Saturn's moon Tethys is a rough, pitted ice carcass of a world, and is in many ways emblematic of most of its fellow moons: Ancient, composed almost completely of water ice, geologically dead, and brutally scarred. It typifies an entire class of satellite in the system, bearing so great a resemblance to its larger, more distant counterparts Dione and Rhea that it's often difficult to tell them apart at a glance.
The progress of our adventure so far (current in bold):
1. The SunThe most distinctive face of Tethys in true color, as the human eye would see it:
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
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)
19. Jupiter (Vol. 1)
20. Jupiter (Vol. 2)
22. Europa (Vol. 1)
23. Europa (Vol. 2)
26. Saturn (Vol. 1)
27. Saturn (Vol. 2)
28. Saturn (Vol. 3)
29. Rings of Saturn
37. Minor Moons of Saturn
46. The Kuiper Belt & Scattered Disk
48. The Interstellar Neighborhood
49. Overview: Human Destiny Among the Worlds of Sol
50. Test Your Knowledge
Tethys is the fourteenth moon of Saturn, and of the seven major moons is the third closest to the planet after Mimas and Enceladus. Its orbital radius around Saturn is about 77% of the average distance between the Earth and Moon. Orbital and gravity well diagrams:
Saturn covers 23.6° of arc across the Tethyan sky, or about 47 times the size of the full Moon as seen from Earth, and larger than Jupiter from Io - not including the rings. So its apparent scale would be roughly like this, although the rings would be much less visible because they would be nearly edge-on:
Tethys is as old as the Saturn system itself, and by extension the solar system in general, with the most heavily cratered regions of its surface thought to be primordial - 4.56 billion years old or thereabouts. Due to the Saturn system's uniquely water-rich environment - caused by chemistry during formation that favored the generation of H2O rather than the ammonia (NH3), methane (CH4), and carbon monoxide (CO) favored at greater distances from the Sun - Tethys formed overwhelmingly out of water ice, and so (unlike Enceladus) didn't have enough radioactive elements to keep its interior heated. Tidal heating due to other moons may have drawn out the process for a time, but two to three billion years ago it finally froze solid, causing it to expand (because ice is less dense than water) and crack, creating the huge rift valley Ithaca Chasma that dominates an entire hemisphere.
At some other point in the past few billion years, a large impactor created the giant crater Odysseus that remains the most distinctive feature of Tethys. The structure of Odysseus has since then gradually relaxed and faded to its present shallowness, with the interior of the crater now conforming to the curvature of the moon rather than being a gouge. Other than these ultra-slow-motion erosive processes, not much else has changed in eons: Tethys was probably already a relic around the time life on Earth started breathing oxygen.
1. Orbital and Rotational Features
The Tethyan month is about 45 hours, which is the same as its day because Tethys is tidally locked to Saturn - making one complete rotation for one complete orbit of the planet. It has no appreciable axial tilt relative to its orbital plane, only a very small inclination relative to Saturn's equator, and barely any eccentricity in its orbit (i.e., the orbit is very circular). As a result of its circular orbit, Tethys doesn't experience much in the way of tidal heating today, although that doesn't rule out the possibility that it may have in the distant past.
The leading hemisphere of Tethys - the one facing forward along its orbit - is constantly being pelted with icy material ejected into space by the geysers of Enceladus, and as a result is somewhat brighter than the opposite face. The trailing hemisphere, meanwhile, is bombarded by the ionized particles and electrons of the diffuse plasma cloud surrounding Saturn (see Saturn (Vol. 3) for more details), which is swept around the planet by its rotating magnetosphere at much greater speed than the orbits of moons. These factors are probably what cause the differences in coloration over the Tethyan surface, but scientists are not sure why or what chemical compounds are involved. However, it's in a relatively benign part of Saturn's magnetosphere for an inner moon (unlike Titan, which orbits in a radiation belt), so the particle flux is moderate. Illustration:
Both of these particle environments apply to all major moons except Iapetus, which is too far away from Enceladus to be significantly affected by its geysers, and too distant in Saturn's magnetosphere to be affected by the plasma cloud. But as Tethys is geologically dead and experiences only weak tidal forces, bombardment from these particles is pretty much the only thing happening to it.
