Saturn, Queen of the solar system, reigns in majesty over a vast retinue of exotic moons and icy rings like a celestial crown. No other planet of our solar system can compete with its domain for sheer beauty: Exquisite alien vistas that boggle the mind, scientific wonders abound, material wealth beyond imagining, and the promise that some day humanity will know this cosmic work of art with our own eyes and join in its mystery. But when we do, it can only be from among its satellites, for though planet Saturn is a picture in grace from a distance, below its clouds an implacable maelstrom rages forever. Welcome to a realm of unparalleled strangeness and visual revelation. We will be remaining here for the next dozen or so parts of this series, as we explore all the various worlds that Saturn has to offer, beginning with the planet itself.
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
Saturn, as we would see it with our own eyes:
I. Context
Saturn is the sixth planet from the Sun, orbiting between 9 and 10 times the Earth-Sun distance (9.05 - 10.1 AU) or about a billion-and-a-half kilometers. It is the second largest and second most massive planet in the solar system after Jupiter, and also has the second largest number of known moons (62), with 7 of them being significantly-sized. The standard adjective for an object of the Saturn system is Saturnian, although I've always thought Saturnine sounds both cooler and more accurately descriptive of this system (look up the definition). So, I'm going to arbitrarily use "saturnine" in all the entries concerning this system, even though it's not a recognized variant of the planetary adjective. Hopefully it will catch on. A diagram of Saturn's orbit:
Saturn doesn't look like much from Earth, although the novelty of its rings certainly excited the early telescope-based astronomers:
But the closer you get, the more interesting things become...
And then they get just plain jaw-dropping (to show just a couple of examples):
Due to the inverse-square law of electromagnetic radiation, the intensity of sunlight arriving at Saturn ranges between 1.2% and 0.98% that received by Earth. In other words, it's a pretty gloomy place. These lighting conditions are about what you would get close to the terrestrial poles near equinox, so something like this (but with a much smaller Sun):
However, it's important to understand that this is the amount of light received at high noon on a Saturnine surface directly facing the Sun - i.e., the maximum direct illumination experienced around Saturn is analogous to the minimum experienced on Earth. We can better appreciate the amount of illumination reaching the surface by removing the sky from the lower image:
Saturn doesn't exactly seem stygian looking at its fully-illuminated globe against the black of space...
...but images from different angles put the planet's light environment in full context:
Anyone can figure out the intensity of Sunlight anywhere in the solar system relative to what Earth receives with some elementary algebra. The basic equation for the intensity of electromagnetic radiation shows that intensity (I) is equal to power output (P) (meaning energy over time in Watts) divided by the square of the distance from the source of the radiation (R2):
You could just plug in numbers directly from there, but if all you want to know is how much more or less intense the light is than Earth receives, that's not necessary - all you need to know is the relationship between the light received by Earth and that received by wherever you're thinking about. Because it's the same Sun emitting the light in both cases, P is the same in both equations, so we can treat it as constant. The only thing that's different is R. This means that all you need to know is how many times the distance a given place is from the Sun than Earth. If it's twice as far, you replace the R for Earth (let's call it RE) with 2RE and square it, giving 4RE2 in the denominator. If it's half as far as Earth from the Sun, replace RE with (1/2)RE and square it to get (1/4)RE2 in the denominator. In other words, just throw in a factor of whatever the distance multiple is into the denominator and square it. Let's use the letter 'a' for whatever that multiple is:
Now, that just gives you the intensity of sunlight at a given distance in space from the Sun, but you would need more involved calculations to figure out how much sunlight a particular place on a particular body gets (due to things like latitude and axial tilt) - and it's even more complicated if you have to figure out the effects of an atmosphere. But that's too much detail for our purposes.
