with respect to the planet, which itself technically rotates in the opposite direction to all other official planets except Venus. The orbits are not in the equatorial plane. The outermost moon is Ferdinand, orbiting on average twenty million kilometres from the planet and taking almost eight years to do so. Margaret is unique among this group in orbiting in the same direction as the large moons.
When the large moons were first discovered they were numbered in order of their discovery. This was then changed to the order of their distance from the primary because of course they’d change the system because it’s “Hamlet” isn’t it? Hence there are two different numbering systems.
It isn’t that the moons are less distinctive or interesting than those of Jupiter and Saturn, although they may in fact be, so much that little is known about them. The larger ones certainly seem to be more similar to each other than those of the two largest gas giants and there isn’t as much interaction between them. They are also rather unlike the moons of Neptune, which include a major anomalous member. The general impression they give is of a system of remarkably unremarkable moons of average dimensions, although in a way this is surprising considering that they all effectively have days lasting seven dozen years.
I’m not sure what to do next. I will probably more on to the rather similar Neptune, but there might be something interesting going on between the orbits of the seventh and eighth planets so I might also consider that.
A Box Of Chocolates 