Dione

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.

That seems to be it for Dione. Next time: Rhea.

Planet Cueball

The first time I saw images of Jupiter’s moon Europa, it reminded me, for some reason, of a softball. I realise it looks a lot more like a cue ball than that, and I can’t explain why I got that association rather than the other. Because I was thinking of a relatively pristine object, it always makes me feel that it’s a bit worn out, scuffed, dirty and in particular scratched, and it makes me feel like I’ve got dusty hands like I’ve just picked up a mucky ball in dry but dirty conditions, as prevailed in our sports hall at school. I may be wrong about this, but my impression of Kent generally is that it’s rather dustier and sandier than the English Midlands, and that does make sense given its slightly warmer, drier climate. Over the channel it seems to become slightly more so, but I don’t know because it doesn’t seem like the difference is that big. The average annual temperature in Canterbury is 11°C and precipitation is 728 mm. Compare this to a place I don’t live (because I don’t want to doxx myself) but do live fairly near, Oakham is slightly drier at 716 mm precipitation annually and slightly cooler at 9.8°C, so in fact it seems not to be true.

But this post is not about the climate of East Kent but if anything, the climate of Jupiter’s moon Europa. Europa is in some ways very Earth-like in a way no other planet (see here for why I’m calling it that) is. It’s the smallest Galilean at 3 126 kilometres in diameter, which makes it slightly smaller than Cynthia. There are of course more than six dozen still smaller Jovian moons and if we could see Europa from the distance we see the lunar surface from, it would look about the same size, but would be four and a half times brighter and lacks the shadows our satellite has due to its flatter relief.

The “accident” of its naming opens it up to comparisons to the pretend continent with a similar name, and it’s also worth explaining why it has the same name, so let’s start with that. Europa the mythical, or possibly historical, figure was King Minos of Crete’s wife. There have been attempts to connect the name to the Akkadian word for “west”, ‘ereb, and that’s quite neat because it then allows Asia to be connected to a word for “east” and Afrika to a word for “south” (I think), but it may not work. It might also mean “wide face”, which is how it sounds in Greek. As usual for these stories, Zeus abducted or raped Europa, and this time he was in the form of a bull hiding in her father’s herds. This was commemorated as the constellation Taurus. The association with Europe is therefore somewhat surprising, but the way it worked was that it was initially applied to cis Balkan Thrace by the Greeks, then became the name of a Roman province including that area, which was then used to supplant the division which had emerged between the eastern and western Roman Empire. I have to say this explanation really feels like it has a lot missing from it. The element Europium is named after it, and just in passing I want to say that Europe is a fake continent. It’s actually just Eurasia’s biggest peninsula, and from that rejection, Asia is also a misleading name. There’s just Eurasia. That said, I regard myself as Northwestern European, while recognising that this doesn’t refer to my origins in a part of a continent but just as from that part of that peninsula. (This may be enlightening). This is the convoluted route whereby Europa came to refer to two such different things.

The surface of the roughly Cynthia-sized Europa is three times the size of the terrestrial region at thirty million square kilometres. This makes the planet’s surface twice the size of Antarctica. Another way of thinking of this is that Europa’s surface is equal in area to the combined area of Antarctica and the Arctic Ocean. We kind of have our own Europa right here, as well as our own Europe, but the Europa orbiting Jupiter is colder even than the South Pole in midwinter, at least on the solid surface, at a temperature of -160°C. The temperature at the equator varies daily between -141 and -187°C. The poles are actually warmer than the equator at night, and the north pole is warmer than the south at those times. This range of temperature happens to be the one (below freezing) where the properties of water ice change most.

Europa is very bright, having a surface of water ice, although it doesn’t reflect as much light as Enceladus as its surface is “dirtier”. Compared to the other Galileans, it’s composed much more like the inner planets, being mainly silicate rock with an iron core. The chief difference is that its surface is solid water ice with an ocean of salt water underneath. Back in a period referred to as the Cryogenian, Earth was in a somewhat similar state with a crust of ice covering a salty ocean over silicate rock and an iron core of course, although Earth is much larger than Europa and it had continents and oceans underneath the ice, unlike the moon, which is probably more homogenous. This was 700 million years ago, and is sometimes thought to have stimulated evolution enough to trigger the Cambrian Explosion.

It’s difficult to talk about Europa without talking about the possibility of life, so I’m going to break my self-imposed rule here and do that. It wasn’t initially clear whether the ice was simply frozen solid or covered a water ocean, but the latter appears to be so. Salt water can be detected by space probes because of its ions, which being charged behaves differently in terms of magnetism than fresh water. The surface, though mainly water ice, is also covered in sulphates and there is some sulphuric acid, but these may well be from Io’s volcanism. Like most moons, Europa faces the planet it orbits at all times, giving it a leading and a trailing hemisphere, and the sulphates, which include Epsom salts, and sulphuric acid are mainly deposited on the latter, indicating that it doesn’t come from the ocean but from Io, or it would be evenly distributed. The leading hemisphere, by contrast, has sodium chloride on its surface. This would lower the freezing point of the water, making it more likely that “life as we know it” could exist there. There is a “found footage” film, ‘Europa Report’, which takes pains with accuracy and depicts complex multicellular life in the ocean, and ‘2010’ also shows complex life there. The main difficulty as I see it is that although the situation isn’t as bad as on Io, the radiation belts are still significant, but I presume the ice provides shielding. As well as the other constituents, there’s dry ice and frozen hydrogen peroxide, the latter of which is thought to be formed by the radiation.

If there is life, it’s likely to derive its energy from deep-sea vents, as also happens on Earth, and like Io, the energy for this volcanism comes from the flexing of the crust and planet from tidal forces of Jupiter and the other Galileans. This is thought to be responsible for the cracks on the surface. Also like Io, Europa’s surface is almost devoid of craters, strongly suggesting that it was liquid more recently than Ganymede and particularly Callisto, the two outer Galileans. When the Voyagers visited, the encounter was relatively distant and the moon wasn’t mapped in as much detail as the others, so the knowledge and research done into the moon lagged behind that on the others. Three types of feature were identified: lineæ, which are the “cracks”, flexūs and maculæ. It was from “macula” used in this naming that I first learnt the Latin word for spot, as in “immaculate”. None of the features are very high or low and the surface is unusually smooth. There are currently forty-five named lineæ, formed when cracks appear in the surface and material seeps up from the interior to fill them, which then freezes. Salt is highest in the lineæ.

Europa takes three days and thirteen hours (plus a bit) to orbit Jupiter. Like most other moons its day lasts as long as its orbit. This period is significant because it’s almost exactly twice Io’s. Roughly every three and a half days, Io and Europa are within a quarter of a million kilometres of each other, making them larger than Cynthia in each other’s skies and this causes them to pull on each other, raising tides in their surfaces and elsewhere and heating each other independently of solar radiation. Perhaps surprisingly, although Europa is the least massive moon of the four Galileans, it has the second highest gravity at 0.134 g, somewhat lower than Cynthia’s. The next moon out, Ganymede, also the largest moon in the Solar System but I’ll come to that later, again has almost exactly double Europa’s period. The Darian calendar, originally designed for Mars, has been adapted for use with the Galileans.

The surface is covered in icy regolith, substantially broken down by the radiation, with grains about the same size as snowflakes, though presumably not so regularly formed. This means it would be possible to ski on Europa, although there are no real slopes. Also the radiation would quickly kill you unless you had really good shielding on your ski suit. Maybe one day. Incidentally, radiation shielding doesn’t have to consist of lead or some other heavy metal, and synthetics work quite well. That said, I don’t know how powerful the radiation is there. It’s weaker than on Io though, and unlike Io, Europa doesn’t have the flux tube. However, although it was long considered quiescent, it does have cryovolcanism. There are domes on its surface which may have volcanic origins and of course it seems to have actual volcanism, or rather volcanism like Earth’s, in the form of deep sea vents. The cracks in the surface, which rapidly freeze over, expose water which evaporates into the atmosphere like steam. And yes, it has an atmosphere, though even thinner than Io’s, but unlike Io’s the main constituent is oxygen. This is generated by the radiation splitting the steam and Europa’s gravity being insufficient to hang onto the hydrogen.

Finally, the Galileo probe was deliberately pushed into Jupiter’s atmosphere to destroy it because of its own discovery of a salt ocean on Europa, to protect any potential life which might exist there.

That’s Europa then. Next: Ganymede.