Saturn's moon Enceladus is a cracked, geologically active ice world with geysers spewing water ice into space from the South polar region. While this often makes for beautiful imagery, what it signifies chemically is far more important: The fact that its geyser plumes have been found to contain organic molecules. Given these observations and the relatively benign radiation environment compared to the moons of Jupiter, Enceladus is arguably the most likely habitat for microbial extraterrestrial life-as-we-know-it (LAWKI) in the solar system, and the location where such life would be most accessible to exploration.
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 Our Family of Worlds
50. Test Your Knowledge
Enceladus looks basically the same in color as it does in monochrome (literally snow white), so black and white images are little different from true color apart from the background:
I. Context
Enceladus is the thirteenth moon of Saturn, and the second innermost major moon after Mimas. Its orbital radius around the planet is about 62% of the average distance between the Earth and Moon. Orbital and gravity well diagrams:
Contextual views:
Saturn covers 29.3° of arc across the Enceladan sky, or about 55 times larger than the full Moon in terrestrial skies - and far bigger than the apparent size of Jupiter as seen from Io. We can get a sense what this looks like by superimposing a scaled image of Saturn into an Apollo image from the Moon:
Standard caveats for the above illustration: The blinding white ice of Enceladus would be a lot brighter than lunar regolith, which is actually very dark. Also, the ring plane would be closer to edge-on. The reason I use a lunar image is that in its original form, the Apollo image shows Earth in the sky as you would see it with your own eyes from the lunar surface, so I've used that image to show the sizes of planets in the skies of their major moons beginning with the Jovian system and will do the same for the rest of the major outer planet moons. This way it's easy to compare and contrast.
---
II. History
The story of Enceladus is like Goldilocks on many levels: Inward toward Saturn, Mimas was too small and lacked sufficient heavy elements in its core to heat the interior, so despite the greater tidal forces acting on it, that moon froze solid and is dead today. Beyond Enceladus, its much larger and more massive brother Tethys is too far from Saturn for tidal forces to keep its interior active while having a lower proportion of heavy elements in its core due to building up a far greater mass of ice around it, and so it too is dead.
But Enceladus gets it just right: Close enough to Saturn and its fellow moons to be flexed and heated by gravitational forces, but far enough away not to be overly warped like Mimas or torn apart like the rings. It also has enough heavy elements in the core to keep water liquid internally, but not so much that the gravity of the core would build up a suffocatingly thick ice shell around it too deep for internal heat to affect the surface. So we have this seemingly paradoxical moon that is small yet plucky, and active under less intense tidal forces than its dormant brother Mimas.
Another part of that paradox is the fact that despite having a rich and storied geological history, Enceladus' active behavior means that a lot of its past can't be known at this point because most of the surface is constantly changing, erasing the record of earlier times. Without the kind of detailed exploration we're able to do on Earth, only the vaguest outlines of its past can be learned: A story written in the cracks, fissures, fault lines and smooth plains of the young regions - terrain potentially not much older than the human species (500,000 years) - and in the numerous collapsed craters of the ancient ones, going back anywhere from hundreds of millions to billions of years.
---
III. Properties
1. Orbital and Rotational Features
The Enceladan month is about 33 hours, which is the same as its day because Enceladus is Tidally locked to Saturn - i.e., it always presents the same face to the planet, and thus rotates once each time it completes an orbit. This is called synchronous rotation, and it's universal to regular moons (including our own) due to tidal processes that tend to slow down and stop any non-synchronous rotation that occurs in a moon's early history. If at some point in the future we learn about moons in other star systems that rotate non-synchronously with respect to their planets, that would indicate either that they're very young or that some massive impact started them spinning relatively recently in their history.
Enceladus is also common in that it has no axial tilt, its orbit is very circular, and there is barely any inclination from its planet's equatorial plane. Major moons that deviate in any of these ways are unusual because the formation process typically gives major moons the same rotational properties as their parent planets, so those that are outside the norm probably experienced some giant impact or extreme orbital resonance that changed their behavior. The fact that Enceladus is so conventional in these terms indicates that its life has been relatively benign.
However, it does have a regular, benign orbital resonance with another major moon, Dione, that maintains Enceladus' very slight eccentricity - which is crucial to sustaining the tidal forces that contribute to internal heating. What this means is that every time it comes near Dione, the latter's gravity nudges it a little bit, and that helps keep the tidal forces acting on it changing so that they can cause friction and heat inside Enceladus. If its orbit were allowed to become perfectly circular, the tidal forces wouldn't change along its orbit, so there would be no "flexing" and thus no friction. Resonances work in the other direction too - Enceladus nudges Dione - but because Dione is so much more massive and farther away from Saturn, the effect is much less significant.
Due to the constant outgassing from its geysers, Enceladus is responsible for the invisible, diffuse E ring of Saturn. The E ring is extremely large, extending all the way out to the orbit of Rhea, but is by far the densest along Enceladus' orbit. You can see the E ring with sensitive low-light photographs, and the little moon's obvious role in it:
Because the five inner major moons orbit Saturn relatively close to each other and with barely any inclination - and because their orbital periods are so short (0.95 - 4.5 Earth days) - visually striking images can often be taken with multiple moons in view. One of the tricky things about these images is that because one moon might be very far away compared to another, their apparent relative sizes may be deceptive: There is no "hazing" effect in vacuum to tell you what is foreground and what is background.
Imagine you had two objects identical in every way but size, with one of them being twice as large as the other: In vacuum, if you put the larger one far enough behind the smaller so that they cover the same angle in a field of view, there is no way to tell them apart visually unless a third thing comes between them to give scale. Of course, with real moons we can recognize them by what they look like, but the surreal, scale-defying properties of space imagery still apply. For instance, in images involving Titan and a smaller, closer moon, the real size difference between them is much greater than it appears. In the images below, only the ring plane in the mid-ground lets you realize just how far away Titan is compared to Enceladus in the foreground:
Without a mid-ground, it's much harder to judge:
Enceladus is clearly in the foreground in the above image, but how far in the foreground? You would have to already know the sizes of the two bodies to figure that out. It can be even more confusing when the bodies in an image are all airless, so they look equally crisp no matter how far away they are relative to each other. Enceladus with Tethys (left):
Enceladus in front of Dione:
In the above image, are both Enceladus and Dione in front of the ring edge, or are they both behind it? Or is Dione behind it and Enceladus in front of it? It's easier when you know for a fact one of the moons is much smaller, like in this image with minor moon Janus:
Or this one with another minor moon, Epimetheus, peeking out from behind it:
Sometimes the configuration helps. Enceladus is the smooth, white cue ball in the following two images:
Enceladus and Dione again:
The fact that Dione is more than twice the size of Enceladus means the former is well in the background of the above image. These ones give a better accounting of the relative sizes of the two:
2. Size and Mass Characteristics
The mass of Enceladus is about 1 x 1020 kg or 0.0018% the mass of Earth - about two and a half times greater than Mimas, and three times greater than the main rings. It's also the second densest major moon after Titan: A fact that first hinted to researchers that it has a proportionally bigger, heavier core than the other inner major moons. In other words, Enceladus is not a chunk of ice through and through, but a packed snowball with a substantial rock inside it.
Surface gravity on Enceladus is about 1.1% that of Earth: Substantially higher than on Mimas, but only about half of what you'd get on asteroid Vesta or dwarf planet Ceres, and only 7% of surface gravity on Luna. A person weighing 150 lbs on Earth would weigh 1.65 pounds on Enceladus, and it would take several seconds to fall 1 meter, so walking wouldn't be the most efficient way of moving around on the surface for future astronauts who explore it: More likely some sort of bunny-hopping.
Enceladus is the second smallest major moon of Saturn, but the sixth largest of all Saturnian moons. It would fit snugly within Germany or Poland, and its total surface area would completely cover Turkey. Rough size comparisons with comparable objects - mouse over the image to see the name of the file if you don't recognize something:
3. Internal Structure
About half the total mass of Enceladus is thought to be rock, which is much more than its neighboring moons. Since rock is denser than water ice, this suggests a rocky core occupying more than half of the moon's radius, with an H2O mantle and crust somewhere in the range of 100 km thick. At some depth within the mantle, heat conducted upward from the core through solid ice is thought to combine with friction produced by tidal forces to create a salt-water liquid ocean layer. One vague model of the interior, showing all H2O layers as a uniform blue region without speculating about solid vs. liquid:
Understanding of the Enceladan interior is still very crude, and a number of models are still being explored. Some models have no liquid ocean, and explain the South polar geysers as being purely a result of mechanical friction between faults in the crust. Others suggest disconnected pockets of liquid water heated by plumes from beneath, which then continue upward through breaks in the crust in smaller plumes of their own. And still others hypothesize a global liquid ocean isolating the core from the surrounding ice, and bulging upward in the South with plumes breaking through the icy crust in places. An example of this last model:
The presence of carbon compounds in the plumes - which we discuss in more detail later - could indicate that the geysers are not merely heated surface ice escaping under friction, but rather result from deeper processes with small amounts of material other than water dredged up from below. Researchers seem to be leaning toward the liquid ocean model, although the specifics are still a wide field of discussion. Why geysers only occur at the South polar region is another mystery that has not yet been settled.
4. Temperatures
As the most reflective body in the solar system, whiter even than Europa, Enceladus is also one of the coldest places because it rejects so much solar radiation. The average surface temperature is 75 K (-198 °C / -325 °F), but can range below 40 K (-233 °C / -388 °F) or as high as 145 K (-128 °C / -199 °F) in places. The hottest, most persistent hot spots are all in the South polar region and are associated with the "tiger stripes" - a parallel series of four fault lines near the pole out of which the moon's geysers erupt. Temperature map of Enceladus, showing the anomalous heat of the South polar region:
A closer look at the South polar hot spot reveals that the pattern of heat closely follows the tiger stripes:
5. Surface Features
With the most reflective surface in the solar system, Enceladus is basically the color of snow with only subtle color differences that are barely discernable to the human eye. It's much brighter than its fellow icy moons because the surface is gradually replenished with material from geyser plumes, whereas dead moons like Mimas, Tethys, Dione, and Rhea continually accumulate layers of dirty, dusty material that are darker in color than their underlying ice. The same material accumulates on Enceladus, but slowly enough that bright plume material continuously covers it. Here is Enceladus in true color, as the human eye would see it in person:
As you can see, it's almost indistinguishable from any of the black and white images used above - you can just barely see subtle traces of blue in parts. Due to its lack of color contrast, not many true color images of the moon exist, because what's the point? Instead, NASA took to artificially exaggerating the barely-visible blue tints in false color images, so this is how Enceladus tends to be depicted in popular media even though this is not what it looks like:
Still, the color enhancement does serve to highlight the South polar tiger stripes (seen at left) and the network of major faults radiating from them. To get a general sense of Enceladus, here are some global views in phase:
We see that Enceladus has three general types of terrain: Old, cratered regions, which are most abundant in the North; sinuous, faulted terrain, which is more prevalent in the South; and smooth, young, relatively uncratered plains, that tend to be associated with the faults, and are also most prevalent in the South. The contrast of terrain is very strong when we compare the North with the South polar regions, respectively:
In the South, craters are almost nonexistent within the ring of faults encircling the tiger stripes at the pole, and are only visible at all within two limited arcs of longitude on either side of the pole. Each fault is known as a sulcus (plural sulci), and the general nomenclature for Enceladan features is to name sulci and plains regions after Middle Eastern places and craters after characters from Arabian Nights. The tiger stripes are thus known as Damascus Sulcus, Baghdad Sulcus, Cairo Sulcus, and Alexandria Sulcus. Labeled map of the tiger stripes:
Cylindrical projection map of Enceladus, with (incomplete) labels:
Some labeled craters in the North polar region:
Other than the tiger stripes, two other distinguishing features of Enceladus are a giant canyon running North-South called Labtayt Sulci, which runs between Sarandip Planitia and Diyar Planitia (but isn't labeled in the above cylindrical map, for some reason), and a distinctive double-crater composed of craters Ali Baba and Aladdin, which are visible in the North polar map directly above. Images of the tiger stripes:
Images of Labtayt Sulci:
Ali Baba and Aladdin craters:
Detailed IAU maps are available here, and a complete list of officially-named Enceladan features here. Various images, zooming in:
Note that even across sulci and craters, there are plain old cracks in the crust. This is thought to be due to the crust collapsing as the interior has cooled over time. A closer perspective shows this phenomenon even more clearly:
As you move in closer, the surface starts to resemble the hide of some large animal:
Eventually you can see individual boulders, and appreciate that Enceladus is an actual place that human beings can some day experience directly:
Given the low gravity, the steep cliffsides, canyons, and rough boulder fields you see above might not be as forbidding as they seem.
