Saturn's moon Rhea is hard to know, like one of those people whose faces just refuse to stick in memory, but everything about it other than its appearance suggests a significant role in far future human events. The second largest and second most massive moon of the system, and orbitally the closest to Titan, Rhea is a giant iceball with very similar features to its smaller neighbors Dione and Tethys, but is itself far less distinct than either. While it has its own unique combination of features, no individual feature apart from its size strongly distinguishes it from the siblings it resembles, and its mediocrity in all but bulk properties makes it a somewhat rare topic.
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. Updates
50. Overview: Human Destiny Among the Worlds of Sol
51. Test Your Knowledge
Rhea in true color, as we would see it if we were there:
I. Context
Rhea is the 20th moon outward from Saturn, and is the fifth major moon from the planet. Its orbital radius around Saturn is 37% larger than the average distance between the Earth and Moon, but is still much deeper in its planet's gravity well than Luna is in Earth's. It is the last of the tightly-packed inner moons before a large gap separating them from Titan, which is more than twice Rhea's distance from Saturn. Orbital and gravity well diagrams:
Contextual views:
From the Near Side of Rhea facing Saturn, the planet takes up 13.6° of arc across the sky, not including the ring plane. This is about 27 times the apparent size of the full Moon from Earth, and a little bigger than Jupiter as seen from Europa. Since Rhea is completely airless, we can superimpose Saturn into Apollo images from the Moon and get a rough sense of what you might see of the planet (though the rings would be nearly edge-on):
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II. History
Rhea accreted out of mostly water ice during the formation of the Saturn system over four billion years ago, and not much is known about its history or internal evolution since. It isn't known whether it formed quickly and hotly, in which case it would have maintained a liquid layer internally, or if it accreted slowly like Callisto and thus never quite had the heat for a persistent liquid layer. Although it does have surface features resembling those of Dione - e.g., "wispy terrain" ice cliffs and chasmata indicative of expansion and subsidence - they are far less extensive and less prominent, so their exact significance in telling the story of Rhea's history is unknown at this point.
Unlike Dione, Rhea's craters are distributed as expected, with the higher concentration on the leading hemisphere facing its orbital direction. And while it has a number of features seen elsewhere, none of them occur conspicuously or incongruously enough to be mysterious. In other words, the great irony of Rhea is that not much is known about it, and yet it fails to generate the same level of questions as other moons of Saturn. However it formed, and whatever the nature of its evolution, the result today is that it's eerily nondescript and forgettable: A generic moon with a lot of small and medium-sized craters that look as craters should, with some vague wispy terrain laying over the trailing hemisphere (the side pointed opposite its orbital direction).
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III. Properties
1. Orbital and Rotational Features
The Rhean month is about 4 Earth days and 12 hours long, which is also its day because it's tidally locked to Saturn and only rotates once relative to the Sun for every orbit of the planet. As with Tethys and Dione, Rhea's leading and trailing hemispheres have subtle differences in color tint due to the different materials absorbed by them. However, these differences aren't nearly as obvious as on Tethys or Dione, probably because Rhea is substantially farther from Saturn (in the case of the trailing hemisphere, which is bombarded by electrons due to the planet's magnetic field), and is on the outer edge of the diffuse ring of Enceladan geyser ice that impacts leading hemispheres. So of all the moons of Saturn examined thus far, Rhea experiences the least intense external influences on its surface.
Based on indirect inference from magnetic field data, scientists in 2008 proposed that Rhea might have an invisibly faint ring system orbiting it. A number of findings both supported and contradicted this possibility, and the surface does show equatorial deposits that might have been accounted for by ring material spiraling inward, but there was no direct, conclusive evidence that such rings existed. Then in 2010, a closer look was taken by the Cassini spacecraft that would have easily detected such rings, but failed to find any trace of them.
The absence of direct evidence where an abundance should exist if there were rings, and a number of theoretical problems identified by ring specialists - for instance, the fact that such a ring could only be created by a highly improbable impact almost co-linear with the equator, and that the resulting ring system would rapidly decay - caused the researchers who originally proposed it to concede they were wrong. However, the data that led to their faulty conclusions is still unexplained: Saturn's magnetic field sweeps around faster than its moons orbit, so it's constantly driving electrons and ionized particles into the trailing hemispheres of those moons (see this section of the Tethys page for an illustration).
On the leading side, the moons cast a shadow into this flow of particles because of what their trailing hemispheres absorbed, and for every moon except Rhea those shadows are sharply-defined and relate directly to the body's radius. Rhea's electron shadow, however, gradually tapers off at some distance from it in a way that looks like what you find with a ring system. And yet there is no ring system. Illustration:
No other moon does this, but - and this is generally true in science - the causes are most likely trivial rather than broadly significant to understanding Rhea. In other words, it's far more likely something to do with its relative circumstances (e.g., motion, location, and speed) rather than anything fundamental to its environment or composition. That's a cruel little trick that science often plays: Illusions are far more abundant than bona fide discoveries. In purely poetic terms, Rhea's creepy lack of personality makes it a strangely appropriate vehicle for scientific disappointment. Still, it does make for beautiful imagery, particularly in association with other moons, Saturn's rings, or Saturn itself:
You can make a little game of figuring out what each object is in the above images based on what you've learned about the other moons of Saturn. Some stop-motion videos of Rhea in action - the timescales are all accelerated:
Quicktime video of Rhea moving into Saturn's shadow (i.e., a solar eclipse):
http://photojournal.jpl.nasa.gov/...
A few screencaps from the video, centered on the Saturn-facing boundary region between the leading (left) and trailing (right) hemispheres:
The fact that the Sun covers only 3.3' of arc in the Saturn system - a tenth its apparent size from Earth - while Saturn covers 13.6° of the Rhean sky guarantees that total solar eclipses are a regular occurrence in the system, although they only happen near equinoxes: A couple of Earth years out of the 29 that make up Saturn's year. However, since these moons have orbital periods measured in mere tens of hours to a few Earth days, they experience hundreds of consecutive, total solar eclipses during these times, followed by briefer periods with partial eclipses. They're probably all quite a thing to see, and I would bet that such spectacles will become culturally significant to far-future humans who live in the system. Just imagine something like this artificial scene (built from two real images - one the Apollo image used earlier, and the other an actual Saturnian solar eclipse) dominating your sky:
Although, once again, the rings would be edge-on, and at that point in the eclipse you probably wouldn't be able to see the ground in front of you except as an interruption in Saturn's illuminated arc. But the best seats in the house would be a lot closer to Saturn than Rhea:
Try to imagine something like that being a familiar, calmly joyous sight rather than an occasion for head-exploding awe and you can really put yourself into that context.
2. Size and Mass Characteristics
The mass of Rhea is about 0.039% that of Earth, and about 3.2% of the Moon. Among the seven major moons of the system, its density is right in the middle of the pack: Lower than Titan, Enceladus, and Dione, but higher than Mimas, Iapetus, and Tethys. This corresponds to only about a quarter of its mass being rock and the rest water ice. Due to its mass being dominated by ice, which is a low-density material, the surface gravity is only 2.7% that of Earth: Only 14% higher than Dione despite having 19% more mass and being 36% bigger. This gravity is about 1/6 of what you would experience on the Moon.
Rhea is about 12% the size of Earth, and 44% the size of the Moon. If it were centered on the intersection of Arizona, Utah, Colorado, and New Mexico, it would completely cover all four and impinge substantially into California, Nevada, Idaho, Wyoming, Nebraska, Kansas, Texas, and Mexico:
Moreover, if its surface area were flattened out and projected on to Earth, it would be comparable to Australia, and would have more than triple the land area of Greenland. The other solar system bodies closest in size to it are Iapetus and the Uranian moons Titania and Oberon, with Oberon being almost identical in size to Rhea. Some rough size comparisons, mouse over to see the title if you don't recognize something:
3. Internal Structure
The internal structure of Rhea isn't settled yet, with debate continuing on whether it's differentiated into distinct layers or has a homogeneous distribution of material like Callisto. Different researchers continue to reach wildly different conclusions. Until this is settled with more detailed and self-consistent data, the nature of its formation won't be known either: Whether it slowly accumulated from a benign agglomeration of dust in the same way that Callisto had, or if it collapsed rapidly and with great heat from Saturn's primordial cloud in the way that Ganymede had from Jupiter's cloud. But we can look at the implications of each possibility in light of the one relatively solid fact about Rhea: Roughly 25% of its mass is rock.
Three things can create enough internal heat to differentiate a massive body's internal structure: Rapid formation that traps the heat of impacts; sufficient quantities of radioactive elements; or tidal heating. If there is strong differentiation, one, two, or all three of these processes would have to be involved to varying degrees. In this case the 25% rock fraction would all be concentrated in a medium-sized core about 350 km in radius while the majority of Rhea's radius would be ice. This could possibly mean the core still generates enough internal heat to create a small adjacent liquid layer, but there is no direct evidence of one. A rough amateur illustration of this possibility:
If there is partial differentiation, where layers are present but poorly defined, there would still be a "core" but it would be more of a region than an actual object, and its ill-defined boundaries would cover a larger volume of the interior. So you would have a core that is pure rock/metal and an upper mantle that is pure ice, but in between would be mixtures that had never settled out. The possible cases of a partially differentiated interior are large in number, with various distributions. A vague, general illustration of the principle:
And if Rhea is undifferentiated, then most of its bulk is a relatively homogeneous mixture of rock and ice. So it would be like this:
4. Surface Features
Unlike Dione, Rhea's hemispheres follow the expected pattern: The leading hemisphere, which always points along its direction of orbit, is heavily cratered and old while the trailing hemisphere is relatively smooth, young, and shows signs of resurfacing in addition to the "wispy terrain" corresponding to shallow subsidence cracks.
The main features to note from the leading hemisphere are the massive impact crater Tirawa in the upper right, and the linear chain of four mid-sized craters running North to South that are tangent to it. The leading hemisphere image is high-resolution, and thus worth a closer look:
The pronounced ridge to the southwest of Tirawa is the rim of a slightly larger, degraded crater called Mamaldi that is vaguely visible. Zooming in to Tirawa and the tangent crater chain:
The only other major zoom-worthy feature of the image is the Northern limb, which shows the rimwalls of some mid-sized craters in beautiful relief:
Rhea also has a bright, prominent rayed crater, Inktomi in the South and on the edge of the leading hemisphere:
Computer-generated animation showing the topography of Tirawa's rim, slightly color-enhanced:
Global views of Rhea, showing boundary regions:
Views in phase:
Zooming in to a couple of features from above images:
Regional views:
South polar region mosaic:
North polar region mosaic:
Closer views:
In keeping with Rhea's lack of specific character, the official nomenclature theme for naming its features are mythic figures and places from mythologies around the world. The IAU pages for planetary nomenclature are currently down for maintenance, but here's an incomplete map of Rhea:
More complete maps and details will be available from the IAU site once it's back up.
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IV. Modern Relevance to Humanity
Despite the unanswered scientific questions about its formation, interior, and effect on surrounding particles, Rhea is effectively a nonentity in the popular imagination: The Wikipedia page for Saturn's moons in fiction only lists two instances of it appearing in sci-fi, and both were over 70 years ago in pulp magazines. Part of it is the same problem with Dione: General boredom engendered by an apparently dead icy moon with no radically unique features. But Rhea manages to go beyond even that level of indistinctness by having such an incredibly non-specific appearance. If it didn't already have a name, it might as well be called "Iceball 5." And yet, as described earlier, it is a big place. Still, it won't mean very much to our species for quite a long time.
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V. Future Relevance to Humanity
Despite its lack of personality, one thing Rhea does not lack is resources: It is a large ice moon, has the second most intense surface gravity after Titan of any Saturnian moon, and is orbitally closer to Titan than any other moon of the system. In other words, my guess is that Rhea's main original relevance in the system would be as a resource and industrial base for infrastructure that supports Titan settlements, and also maybe a waystation for craft headed inward toward closer moons or Saturn cloud cities. Minor moon Hyperion comes closer to Titan in terms of distance due to its elliptical orbit of Saturn, but it has a nontrivial inclination relative to the larger moon, so it's not clear whether it has any advantage for reaching it compared to Rhea.
Rhea, however, has no intrinsic attractions for permanent human settlement, and its surface gravity is too low to be healthy as a long-term living space without significant medical intervention. That said, a lot of places on Earth were only ever settled not because they themselves attracted anyone, but because they were convenient points along a path to somewhere that was and developed a local economy with a continuing population by happenstance. For instance, from a high-tech perspective it would be a little puzzling why anyone would choose to live in Nebraska or Kazakhstan given more promising alternatives, but the way human experience works, some people invariably just find themselves in a nondescript place and decide it's "good enough."
It's a bit funny to realize, but that will account for quite a lot of the space settlement activity that occurs in humanity's far future: Not grand Manifest Destiny plans, but just people following jobs, running from disasters or personal misfortunes, and deciding that this - wherever this happens to be - is a good enough place to stay put. So while places like Mars, Callisto, and Titan probably will have the big, dramatic migrations and whole civilizations focused on their development, other places will be settled as a slower, weaker secondary consequence of those efforts. Rhea's a big place and a major store of resources, so eventually there will be societies there, but perhaps every bit as generic and nonspecific as the moon itself: Exurbs of Titanian civilization.
More generally though, the eclipse video above started me thinking about how civilizations mark time. On Earth we mark time by the astronomical events most relevant to our lives, or at least to the lives of our ancestors: Daylight cycles, lunar cycles (the origin of months), and the back-and-forth seasonal migration of the Sun across the sky due to Earth's axial tilt, with one complete migration making up one year. So I wondered, what kind of events would be most culturally significant to civilizations orbiting Saturn? Well, daylight would still be quite significant, just because it's visually different from night: Although the system is a gloomy place, the visual difference between day and night is still drastic. However, the days are different on each moon, so they would mostly be locally significant units of time.
The eclipse seasons are another matter: Although there would be slight timing differences due to Saturn's apparent size being larger or smaller, it's mostly the same for all major moons because it corresponds to equinox season - roughly one Earth year out of 14 and a half experiences eclipses every local day. And as the imaginative illustrations showed that utilized a real image of a Saturn solar eclipse, they would be spectacular. So I would bet that the eclipse seasons end up being a major marker of time in cultures that develop around that system, probably with holidays, celebrations, and other cultural practices surrounding them. Following the pattern of human history, people would choose to take major personal, cultural, and political actions at these times to correspond with their perceived significance, further adding to that significance and making them even more fraught with meaning.
Other possible units of time that might become significant would be based on launch windows between the major moons of Saturn, probably with those relating to Titan having the greatest economic and therefore cultural weight. Travel around such a system would be a lot more regular than space travel is today, but unlike our closest modern analog - air travel - the schedules would be strictly governed by orbital mechanics, meaning that launch windows are cyclical and relate mathematically to the relative orbital periods of the origin and destination moons. Additionally, the fact that these cycles could be visually related based on the appearance of other moons in the sky makes it more likely to develop cultural significance. Both in exploration, economics, and perhaps eventually military conflicts within the system, these launch windows would at times be the difference between life an death, and thus build up great emotional significance for societies and individuals.
Now, these units don't neatly factor into each other - e.g., there's no neat integral way to relate Rhea-Titan launch windows and solar eclipse seasons - but then the days, weeks, months, and years we use on Earth don't relate very well to each other either. But we still use them all because they're all convenient on different scales of time, so I would bet strongly that both eclipse seasons and launch windows would become very important units of time to civilizations around Saturn, and they would probably have their own words for them.
On the longest timescales, Rhea probably would be stripped of resources, although - and this is true of any such moon - there is still a possibility that societies developed there would still maintain surface appearances, in whatever way "Rhea" comes to be culturally associated: Certainly not its natural appearance as we see it today though.
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VI. Future of Rhea
If its interior is undifferentiated, the loss of Rhea's ice as the Sun expands would leave behind a smaller, porous rocky moon made up the kind of sloppy terrain you get when it's the product of sublimating volatiles leaving behind higher-temperature materials like rock. The same would probably be true if its interior is partially differentiated, because there would still be the same surface effects of sublimation. But if it is strongly differentiated, then the exposed core would probably look the same as Dione's: A relatively smooth, solid rock. I wouldn't hazard a guess at what happens after that: Rhea's in a middle region of Saturn's gravity well, so whether it ends up spiraling inward, being perturbed into an eccentric orbit, or being thrown out of the system altogether would just depend on the details of the system's future evolution.
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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)
