Saturn's moon Dione (Die-OH-Nee) is a world of two faces: One with a network of brilliant ice cliffs criss-crossing dark terrain, and the other a bland craterscape of shallow impact features not that different from the surfaces of neighboring moons Tethys and Rhea. As we learn about the major moons of Saturn, we find that what you see on one side is not always what you get on the other, and Dione is a prime example of the principle.
The progress of our adventure so far (current in bold):
1. The Sun
2. Mercury
3. Venus
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
10. Luna
11. Mars (Vol. 1)
12. Mars (Vol. 2)
13. Mars (Vol. 3)
14. Phobos & Deimos
15. Asteroids (Vol. 1)
16. Asteroids (Vol. 2)
17. Asteroids (Vol. 3)
18. Ceres
19. Jupiter (Vol. 1)
20. Jupiter (Vol. 2)
21. Io
22. Europa (Vol. 1)
23. Europa (Vol. 2)
24. Ganymede
25. Callisto
26. Saturn (Vol. 1)
27. Saturn (Vol. 2)
28. Saturn (Vol. 3)
29. Rings of Saturn
30. Mimas
31. Enceladus
32. Tethys
33. Dione
34. Rhea
35. Titan
36. Iapetus
37. Minor Moons of Saturn
38. Uranus
39. Miranda
40. Ariel
41. Umbriel
42. Titania
43. Oberon
44. Neptune
45. Triton
46. The Kuiper Belt & Scattered Disk
47. Comets
48. The Interstellar Neighborhood
49. Overview: Human Destiny Among the Worlds of Sol
50. Test Your Knowledge
Dione in true color, as the human eye would see it:
I. Context
Dione is the seventeenth moon of Saturn, and is the fourth closest major moon to the planet. Its orbital radius around Saturn is 98% of the average distance between the Earth and Moon, so the distances involved are comparable - although Saturn's gravity well is much steeper, so a lot more fuel would be needed to travel between Dione and Saturn than between the Earth and the Moon. Orbital and gravity well diagrams:
Contextual views:
Saturn covers 18.4° of arc across the Near Side Dionean sky, or about 37 times larger than the full Moon seen from Earth - not including the rings, which would be barely visible anyway (they would be seen virtually edge-on). So this would be its apparent size, superimposed into an Apollo image from the Moon:
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II. History
Like all other known major moons, Dione is tidally locked to its primary - i.e., one hemisphere always faces the planet, and the opposite always faces away. This also means that one face is always pointed in the direction of orbit (the leading hemisphere) and one opposite (the trailing hemisphere). Typically the leading hemisphere ends up more heavily cratered because the moon contributes its own velocity to the energy of collision, while the trailing hemisphere is kind of "running away" from the things that impact it. But with Dione the reverse was found to be true: The leading hemisphere is less cratered than the trailing hemisphere, and in an exact way that each side resembles what would be expected of the other.
This causes scientists to hypothesize that the two hemispheres were originally in an opposite orientation, cratered as normal, and then an impact at some later point started the moon briefly spinning before tidal forces returned it to a locked state in the current configuration. However, the fact that the hemispheres seem exactly reversed rather than being off by some angle has puzzled researchers. It may be that these same tidal forces would tend to nudge a rotating moon toward one of these two orientations relative to its original formation - like a weighted casino wheel - and it just happened to resolve on the opposite half. Illustration of what could have occurred:
Like most icy moons, when Dione originally formed it had some amount of heat trapped internally from the impacts that created it, and also was warmed due to the radioactive decay of metallic elements in its rocky core. This would have temporarily maintained an internal liquid water layer, but eventually the heat wasn't enough to keep it that way and the moon froze completely solid. Although it has a relatively large core, this alone wasn't enough to keep its interior heated, and it doesn't have the right orbital characteristics for tidal heating to make up the difference. As a result, Dione is geologically dead today. When it finally froze completely, the icy mantle expanded somewhat (since ice is less dense than liquid water), causing several deep, long rift canyons (called chasmata) that scar its surface.
At some later point, after most of the impact cratering and chasmata formation had already happened, the surface ice on the trailing hemisphere sank back to some degree and created the distinctive pattern of "wispy" terrain seen today: Bright cracks that contrast sharply with the darker surrounding material, and are not as deep as the chasmata. What caused the subsidence isn't known, and it's also a mystery why it only happened to a major extent in the trailing hemisphere, but the fact that the cracks cut across so many existing craters means that their formation was apparently the last major event in Dione's history.
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III. Properties
1. Orbital and Rotational Features
The Dionean month is about 2 Earth days and 18 hours, which is also its day because it's tidally locked to the planet, so it only completes one rotation relative to the Sun for every orbit of Saturn. Like Tethys, its leading and trailing hemispheres are tinted different colors due to the different kinds of diffuse material constantly being absorbed by them, although it's not entirely clear why the trailing hemisphere is so much darker than the dark regions of other moons. However, the brightness of the leading hemisphere is almost certainly caused by absorbing ice particles from the E ring - material ejected into Saturn orbit by the geysers of Enceladus.
Also like Tethys, Dione has two Trojan companions that share its orbit of Saturn, 60° ahead of and behind it: Helene and Polydeuces, respectively. How this kind of orbital relationship works (see the Wikipedia article on L4 and L5 Lagrange points) is a complicated balance of constantly changing force vectors in a rotating reference frame, and is hard to visualize because it's only stable over time. Rough orbital diagram, not including minor moons inward of Mimas:
Helene has been seen in detail:
But not Polydeuces:
As a highly regular and relatively close moon, alignments between Dione and other moons of Saturn are common. Here's one with minor moon Pandora crossing in front of it, and the ring plane slicing across the field of view:
Here minor moon Epimetheus is in the background, and you can just barely see the night-darkened limb of Saturn interrupting the far side of the ring plane:
The limb of Dione with Prometheus and Epimetheus against the ring plane:
With another minor moon, Janus:
Mimas in the foreground moves in front of Dione while the latter's night side is illuminated by Saturn-shine:
Mimas goes behind Dione and then emerges:
Enceladus shows Dione's relative darkness:
With Tethys:
Dione with its bigger doppelganger Rhea on the left and in the background of the first image, then the left and foreground of the second:
The limb of Rhea huge in the foreground, and Dione behind the ring plane:
True color image of Dione moving behind Titan, with the ring plane and Saturn in the background:
Images like the above really give a sense of the general gloom of the Saturn system, and also the surreality that often makes it hard to realize these are photographs as tangibly real as any family portrait on your wall. If you've ever been awed by a thunderhead on the horizon, just try to imagine the above image on the same visual scale (because human eyes can't distinguish bigger scales than that - they all just look Really Really Big). If you can picture that and relate to it, you probably understand why space exploration is not an optional hobby - it's more important by far than most of what human beings do. With Titan in the far distance:
Again with Titan in the background, with the ring plane and Pandora in the foreground - you can also just barely make out the tiny moon Pan as a handful of pixels interrupting the leftmost ring gap:
Here's a handy thing about Dione: Because its leading and trailing hemispheres are so different, you can tell whether it's in the foreground or background of an image and where it's heading if the ring plane or Saturn are visible. In the above image, you can see the dark terrain and network of ice cliff cracks of the trailing hemisphere, so you know it's on the far side of the ring plane and headed to the left and away from the viewer's perspective. Conversely, the image before that shows the smooth leading hemisphere, so you know it's headed toward the camera. That is, assuming North is oriented "up," which space agencies typically do in wide shots of astronomical bodies for the sake of convenience - but be aware that images are sometimes reoriented by secondary sources (like me) for aesthetic reasons. More with Titan from various perspectives:
Video of Dione moving into Saturn's shadow, which is only possible at certain times of the Saturnian year due to the 26.7° tilt of the system's axis relative to the planet's solar orbit:
2. Size and Mass Characteristics
The mass of Dione is about 0.0328% that of Earth, and about 1.9% the mass of the Moon. It has a density significantly greater than water ice, indicating that about 46% of its mass is rock - which is a lot for a moon of Saturn. Of the seven major moons, only Titan and Enceladus are denser. This gives it a relatively hefty surface gravity of 2.4% that of Earth, which is the third strongest in the system after Titan and Rhea, and about 1/7 of what you'd experience on the Moon. So a reasonably athletic person could jump off a cliff the height of a 20-story building on Dione without injuring their legs, though probably not recommended.
It's about 8.8% the size of Earth and a third the size of the Moon, which translates locally to 6% bigger than Tethys and 6% smaller than Iapetus - the moons closest in size to Dione. The same rough size comparisons with political regions on Earth apply here as with Tethys: Its diameter would cover most of Texas, and its surface area would completely cover India. Rough visual size comparisons - mouse over to see the title if you don't recognize something:
3. Internal Structure
As mentioned earlier, about 46% of the mass of Dione is rock, which allows for some sloppy amateur calculations of the proportions of its internal structure. Using 2.7 g/cm3 as a typical density of silicate rock (according to Google), the result is that about 60% of Dione's radius is taken up by the rocky core. It would be a bit less if there's more metal, and a bit more if there's less metal, but these are more or less the internal proportions with the given rock fraction:
I'm sure professional models are floating around somewhere, but I haven't found one with a casual Google search. The main thing to take away from it is that Dione's pretty solid compared to, say, Tethys, and that might have something to do with why it looks less "ruined" despite being of comparable size, extremely similar surface composition, and experiencing basically the same external influences. This is also why it has relatively significant surface gravity, and would be a substantial moon even if it had no ice whatsoever.
4. Surface Features
To summarize what we've already seen, Dione's most distinctive face is its heavily-cratered trailing hemisphere with a network of bright cracks interrupting dark terrain, and is contrasted with a relatively smooth, young, and bright leading hemisphere with far fewer craters:
More global views:
What you'll notice in the image immediately above - which shows a boundary region between leading and trailing hemispheres - are the faint outlines of a massive impact crater in the South that has faded almost completely away. This is called Evander. As it bears a lot of resemblance to Tethys' Odysseus crater, Evander gives an interesting opportunity to compare the two moons. Both craters are old and eroded, but Evander much more so. This could partly be because of the 60% higher gravity on Dione, or it might relate to the same reasons why the rest of the leading hemisphere is so young and smooth. Closer view:
Views of Dione in phase, highlighting various features:
Closer, regional views - the long arc of Padua Chasmata, Northern and Southern parts:
Subsidence cracks, creating ice cliffs up to several hundred meters high:
Various craters:
Boundary region, showing craters and chasmata in stark relief, with subsidence cliffs visible as bright wispy terrain:
Erulus crater:
North polar region mosaic:
South polar region mosaic:
Zooming in to even closer regional views:
The Cassini spacecraft took some of the most spectacular imagery of an airless body ever when it took a high-resolution image of the crescent limb of Dione. You can't really tell from the widest perspective...
But look what happens when you zoom in - suddenly you're there, looking at the hills, ice cliffs, and crater rims...
The cliffs you see, particularly in the third image, are bright because they're exposed ice while the surroundings are covered in darker material. But you may have noticed in the second image something very rare - a vertical crater in the cliff wall! Something skimmed along just over the surface at a tangent and almost went right past Dione, but then ran smack into that cliff wall:
Dionean features are named after people and places from Virgil's Aeneid, and there are 93 features officially named by the International Astronomical Union (complete list here). Labeled map of Dione, showing the most significant features:
More detailed maps can be seen here (PDF page).
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IV. Modern Relevance to Humanity
As one of the dead, "boring" iceball moons of Saturn, Dione doesn't attract a lot of scientific attention. The puzzle surrounding its apparent hemispheric reversal is mostly interesting to planetary geologists specifically studying the formation of icy moons, and even within that sub-discipline is studied mainly as one example among many - and not an especially important one. As a result, it doesn't get much play in science fiction either: To the best of my knowledge, it's never even been mentioned in a sci-fi TV show or movie, and I personally have never seen it mentioned in literature either (although there are a handful of examples listed by Wikipedia).
In fact, if you don't take a specific interest in space exploration, you've probably never heard of Dione - which is a little bizarre given its size, as shown in the comparisons earlier. I don't recall even hearing the word "Dione" spoken by anyone, ever - not even in college when I was studying astronomy. And remember, this is a major moon of Saturn with more surface area than India. It just never came up, and I'm sure that will be the story of Dione for a long time to come.
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V. Future Relevance to Humanity
Dione and the other major moons of Saturn will only start getting major attention when there are people living in the Saturn system, which is probably several centuries from now. Once that happens, however, interest would be mainly economic: Its ice would be a valued commodity in a fuel-hungry, spacefaring civilization. I doubt there would be permanent colonization - at least not planned as such - due to the gravity being too low for long-term human health, but it could sustain a lot of industry, trade, "company towns," etc. Basically the kind of things that happen in bleak places that are rich in some potent resource but have few other attractions. Then again, those ice cliffs are pretty amazing, so maybe there could be tourism and a substantial but transient population.
The economic position of Dione in such a civilization isn't entirely clear: It would depend where the heaviest concentration of wealth occurs - if most of what gets done is for the benefit of economies outside the Saturn system, Dione is pretty deep in the planet's gravity well compared to quite a lot of alternatives, and thus it would be more expensive to export its commodities. On the other hand, Rhea - which is extremely similar - is closer to Titan, if that's where the economic locus occurs. And if the main focus of the system is in cloud cities floating around Saturn itself (see this section of Saturn (Vol. 3) for a discussion of this possibility), Dione's not as close to the planet as Mimas, Enceladus, and Tethys. So it might be a backwater for a while even after there are large numbers of people in the system.
On the other hand, places that can sustain themselves and yet aren't subject to overwhelming economic pressures from outside often develop the healthiest, most stable civilizations. Consider Norway with its oil revenues: Unlike the Middle Eastern oil exporting states, Norway's petroleum resources don't destabilize its politics because modern Norwegian society wasn't built entirely around the exploitation of that resource.
So if medical science finds a way to allow humans to live permanently in low but nontrivial gravity without major health problems, the relative lack of external economic imperatives - at least for a while - could allow interesting, stable human cultures to develop on Dione, and then once its resources do become in demand, that would merely feed its growth rather than threatening or corrupting it. This is all supposition, obviously - it could just as easily end up like Svalbard: Industrial territory of a culture located somewhere else, and not much more.
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VI. Future of Dione
Provided it survives human economic hunger for ice and metal, Dione would lose the former as the Sun expands over the next few billion years, making the Saturn system too warm for water ice to survive in vacuum. This doesn't mean it would be above freezing - just that the rate at which material sublimates into space in daylight would exceed the rate of refreezing at night. So eventually the large rocky core would be exposed, leaving a smaller rock moon where a large ice moon once was. Given its relative closeness to Saturn, it might end up spiraling into the planet from there, but as far as I know it could just as easily be perturbed into an eccentric orbit or even be thrown entirely out of the system.
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VII. Catalog of Exploration
1. Past & current probes:
Voyager 1 (USA - 1980 flyby)
Voyager 2 (USA - 1981 flyby)
Cassini (USA and Europe - entered Saturn orbit 2004, currently operating)
2. Future probes:
(none planned)