---
IV. Geysers
Numerous images of Enceladus have shown wispy plumes of diffuse material emanating from the South pole, which are visible even from a great distance in the right lighting conditions:
At first the fine structure of the Southern plume wasn't clear, but as the Cassini spacecraft made closer flybys of Enceladus, it showed that the plume was actually fed into by several distinct jets - which are visible in the images above taken closer to the moon. After some debate, it was decided to fly Cassini through the plume, and the results were images like these:
All known plume jets come from the tiger stripes, and over 90 jets have been observed. Locations of the most prolific sources superimposed over the earlier heat map:
As mentioned, a number of models have been created to explain geyser activity, and they fall into four distinct categories, two of which posit liquid water and the other two of which involve a viscous ice slush. One model that involves liquid water proposes a global ocean layer heated by radioactive decay in the core and tidal heating, causing local regions of liquid to upthrust into pressure chambers in the crust and vent to the surface through the tiger stripe faults. Illustration:
Another model proposes that liquid water occurs in limited pockets within the ice layer, heated by hot spots on the rocky core that experience tidal heating. The water in these pockets is under pressure due to the heat rising from below, and vent to the surface through cracks in the ice above. Illustration:
Instead of liquid water, the remaining two models propose a viscous layer of water-ammonia warmed directly by tidal heating and also from below by radiogenic heat from the rocky core. Both models propose that that this slush oozes up to the surface through the faults, but they differ on what happens then. One proposes that the exposed slurry immediately begins jetting out by sublimating (turning directly from solid to gas); the other suggests that due to the hypothetical ammonia content, the water has "antifreeze" properties and flows to some extent over the surrounding terrain before sublimating. The two "slush" models, respectively:
The "flow" model above has commonalities with the behavior of volcanic plumes on Io, which are created when flowing magma encounters sulfuric ice deposits and releases large, rapidly-expanding clouds of gas and entrained dust as a result. However, the geologic behavior of a small iceworld like Enceladus is very different from that of a massive rocky body like Io, so we shouldn't read too much into analogies.
The science of other worlds moves relatively slowly, so even though researchers seem to lean in the direction of the global ocean layer model, they are taking their time being definitive about it - a fact driven by the periodic nature of Cassini flybys of Enceladus, and the incomplete data that can be generated by each encounter. Each of these models can be diversified even further with theories regarding the structure and behavior of the surface vents themselves:
Since scientists knew almost nothing about any of this prior to Cassini, the Enceladan geysers have been an active area of research and discussion, and by far one of the greatest contributions of the Cassini mission. It should be noted that theories can overlap and trade bits and pieces, so a number of ideas consistently float around. One that appears to be taken seriously enough by NASA to post it as an official graphic illustrates the idea that the plumes arise from a bubbly ocean of "Perrier" water, whose dissolved gases help drive their escape through the plumes.
Enceladus is only one of four bodies in the solar known to have geyser activity, the other three being Earth, Io, and Neptune's moon Triton. They are all very different, and the features that distinguish Enceladan geysers make them today the most attractive location in the solar system to look for extraterrestrial life.