Tethys has two small co-orbital moons, that stably share the same orbit of Saturn with it at 60° ahead of and behind it: Telesto and Calypso. They occupy the Saturn-Tethys L4 and L5 Lagrange points, respectively, and as such are sometimes known as "Tethys Trojans" (see Asteroids (Vol. 1) and Vol. 2 for a more detailed discussion of Lagrange points and Trojan orbits). What this means is that the gravitational forces of Tethys and Saturn are in a dynamic balance at these points with the orbital motions of these two smaller moons, such that they are kept in the same general location relative to Tethys, although from a closer perspective they move in more complicated ways. Illustration:
Telesto and Calypso themselves, respectively:
As one of the inner major moons, there are plenty of opportunities for visually interesting alignments with other moons. It's important to note that it's never clear how much closer or farther away from the camera one moon is compared to another unless you already know how big each is, so don't take these images as indicating relative sizes. With Titan and the ring plane:
With the ring plane, Enceladus, and Titan:
With just Titan - the first is two consecutive images showing Tethys moving behind and then emerging from the limb of the much larger moon:
With Dione - typically the darker partner, but may look brighter than Tethys as a crescent:
Dione and the ring plane, with the addition of minor moon Pandora in the second image:
Enceladus moving behind Tethys:
2. Size and Mass Characteristics
The mass of Tethys is about 0.0103% that of Earth and 0.8% the mass of the Moon, making it the third lightest of the seven major moons of Saturn after Mimas and Enceladus. It also has the lowest density of the seven, and thus is known to be overwhelmingly composed of water ice with a relatively porous structure.
Because of this low density, despite being more than twice the size of Enceladus and having nearly six times the mass, its surface gravity is only 36% greater - 1.5% of Earth's. This is about a tenth of what you would experience on the Moon, so objects on its surface would not only weigh much less, but would accelerate downward more slowly. While people often make the mistake of thinking heavier weights fall faster on Earth, all weights fall at the same rate in the same gravity, but at higher gravity everything falls faster, and at lower gravity everything falls more slowly - it isn't just the weight that changes. If you dropped something from your hand on Tethys, it would take several complete seconds to reach the floor. So human explorers on its surface would move around by bunny-hopping or going hand-over-hand if they didn't feel like waiting to come back down with each jump.
Tethys is 1,062 km wide - about 31% the size of the Moon, and 8.3% the size of Earth. Its diameter would completely cover France or almost cover Texas, and its surface area would more than cover India. Rough size comparisons with comparable objects - mouse over the image to see the file name if you don't recognize something:
3. Internal Structure
Not much is known about the internal structure of Tethys, and in truth it doesn't draw much scientific attention compared to exotic moons like Enceladus and Titan. The main thing about its interior that distinguishes Tethys is that the amount of rock in its composition has been constrained down to a maximum of 6% of total mass, as compared to a major fraction (up to half) of the mass of Enceladus and Dione.
In fact, it's not even known whether Tethys is internally differentiated into discrete layers or is homogeneous like Jupiter's moon Callisto, but if so the largest rocky core it could have given the 6% rock constraint would be 145 km in radius - only a little over a quarter of the total radius. So if its interior is differentiated, and if it had the maximum rock-fraction of 6% (the actual figure is probably smaller), these would be the rough proportions of the interior:
There is very unlikely any sort of liquid layer or other dynamic process happening beneath the surface. Either Tethys is frozen solid into an unchanging layered form, or else is a ball of static, uniform slush whose rock content is distributed throughout the mantle in blobs and chunks from the impacts that originally delivered them.
4. Surface Features
Tethys is the second brightest moon of Saturn after Enceladus, partly because of Enceladus: The other moon's geysers eject considerable amounts of water ice into orbit around Saturn, some of which ends up peppering the surface of Tethys - particularly the leading hemisphere. As a result of this material and its chemical interactions with Saturn's plasma cloud, there's a subtle but visible greenish tint to the trailing hemisphere, and a vaguely noticeable bluish tint to the leading hemisphere. Similar patterns are also visible on Dione and Rhea, so it's not unique to Tethys. The phenomenon can be seen in true-color images:
Images of Tethys often show Odysseus and Ithaca Chasma, but sometimes neither are visible and it's hard to know what moon you're looking at. Also confusing is that Tethys has a large (but not gigantic) impact crater near the South pole, Melanthius, that bears some resemblance to the larger crater. General views:
Various images in phase and limb shots:
Zooming in for some regional perspectives, first to get a closer look at Odysseus:
Odysseus is 445 km across - so big it could almost fit Mimas inside it. However, it's relatively shallow, sinking only 3 km below Tethyan mean radius (a liquidless body's equivalent to "sea level"), with a rim only 5 km above mean radius. This strongly distinguishes it from the other "Death Star" crater in the Saturn system, Mimas' Herschel, which is deep and gouge-like, with a prominent central peak. Tethys is relatively flat in general, without a great deal of topographic contrast. Relief map:
As you can see from the size comparison images earlier, Tethys as compared to Mimas has more of a "pitted" surface as opposed the "hammered" look of Mimas, so that's one way to tell them apart from a distance if you're looking at a bright moon with a huge crater in it. Telling it apart from Dione is easy from some faces, but difficult from others, although Dione is generally darker. Rhea, however, is very similar in appearance. The two "Death Star" craters of Tethys and Mimas, Odysseus and Herschel, side-by-side and shown roughly to scale:
A regional view of crater Penelope (in shadow), the most prominent feature of the trailing hemisphere:
Telemachus looks a lot like Herschel, and is likely among the younger craters given the sharpness of its features:
South polar region - Penelope is visible at the mid-latitude limb of the image, and Ithaca Chasma is seen running out of the frame from near the pole:
North polar region - Odysseus is visible at mid-latitudes:
A regional view of the trailing hemisphere shows the characteristic "pittedness" of the Tethyan surface:
The southernmost region of Ithaca Chasma, showing its steep ice cliffs:
This crescent view of the limb gives a sense of the terrain, but...
