Dione is the first moon I think of when I consider the moons of Saturn. There are certainly more famous moons and moons with a lot more going on for them than Dione, but for some reason this is always the one that springs to mind. It has a sort of average look to it, except for one thing: it has whispy bits of what seem to be frost on its surface, next to a vaguely reddish mildy cratered surface more generally. It’s slightly bigger than Tethys at 1 120 kilometres diameter, making it the biggest moon so far I’ve mentioned in detail which orbits Saturn. I get the impression that its craters are more like dents than having raised rims and central peaks, and also that they’re fairly shallow. It also seems to be partly responsible for the activity inside Enceladus by raising tides within it. Like Tethys it has two coörbital satellites, one of which I know as Dione B but is now called Helene and was discovered from Earth, and Polydeuces, which was discovered using data from the Voyager missions after the probes had left the Saturn system. Polydeuces is absolutely minute at around three kilometres in diameter, and Helene larger at around forty kilometres, making it the biggest trojan moon of all in any known system as far as I’m aware. There seems to have been a lot of preliminary astronomical observation of the Saturnian system just before the Voyagers reached it, presumably to make the most of the visit in advance.
Dione has the biggest contrast of brightness on its surface except for Iapetus, which will be covered in future. This is because of its streaks of white frost, which are actually even brighter than the surface of Enceladus. All of the frosty features, referred to as “lineæ”, are on one hemisphere, which is also less cratered than the other. They’re centred on a feature almost completely covered in ice called Amata, which has a diameter of 240 kilometres, but it isn’t clear if it’s a crater or something else. The situation is not like the rays of some lunar craters and they seem to be ice-filled canyons, similar to the tiger stripes on Enceladus but having had longer to develop due to the fact that they aren’t being constantly messed about by tidal forces. One possibly simplistic way of thinking about the place is that it’s intermediate between an Enceladus-type moon, which is basically Enceladus of course, and a Tethys and Mimas-type moon, where there’s an internal heat source, probably radioactive, which kept it warm for longer in its early history before it froze through completely. This kind of range of differences, with Enceladus at one end, Dione in the middle and Tethys and Mimas at the other, could be repeated over and over again throughout the Universe in any places where worlds such as these can exist, often around gas giants about as irradiated as Saturn where they’re close enough to influence each other gravitationally or in circumstances as crowded as the TRAPPIST-1 system, where three or four planets orbit within the habitable zone of a red dwarf star. Not that system itself, because they’re too warm, but perhaps a brown dwarf.
Ceres is somewhat smaller than Dione, meaning that by the questionable 2006 IAU definition of a planet, if this moon was orbiting the Sun alone it would definitely qualify as a dwarf planet. Although it’s a lot icier than Ceres, it’s one of the densest satellites of Saturn at around 48% greater than that of water, meaning that it’s likely to contain more rock than the likes of Enceladus. The only denser moon is Titan. Dione’s distance from Saturn’s centre is comparable to Earth’s and Cynthia’s, although of course Saturn is a lot larger and its system much more elaborate than cis lunar space. Surface gravity is about a fiftieth of ours.
There’s something peculiar about the crater distribution. Normally a moon would be expected to have more craters on its leading hemisphere because that’s the one which gets there first and is more likely to be hit. A front windscreen is more likely to be shattered by a stone than a rear one. Dione, however, has more craters on the trailing hemisphere, suggesting that some major disruptive event in the past twisted it round and it’s now “in reverse”. The trailing hemisphere is also darker. The average temperature is -186°C, at which water ice is rock-hard. About a third of the moon is likely to comprise a rocky core, above which there may or may not be a water ocean, or the whole body could be frozen solid. It is, however, more likely to have an ocean than Rhea, the next moon out.
All Saturn’s moons smaller than it added together are less than this moon’s own mass but it’s far from the largest, being only fourth in size after Titan, Rhea and Iapetus. Its surface area is larger than all but six countries and bigger than India. Ceres is between it and Tethys in mass and its gravity is slightly higher than that of Iapetus, a moon which shares with it, Tethys and Rhea the appellation Sidera Lodoicera, so named by Cassini after the “Medician Moons” of Galileo, in honour this time of Louis XIV of France. However, they don’t form as straightforward a grouping as the Galileans because they’re not consecutive and Titan comes in the middle and isn’t counted as one. Their official names arrived in 1847 CE.
The mythical Dione is the daughter of Tethys and Okeanos, and therefore a water-nymph. It kind of makes sense that watery moons such as Tethys and Dione should be named after water sprites. The name is used elsewhere in Greek mythology but often for water spirits.
Mimas is special. In fact, I hope all worlds described in this series are special, but to me, Mimas is special. I’m no fan of ‘Star Wars’, so I note in passing that it looks a bit like the Death Star but may not go too much further in commenting on that. Or I might.
The Death Star is 160 kilometres in diameter, and apparently (I know little of the franchise) was replaced by another one 200 kilometres across. If that’s so, the scale on the above picture is approximately correct because Mimas has a diameter of 396 kilometres. That’s slightly smaller than mainland Scotland, although obviously its surface area is greater.
Do I need to say it’s a moon of Saturn?
Mimas is a kind of landmark in the Solar System, and in fact in terms of size generally. It’s the smallest body which has achieved approximately spherical shape by means of hydrostatic equilibrium. Most or all bodies smaller than Mimas are far from being round, and most or all bodies bigger than it are round. However, this isn’t inevitable, for two reasons. One is that objects smaller than Mimas can still just happen to be round, and considering the huge number of smaller objects there are, I wouldn’t be surprised if some of them just happened to turn out to be round by chance. It’s also possible that a larger body than Mimas could turn out to be irregular in shape due either to having lower density or being made of stronger materials. Vesta, for instance, is larger than Mimas but isn’t anywhere near as round. Taking this the other way, there can also be smaller but denser or weaker bodies which are round for that reason. An extreme example would be a neutron star, which would be only ten kilometres in diameter but would be so perfectly round it would act as a mirror. Another factor which might or might not come into consideration is surface tension. If an object made of liquid water were able to hold together in a vacuum, the chances are it could be a tiny fraction of the size of Mimas and still be spheroidal. Hence I can ask a question I don’t know the answer to: is the surface tension of molten lava sufficient to make a body spherical at a much smaller size than Mimas?
A few bits of maths can be done with this moon before actually considering anything else about it other than its rough shape and size. It has a surface area of 493 650 km2, slightly smaller than Spain, although the climate is somewhat cooler and there’s no rain at all, on the plain or otherwise. Its volume is 32.6 million cubic kilometres, which makes it less than a five hundredth the volume of Cynthia or five times the volume of all the water on this planet. And it is in fact substantially made of water ice, at a cold enough temperature that it will have contracted from the volume it would’ve been at freezing point.
Before the Voyager probes, nobody had any idea that Mimas looked like the Death Star, and since they got there in 1980 CE and ‘Star Wars’ started in 1977, there couldn’t have been any conscious inspiration, but it does make me wonder if these things sometimes happen in other ways, but I imagine this is not the kind of thing which comes up much in astronomy. Pioneer 11, with its poor camera, had approached the 400 km satellite to within 104 263 kilometres, too distant to pick out any details, even the absolutely bloody massive one of the enormous crater Herschel which is the first thing anyone notices about it. However, Voyager 1 didn’t get much closer than Pioneer 11 at 88 400 kilometres, and I don’t know about now but at the time only about half the moon was seen.
Herschel is a third the diameter of Mimas itself, with walls five kilometres high and a central peak six kilometres in height and twenty by thirty kilometres across. To scale on Earth, it would be wider than Canada. It’s centred on the equator, which makes me wonder if that’s significant. None of the other craters on the surface are more than fifty kilometres across. There’s also a distinctive distribution of craters, where more than an entire hemisphere only has small craters less than twenty kilometres in diameter and the other hemisphere, which of course includes Herschel, has larger craters. There are also valleys.
The name can be pronounced either “My mass”, which is what I say or “Me mass”, which is closer to the classical pronunciation. The way I pronounce the names of astronomical bodies reflects a time before I knew much about the way Greek and Latin were spoken and therefore I often say them as if they were English words. The adjective, a little surprisingly, is “Mimantean”, like “Atlas” and “Atlantean.” Mimas the mythological figure was the son of Gaia and born from the blood of Uranus’s castration. I’m not quite sure how they could be both.
Herschel is of course mainly flat, meaning that the horizon from anywhere on its surface is further away than the horizon at eye level on Earth. From the central peak it would be even further. This probably means that of any spheroidal body in the system, the central peak of Mimas is the record-breaking location for seeing the maximum portion of any world at something like a twentieth of the moon’s surface. From the rim, it’s possible to see all the way across the crater, a distance of up to thirty kilometres, but also, because the rim is raised five kilometres above the “geoid”, it’s possible to see the rim from more than twenty kilometres away. To some extent it’s mysterious that the moon managed to hold together at all from the impact which formed the crater. Although there are much larger impact basins elsewhere, only the one on Mimas has a practically flat surface because of the small size and low gravity of the moon.
The way Mimas moves suggests that it contains a liquid ocean, but some scientists consider this unlikely because the moon is so small one would normally expect it to be frozen solid, so it isn’t known for sure if this is so. It’s more likely that the reason is either that the core is not spherical, not at the centre, or that HersUnlike many other places, Mimas has no ray craters. These are craters whose rims have lines radiating out from them such as Tycho, as can be seen clearly on Cynthia. This is thought to be due to the extreme brightness of the surface, which reflects 96% of the light falling upon it, thought to be due to it being covered in the kind of frost found on Earth. However, there are chasms around ten kilometres wide, around one to two kilometres deep and up to ninety kilometres long. I would imagine these are cracks caused by the impact, and on another Saturnian moon, which I will cover in future, it’s thought that an impact broke the entire moon apart and it fell back together again. I would expect the same to have happened to Mimas, although its lower gravity might have stopped this from happening.
Mimas is at least sixty percent water ice. What isn’t is probably due to impact by non-icy meteorites becoming embedded and gradually sinking into the interior.
It has exactly half the orbital period of the more distant moon Tethys and orbits twice for every three orbits of the moon Pandora, which is a shepherd moon. It’s also responsible for the Cassini Division.
The only other thing I can think of is that the map of the moon’s surface temperature looks like Pacman.
Because I just spent two posts not talking about the Solar System, for reasons I hope make sense, tomorrow’s post will be about one of the most interesting moons of all: Enceladus.
No, King John did not sign the Magna Carta here. Buddy Holly is not alive and well here. Christmas has never been celebrated here. Nor is this in Surrey. That’s Runnymede. Incidentally, Buddy Holly doesn’t live there either, although Christmas has definitely been celebrated in it. However, this is Ganymede, the largest moon in the Solar System and therefore the largest Galilean. It’s larger than Mercury and Pluto, but smaller than Mars. That said, it’s only 45% of Mercury’s mass.
To explain the rest, ‘1066 And All That’ claims that King John signed the Magna Carta on Ganymede. This opens up the possibility of a weirdly transposed version of English or British history where all the stuff that went on in our Middle Ages could be copied and used to tell the tale of a British version of the entire Solar System, perhaps with Jupiter as its capital. That makes me wonder where Scotland is. Ganymede also turns up in the rather startlingly entitled ‘Buddy Holly Is Alive And Well On Ganymede’, a novel which recounts the tale of one Oliver Vale, conceived at the moment of Buddy Holly’s death, who thirty years later finds that all the TV stations in the world have their signal hijacked by a broadcast of a rather bemused Buddy Holly on Ganymede who knows nothing of his situation except that there’s a TV camera pointing at him and a sign next to it reading
For assistance, contact Oliver Vale, 10146 Southwest 163rd Street, Topeka, Kansas, U.S.A.
It’s actually quite a good book.
As for Xmas, Isaac Asimov wrote a 1940 CE story called ‘Christmas On Ganymede’ about a mining company on said moon whose employees hear about Christmas and proceed to go on strike until visited by Santa in his flying sleigh pulled by reindeer. This version of Ganymede is much denser than the real one and has an almost-breathable atmosphere containing oxygen. I suspect Asimov already knew Ganymede wasn’t like that.
I’ve always called it “Ganymeed”, but it’s supposed to be pronounced “Ganymeedee”, which is how Buddy Holly says it in the book so it must be true, but I think both are acceptable pronunciations. Ganymede, or Ganymedes, himself, was a Trojan prince abducted by Zeus to serve as a cup-bearer to the Olympians. This means Ganymede was Zeus’s sexual partner in a pederastic setting, so the situation is mixed. On the one hand, we have a moon acknowledging homosexuality, but on the other current values place him as a victim in the same way as Io and Europa are of Zeus’s insatiable lust. He’s the basis of Aquarius, but the constellation Crater has nothing to do with either. I don’t know why Ganymede was the name given to the largest moon and I’m now wondering if Kepler or Marius, who named them in 1614, was secretly gay.
The next largest moon is Saturn’s Titan, which is also larger than Mercury. This makes it the ninth largest known object in the system. It’s the tenth largest by mass, again just ahead of Titan and giving it a larger surface area than Eurasia by quite a margin, and slightly larger than the Atlantic. It also contains an internal ocean with more water than exists on Earth. It takes four times as long to orbit Jupiter as Io does, and twice as long as Europa, so once again it’s in orbital harmony with other Galileans. It’s also the most massive moon, which puts it in a slightly odd position as its surface gravity is lower than Io’s or Europa’s, because it continues the trend of decreasing density with distance from Jupiter. It gets closest to Europa, at 400 000 kilometres, just over the distance between Earth and Cynthia. Callisto is somewhat apart from the others. Io and Europa taken together are less massive, but the imbalance between a single large moon and several or many smaller ones whose combined mass is less doesn’t apply in the Jovian system. In a way, Ganymede is the Jupiter of Jovian moons. It also, perhaps surprisingly, has the lowest escape velocity of all the Galileans, meaning that it won’t be able to hold onto anything like a proper atmosphere, or even the kind of atmosphere the inner two have. Like those though, it orbits within the radiation belts. Until the outer planets and moons were more thoroughly explored in the 1980s and more recently, it wasn’t clear out of the three moons of Ganymede, Titan and Triton which one was the biggest.
The moon was big enough for large Earth-bound telescopes to make out at least one of its surface features, Galileo Regio. The rather vaguely named regiones are Galileo, Marius, Perrine, Barnard and Nicholson, and are the dark patches. They also have sulci across them, of which there are over a dozen. Unlike the two inner Galileans, Ganymede’s surface has not been extensively reworked due to tidal forces and it therefore has a fair number of craters, though not as many as somewhere like Mercury. It’s the brightest of all the moons in our sky other than Cynthia, although it’s dimmer per unit area than Europa, because it’s also the largest. To some extent it resembles Cynthia, as the darker regiones are like the maria (seas) and there are also craters, but the broad sulci are not found on the lunar surface. Due to the surface being largely ice and at this temperature being softer than rock as we know it, it isn’t as craggy either, although it’s not as smooth as Europa. The gravity being lower might contribute to this. The maximum elevation is found among the sulci, which reach about seven hundred metres above the surface.
Galileo Regio is the size of Antarctica. It covers a third of the hemisphere facing away from Jupiter. Putting this into perspective, this means that as far as Earth is concerned, our continents and oceans would mainly be visible from Jupiter with a good telescope, although Australia might not be, and Jupiter is almost our neighbour in cosmic terms. All we’d be able to do from that distance is discern that the continents and oceans existed and were differently-coloured from each other. The most distinctive feature of the moon, and let’s once again affirm that by a more recent view than the 2006 IAU definition Ganymede is also a planet as much as Pluto is, is its grooved surface and the stripe-like features where they’re bundled together. These lighter sulci are newer than the dark regiones. It and Earth are the only known bodies which have lateral faulting, that is, where a fracture in the ground leads to the surfaces sliding along the fault rather than subsiding or rising. These sulci divide the terrain into polygonal blocks, the regiones, up to a thousand kilometres across. Although the moon is not currently active and drifting doesn’t occur, it has done in the past and this arrangement of plates separated by lateral fault zones is similar to Earth’s continental plates, making Ganymede the only other world in the system which has this kind of arrangement. Not even Venus, which is geologically quite like Earth in many ways, has this feature.
The crust is somewhat weak and can’t stand heavy weights upon it, and is underlaid by a much deeper layer of water. This leads to “drowned” looking craters which look quite similar to the ones on the lunar surface which became flooded with lava in æons past, but unlike them their origins are not associated with flows of liquid but mere collapse into the surface due to the weakness of the material. Most craters are on the regiones because they’re older. About half of the bulk of the planet (let’s call a spade a spade now we’re allowed to again) is ice and half is rock, although it isn’t clear how this is distributed. It does have a very large rocky core under the deep oceans, and also its own magnetic field, practically guaranteeing an iron-rich centre like Earth’s. Although one way of looking at the interior is as a frozen-over deep ocean of salt water over an ocean bed, it could equally well be described as a planet where ice and water replace our rocks and magma, with a mantle of water rather than molten rock. However, because the gravity is so low there, the pressure at the bottom of the ocean/mantle wouldn’t be excessive. As well as ice, there is clay mixed in with the crust, and there may also be ammonia ice. There’s also more dry ice at the poles, and as with several of the other moons the leading and trailing hemispheres of the planet have different surface compositions, probably because of Io again as the trailing side has more frozen sulphur dioxide. I have to admit that I don’t understand why these moons have deposits from Io on the trailing hemisphere rather than the leading one because it seems to me that they’d be entering a cloud of the stuff, which would then land on the “front” of the moon.
The crust is eight hundred kilometres deep and contains the kind of ice we’re familiar with here along with, as I’ve said, clay, but may also contain bubbles of the same kind of oxygen as we have in our atmosphere. Above it is, for the same reasons as on Europa, an extremely thin atmosphere of oxygen and ozone, and I’m guessing the ozone is formed by Jovian radiation in the same way as an electrical spark forms ozone here. This is a small fraction of a nanobar in pressure. Deep in the crust are large clusters of rock, which might either be piles at the bottom due to its inability to support their weight or embedded in the crust due to its ability to support it! There is then what may be a further hundred kilometres of water, ten times deeper than our Marianas Trench. This is salty, as can be seen by the way aurora behaves on the planet, influenced by the magnetic behaviour of the brine. The fact that there is so much water seems appropriate for a planet named after Zeus’s water-carrier. Ganymede is the only moon in the system with a magnetic field.
Beneath the ocean lies a layer which makes the existence of life as we know it unlikely. Although the lower gravity reduces the pressure, the ocean is so deep that it manages to compress the water back into ice, but of a different kind than we would come across here: tetragonal ice. There is more than one kind of tetragonal ice, and this one is referred to as “ice VI”. The ice we encounter close at hand on Earth’s surface is hexagonal, as can be seen for example in hexagonally-symmetrical snowflakes and the hexagonal columns which form in frost and elsewhere. Ganymede’s lower layer of ice is heavier than water and more akin to the normal behaviour of freezing materials than our ice, because it contracts on freezing. Its crystals consist of elongated cuboids composed each of ten water molecules. Depending on the pressure, its melting point can be as high as 82°C or as low as 0.16°C and it’s 31% denser than water. This kind of ice turns up in the interior of some icy moons. In Ganymede’s case it seems to rule out the existence of thermal vents which could provide energy for life, as it probably does elsewhere in the Universe in many ocean planets, because it forms a thick layer on top of the rocky surface below it which volcanism wouldn’t be able to penetrate.
The structure of the ocean may not be that simple though. The ocean may in fact be arranged in four layers separated by shells of ice of different kinds. Beneath the “ordinary” ice crust on the outside, there may be a relatively shallow ocean on top of a layer of ice III snow. Ice III has a similar crystal structure to ice IV, but because this is snow it would consist of non-hexagonal flakes, perhaps more like needles than hexagons or six-pointed stars. This could be floating on top of a second, deeper ocean, below which is a layer of ice V. This is tens to hundreds of kilometres deep and is monoclinic in structure – that is, two of its axes of symmetry are at 90° but the third is slanted. Examples of monoclinic minerals include gypsum (blackboard chalk), jade and some feldspars (which can be enormous crystals the size of buses found in caves). Then there’s a final layer of water followed by the aforementioned ice VI base.
Below the rocks is a liquid outer core consisting of a mixture of iron pyrites (fools’ gold – this is a sulphide of iron) and iron, and the final solid inner core is made of iron.
A few other bits and pieces. The radiation on the surface is sufficiently weak to be fatal to unprotected humans after a few weeks, but it still wouldn’t be a good idea to go there. There are ray craters like those we see on Cynthia, such as Tycho, which may have light or dark rays depending on where the impact occurred. The “drowned” craters are described as “palimpsests”, after the faint remnants of writing seen in old documents which have been over-written later. Nobody understands why there is a strong magnetic field.
For such a large moon I find it a little disappointing that Ganymede isn’t better known. It feels like there’s either a lack of information on the place or that it’s overshadowed by its more exciting neighbours. Io has the hyperactive volcanism, Europa the possibility of life. Ganymede has if anything a greater right to be thought of as a planet than any other moon in this solar system, being larger than Mercury, and might be expected to be either more interesting or a better-studied place but it definitely comes across as more placid. Also, for its size it’s surprisingly light. This lack of knowledge is likely to change in the next few years when the European Jupiter Icy Moons Explorer (JUICE) is launched to investigate Europa, Ganymede and Callisto, excluding the non-icy Io. This will ultimately orbit Ganymede for around two years before being crashed into its surface. There’s a lot of that, isn’t there?
A Box Of Chocolates 