---
V. Potential for Life
The Goldilocks analogy used earlier may prove even more apt if the potential for life beneath the surface of Enceladus proves out: It has an unusual combination of environmental factors that make it a serious potential abode for microbial life despite orbiting a planet far beyond the Sun's Habitable Zone. If that turns out to be the case, humanity will learn something crucial about the nature of life: That as long as the end-result is a sustainable heat source, liquid water, and organic materials, it doesn't matter what it is that produces them - be it an Earth-like planet in a star's habitable zone, a tidally heated iceball moon in the frigid outlands of a solar system, or something even more exotic.
In the search for life-as-we-know-it (LAWKI) off Earth, Enceladus is currently the prime target among all other candidate worlds in this solar system. Mars, despite its high gravity, relatively manageable temperatures, and Earth-like deep history, is water-poor and exposed to intense radiation due to its lack of a magnetic field that sterilizes and chemically alters the surface. While Enceladus has no magnetic field of its own, it is deeply embedded in Saturn's magnetosphere and well-protected by it - as well as by the diffuse E ring its own geysers create, which tends to absorb high-energy particles. Europa, despite likely having a global subsurface ocean, covers that ocean in a thick ice shell that would be difficult to penetrate, and its surface is bathed in a lethal level of radiation. Titan, although a stew of complex organic chemistry, doesn't appear to generate liquid water, so if life did exist there it would have to be chemically exotic - i.e., not LAWKI.
A number of hydrocarbon compounds have been observed in Enceladan plumes, including propane (C3H8), ethane (C2H6), and acetylene (C2H2), among others. If indeed the geysers result from liquid water regions with sustained heat sources and organic chemicals present, and no show-stoppers are identified, there's no reason why basic microbial life would not have arisen there and still exist. Observed concentrations of plume material:
---
VI. Modern Relevance to Humanity
The potential for LAWKI on Enceladus is the overwhelming driver of its significance today, and one of the most important contributions of the Cassini mission over its many years of service. But beyond that, it has also taught us a lot about ice-world evolution in general, which may help in understanding other worlds such as Europa, and perhaps give us a head's up in understanding more distant, poorly-understood worlds like Triton that are also icy and geologically active. Enceladus, like Titan, can be called a "frontier of a frontier" - an exotic place even by the standards of the Saturn system.
Unfortunately, the Cassini mission is slated to end in 2017, and no space program has any firm plans to send future missions to the Saturn system, so unless something changes the data gathered so far and over the next four years will have to be digested over the next decade or more without additional input. Furthermore, the next several generations of Saturn system probes are unlikely to be designed for landings (unless on Titan), let alone directly searching for life on Enceladus, so it is highly unlikely that such would be discovered in the foreseeable future even if it were abundant and located in the most obvious places. The potential for life, if it sustains further analysis, will remain only a possibility for a very long time.
Enceladus hasn't played much of a role in science fiction, probably due to how recently humanity became aware of its significance. But one of the rare places it did play a role, that I personally think very highly of and enjoyed a lot as a kid, was the quasi-anime series Exo-squad: An animated science fiction series told in a serial format with continuous expansion of the field of action, character growth, and an exciting background spanning numerous places throughout the solar system. Enceladus in the series is a fortified base held by a loose alliance of interplanetary pirates, and is the setting of a battle between them and Exo Fleet (the solar system's official peacekeeping military). Although vague, the depiction was pretty decent, and at times showed a number of cliffs and canyons:
The above screencap is from episode 3, "Hidden Terrors." You can watch part of it on Youtube here and judge the quality of the series yourself, but I highly recommend it: Although it was targeted at kids and was aired in the early '90s, it has aged very well both in terms of decade and audience. However, as usually happens in science fiction in general - and even more so in "youth" programs - space settings are little more than background without much attempt to teach anything about them specifically, so we don't learn anything about Enceladus from this depiction other than that it's a moon of Saturn (which I suppose is educational enough in a kids' show).
---
VII. Future Relevance to Humanity
There are few clear economic attractions to Enceladus other than scientific curiosity: Saturn's rings and Mimas could serve the material needs of colonies in cloud cities in the Saturnine atmosphere, and more distant moons would easily be able to serve most of their own needs with far greater masses of ice than Enceladus possesses. If there were a need for organic material, Titan would be the most obvious and abundant source, and it wouldn't be necessary to land on it to gather such resources - you could just skim the atmosphere. In other words, there is no clear, intrinsic reason for human development on Enceladus other than that designed to support research into it. About the only hypothetical economic attraction it might hold is in the relative largeness and accessibility of its rocky/metallic core for metal and mineral mining: Something that might be useful in a satellite system where most of the mass is bound up in Titan and the vast majority of the remainder is water ice.
Barring metal/mineral mining, if life were discovered there at some point, the scale of scientific endeavors could become quite large and support industrial-scale operations similar to what occurs in Antarctica. But paradoxically, the increase in interest would breed familiarity, and as the saying goes, familiarity breeds contempt: The novelty of extraterrestrial life would wear off if it became clear that it was similar to microbes we already know, or if life was also found elsewhere in the solar system. At most, I would imagine that the geyser plumes themselves would be cordoned off, but the rest of the moon ignored / allowed as fair game to whoever found a convenient use for it.
I doubt there would be any purpose to establishing actual settlements on it: Enceladus has the second lowest surface gravity of the major moons of Saturn, to a degree that would definitely be unhealthy; it's too deep in Saturn's gravity well to be convenient for trade with other parts of the solar system, but not deep enough to be useful for economies in the planet's cloud layer; its surface commodity, water ice, is overwhelmingly abundant in the system; and if low gravity is desirable, there are many, much smaller objects that provide it without the annoyance of still being noticeable. As for tourism, that's also doubtful - at least for the surface - because it's unlikely you could see anything standing right next to the plumes: The material is very diffuse. At most I would expect tourist flights that fly by in Saturn orbit from certain angles to catch the sunlight through the plumes.
Of course, people always, ultimately spread to wherever they can, so even if there is no compelling reason to settle Enceladus, and plenty of reasons why it's sub-optimal, you would eventually get people living there on a permanent basis for whatever reason. Ultimately, once the number of people gets above a certain size, others start immigrating because they like something about the people who live there or think there are opportunities to be had, so then you reach critical social mass and end up with a civilization even if its location has no special attraction. Plenty of surprising local cultures have developed over the past few centuries on Earth, and there will probably be more as humanity expands out into the solar system.
---
VIII. Future of Enceladus
Although Enceladus constantly loses material due to the geysers, a lot (if not most) of it ultimately comes back home - either by falling back to the surface directly as snow (which accumulates at a rate of half a millimeter per Earth year), or else orbiting Saturn for a time until it encounters the moon again. Some of it does escape completely, ending up absorbed by other moons or part of the diffuse cloud surrounding Saturn out to a large distance, but the fact that a lot of it comes back on extended timelines means it's not a simple calculation to figure out how fast Enceladus is losing net mass.
The rate of material ejected by the geysers is estimated at 150 to 360 kg/s, which sounds like a lot but at the higher figure would actually take about 4.8 billion years to fully deplete Enceladus if half its mass were water and none of it came back. Since we know it overwhelmingly does come back, the Enceladan ice would last longer than the expected future lifespan of the Sun if the rate of depletion were constant and external factors are ignored. However, in reality the heavy elements in its core that drive radiogenic heating are gradually decaying, and will contribute less to Enceladus' internal heat in the future, so at some point it will freeze solid despite the continued tidal flexing.
Then, as the Sun expands into a red giant several billion years from now, the environs of Saturn will be become warm enough that Enceladan ice will sublimate and leave behind its exposed rocky core. As this happens, it may be slowed down in its orbit by the greater amount of material surrounding Saturn and consequently spiral inward, possibly wandering into Saturn's Roche limit and breaking up into a rocky ring that is itself ultimately swallowed. This is just my understanding, anyway, and I defer to any experts in the house on the subject.
---
IX. Catalog of Exploration
Voyager 1 (USA - 1980 flyby)
Voyager 2 (USA - 1981 flyby)
Cassini-Huygens (USA and Europe - entered Saturn orbit 2004, currently operating)
2. Future probes:
(none planned)