...as you zoom in, you find Tethys to be a science-fiction wonderland of white ice hills beneath the black of space:
This is the closest view of the Tethyan surface yet taken, and has a resolution of 110 meters per pixel:
A more detailed map, including the few named polar features, can be had here (PDF page, may take some time to load).
IV. Modern Relevance to Humanity
Due to its apparent mediocrity, Tethys has not played a major role in scientific investigations of the Saturn system or in science fiction. In the latter, its most prominent role was to be briefly and inaccurately portrayed as an irregularly-shaped moon in the otherwise superb US anime series Exosquad. Although it spends little time on this particular moon, I would recommend the series for all ages: The writing and the events depicted are truly witty, creative, and insightful, and I don't say that just "for a cartoon" - it's amazing for any TV show, and ranks as a classic in my view. It's one of the few sci-fi epic TV shows whose setting is limited to our own solar system rather than imagining warp drive / interstellar travel, and although it relies on a number of entertainment tropes that aren't scientifically accurate for the sake of action - e.g., sounds in space, banked turns in space, etc. - overall it's remarkably respectful of reality, and more intelligent by far than a number of other science fiction shows with cult followings.
V. Future Relevance to Humanity
Although its surface gravity is too low to be healthy for permanent settlement, the very mediocrity and thorough iciness of Tethys that causes scientists to overlook it could make it attractive to industrial activity once the Saturn system has significant human populations. It has no specific attraction greater than Dione or Rhea, but all three would probably end up playing host to the same kinds of developments: Water ice mining, obviously, and the derivatives of it - hydrogen and oxygen.
Collectively, I could see them being waystations between colonies and/or deuterium/helium mining operations in Saturn's clouds and more distant settlements on Titan or Iapetus, which you'll realize would be very necessary given the distances involved when you review the first orbital diagram up at the top. The distances and gravitational climbs involved in going from Titan or Iapetus to Saturn or vice-versa are way beyond those involved in going from Earth to the Moon and back, so fuel generation, transport, depoting, and expenditure in flight will have to reach industrial scales that are unimaginable today - but which Tethys, Dione, and Rhea are suited to enable.
As with all topics related to the large-scale human colonization of Saturn and its moons, the timetable we're talking about is about 500-1000 years in the future, giving ample time for earlier waves of colonization and development of more accessible locations in the solar system. The near-future prospects for robotic exploration of Saturn's moons are not good: Outer solar system probes require nuclear power, and thus will remain extraordinarily expensive and difficult even if/when well-funded commercial organizations like Planetary Resources succeed in drastically reducing the cost, and radically increasing the numbers, of inner system probes powered with solar panels. Unfortunately, due to budget cuts, NASA robotic exploration is retreating from the outer solar system, and its operations in the region all concern missions already underway with none further planned.
So there is likely to be a gap of some years - possibly decades - between Cassini's last view of Tethys and its fellow icy moons in 2017 (when it will descend into Saturn), and the day when another probe lays eyes on them. However, in context this is not actually a radical departure from the normal pattern of Saturn exploration: Cassini first arrived at Saturn 23 years after the last spacecraft (Voyager 2) had visited it, and unlike its predecessor it stayed there, so perhaps the successor to Cassini will be more impressive still. And while it's unlikely, it remains possible that human beings could visit the Saturn system in our lifetimes, in huge nuclear-electric propulsion ships, if humanity saw fit to invest the unimaginable resources that would be required. More likely, however, that will be in the next century.
Over the longest term, Tethys is probably doomed by the fact that it's basically just a giant ball of unprocessed fuel sitting there for the taking. While it's as amazing as any astronomical body, it's nothing special that would drive people to protect it: When human beings are flitting around the solar system by the trillions in powerful, insatiably hungry fusion rockets, objects like Tethys probably have no future: They will be food for human civilization and whatever bizarre descendent civilizations follow from it.
VI. Future of Tethys
Barring the most likely, human-based scenario of being mined out of existence over however many thousands of years, not much will change for Tethys for billions of years to come until the Sun expands. Then the environs of Saturn will become too warm for water ice in vacuum, and the icy material of Saturn's moons will increasingly sublimate away, turning directly from solid to gas and filling the system with a much denser medium of ice particles. Tethys, being relatively close to Saturn, might then be gradually slowed in its orbit by passing through this material and spiral inward. If it instead survived, it would do so as a pitiful little rock a fraction of its current size, perhaps perturbed into an irregular orbit around whatever remains of Saturn after the Sun blows away most of its atmosphere in its stellar death throes.
VII. Catalog of Exploration
1. Past & current probes:
2. Future probes: