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.