At perihelion (closest approach to the Sun), the Sun is only about 0.06° in angular size as seen from Saturn, which is slightly more than 10% of its size from Earth:
To get a sense of this, we can resize a picture of the Sun in space taken from Earth orbit and put it into the black-sky image of Antarctica at equinox above:
The Sun size comparison has been a standard feature of this series for every entry dealing with a planet, so if you're curious to know how it's derived, all it takes is basic trigonometry:
This doesn't give you an exact figure, since the apparent size of the Sun is enlarged by glare to somewhat more than its actual radius would suggest, but as long the point is simply to compare how it looks from different places in the solar system, this is more or less just as good as having precise figures.
The dimness of the Sun at Saturn means it's nowhere near bright enough for solar panels to be practical, so exploration of Saturn and points beyond is limited to nuclear power sources - a fact that makes it increasingly challenging to explore robotically under tight budgets and dwindling supplies of scarce materials for radioisotope thermoelectric generators (RTGs). It also makes the planet and its system completely inaccessible to human exploration until we develop much faster means of interplanetary travel and radical advances in nuclear power, making Saturn something of a milestone in future human history.
One more useful contextual comparison is a representation of the solar system's nested gravity wells, which I've modified from a popular xkcd comic to enhance simplicity and clarity - you can see how Saturn compares to Jupiter and the two outermost planets in this department, although the Sun's gravity well is too deep to fit in the picture:
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II. History
Saturn is thought to have formed not long after Jupiter by accretion of rocky/metallic bodies in the early solar system (about 4.6 billion years ago), which then swept up what remained of the protoplanetary disk in its region of the solar system until it built up a massive gaseous envelope. However, due to the relative diffuseness of this material compared to the region of Jupiter, both the speed and heat of accretion were considerably lower for Saturn, meaning that it began cooling and contracting earlier in life than its counterpart.
Due to vagaries of temperature and pressure that can allow part of an atmosphere to be much colder than layers both above and below it, at some point the upper atmosphere of Saturn cooled enough that helium precipitated into liquid and "rained" down into the core. The downward passage of this precipitation created friction that stirred up heat not present at formation, and that energy - rather than the collisions that formed the planet, or the decay of radioactive materials in its core - is thought to be responsible for most of Saturn's present-day heat emission: An amount 2.3 times greater than it receives from the Sun.
One counterintuitive result of Saturn's atmosphere being cooler than Jupiter's is that Saturnine winds are somewhat less turbulent, and can thus achieve substantially higher speeds - upwards of 1,800 km/h (1,100 mph). This makes its atmosphere more efficient at expelling internal heat than Jupiter's, and creates a self-reinforcing cycle of more rapid cooling. In fact, this appears to be a general principle among gas giants, since Neptune is even colder than Saturn and achieves even more rapid wind speeds. Sadly, the more efficient and smooth a gas giant's winds are, the less "whorlsome" its clouds appear, which is why Saturn's cloudtops are often bland - particularly in the equatorial region:
The gas giants of our solar system were not thought to have formed at exactly the orbits they have today, but rather to have migrated to some extent, and Saturn's migration - in association with that of Jupiter - may have played a very significant role in planetary history. According to the Nice Model of solar system formation, the outer planets and Kuiper Belt originally formed in a relatively compact system with highly circular orbits and Neptune originally being the 7th planet while Uranus was outermost (accounting for its being least massive of the gas giants).
But according to the theory, the migrations of Jupiter and Saturn put them into a 2:1 orbital resonance that had a disruptive gravitational influence on everything else, sending Neptune out beyond Uranus, scattering the Kuiper Belt into a much larger volume of space, and also directing a large number of objects into the inner solar system. This period may correspond to an epoch of solar system history known as the Late Heavy Bombardment - a time associated with an unusually large number of craters on the Moon, and less concretely with impacts elsewhere. A diagram of the Nice Model is shown below - note that the eccentricity of Saturn is somewhat exaggerated from reality in the final frame, which I don't know if that's just an artifact of the illustration process or an actual outcome of the model:
Saturn's most distinctive feature, the ring system, is thought to be primordial despite its apparent delicacy, and two theories are offered for its formation: (1)That it either consists of leftovers from the planet's formation that were never accreted due to being within the Roche limit - a region where tidal forces rip an object above a certain size into pieces, or (2)that a small moon a few hundred kilometers in diameter wandered into this limit early in Saturnine history and was torn apart. Some of the thinner, less substantial rings are known to result from material being continuously stripped off of existing moons, but the main rings are ancient rather than being dynamically-generated.
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III. Properties
1. Orbital and Rotational Features
As with any fluid body, the rotational period of Saturn is not a simple topic: Different latitudes circle the planet at somewhat different rates, which is what causes gas giants to have latitudinal cloud bands (see the discussion on this phenomenon in Jupiter (Vol. 1) for further details). The Saturnine day varies by about 24 minutes between the equator and high latitudes, ranging from 10 hours, 14 minutes to 10 hours, 38 minutes. In other words, it's not radically divergent over a single rotation, but the difference adds up over time, causing shear forces that pull apart cloud structures - most intensely at mid-latitudes.
Saturn has a pronounced axial tilt of 26.7° - several degrees greater than Earth's, and the second biggest tilt of any planet in the solar system after Uranus. While there may be seasonal effects in the upper atmosphere due to this fact, what's most interesting for our purposes is the consequences for lighting of the ring system: Nearer to Saturn's solstices, the ring plane is either facing toward or away from the Sun, and thus gets maximum illumination while casting deep, wide shadows on to the planet's mid-latitudes - like this:
But closer to the equinoxes, the ring plane is edge-on relative to the Sun's rays, so you get only weak illumination of the rings and they cast only a very thin shadow on to the equator:
In the above image, you can appreciate how dim the rings are at this time of year because seeing them at all required an exposure that somewhat over-exposes the planet, causing it to look white and uniform. And since the Saturnine year is 29.5 Earth years long, the different illumination periods for the rings need to be appreciated when they come around. A diagram of ring light seasons:
2. Size and Mass Characteristics
One of Saturn's more extraordinary features is how greatly its equator bulges relative to its polar axis - a measure called flattening or oblateness. The equatorial diameter is almost a full Earth-size larger than the polar diameter, fitting almost 9.5 Earths into its width as compared to about 8.6 through the poles. In fact, Saturn is the most oblate planet in the solar system, owing to a combination of having a rotational period comparable to Jupiter's with a diameter only modestly smaller while having less than a third of the mass: Its gravity just can't fight the outward inertia as much. The resultant warping is obvious in any of the full-globe images above.
Another fascinating fact about Saturn is that in the upper atmosphere with Earth-comparable pressures, gravity is about 1.07 g - only 7% higher than we experience here. What this means is that if you were to float around the Saturnine clouds in a balloon, you would weigh basically what you do on Earth, with very similar speed of fall and fluid pressures experienced by internal organs. This is hugely different from the case of Jupiter, where every tissue in your body would strain continuously under 2.5 g - which would be dangerous over extended periods of time. So, provided the immense, currently absurd amount of rocket fuel needed to both descend to and then leave the upper atmosphere of Saturn, human beings could very well directly explore its clouds in balloons or dirigibles. The cost/benefit of such a thing seems dubious, but we won't know for quite a long time what is worthwhile where this planet is concerned. More on this in Volume 3, when we explore what Saturn may mean for humanity in the future.
As noted earlier, Saturn is the second largest and second most massive planet in the solar system, making it third largest and third most massive object overall (including the Sun). It has a mass equivalent to roughly 95 Earths, about 30% the mass of Jupiter, and about 0.0029% the mass of the Sun. Going by equatorial girth, it's 8.7% the size of the Sun, 84% the size of Jupiter, 2.36 times larger than Uranus, and 2.43 times the size of Neptune. Size comparisons of comparable bodies, with the addition of Earth:
(Continued in Volume 2)
