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The rings of Saturn are one of the most spectacular and defining wonders of the solar system, and have mesmerized science fiction artists and puzzled astronomers for centuries.  Despite being made of little more than orbital fields of snowflakes and ice chips, the large-scale results are so beautiful and exotic that actual photographs of them look like the surreal dreamscapes of a mathematician.  Saturn's ring plane is a region of such dynamic majesty that there doesn't need to be any greater justification for exploring it: We want to see it because it's there, and have been richly rewarded every time that probes sent to that system have shown us the jeweled crown surrounding Saturn.

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
Rings of Saturn in true color, just as our eyes would see them, and sporting a "ringbow" due to the angle of sunlight:

rainbow

I.  Context

The rings of Saturn are by far the largest, densest, most visible, and most complex in the solar system, with those of Jupiter, Uranus, and Neptune being little more than thin, barely-visible halos.  They are so prominent that the light they reflect from the Sun significantly contributes to the brightness of the planet, and so broad that they make the planet appear to have "ears" to either side in low-resolution images when the ring plane can be seen from an angle.  A cheap commercial telescope will let you see this from Earth:

Saturn (Amateur)  

The visible ring plane begins at a distance of about 7,000 km over the equatorial cloud tops of Saturn - about 67,000 km from the planet's center - then extends out to a radius of 140,000 km at the edge of the faintest visible region.  The thickness of the ring is only a few meters to a kilometer wide, with an average thickness on the low end, so it's practically two-dimensional on the scale of Saturn.  A polar-ish view of the complete ring system giving a sense of relative proportions:

unlitsidehalf

If we replace Saturn with Earth at the center of a polar image, we get a sense of perspective about just how big the rings are:

RingsEarthComp (Polar View)

In the above image, the rings would begin almost twice as high as our geosynchronous satellite constellations and end a third of the way to the Moon.  Another comparison:

RingsEarthComp2 (Angled)

It isn't certain how the rings would look from Saturn's cloud tops, but you would probably see them better from certain altitudes, latitudes, and seasons than others.  For instance, you likely couldn't see very much from the equator or during equinox, when the ring is a thin line passing through the Sun, but if you were in a mid-latitude region in the shadow of the rings at solstice, when the plane is inclined relative to the Sun, you might see quite a lot.  Even more spectacular ringbows than the one shown at the top might not be uncommon.  But most of what you see in any case has to be from far away because ring material is so diffuse: If you were relatively close to the rings, this is what you'd see:

PIA10598

The rings are often so bright that they can illuminate the night side of Saturn, as seen in the image below, so one has to imagine they must be spectacular at some times of year in Saturn's night sky:

PIA10476

Long-lived ring systems are found deep within gravity wells, well beneath the orbits of large moons, and Saturn's is no exception:

SaturnRingsGravityWellComparison

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II.  History

Persistent ring systems exist because their material orbits within a planet's Roche limit: The boundary within which tidal forces tend to overcome the gravity holding an object together, tear it apart, and stretch the resulting cloud of material out along its entire orbit into a ring.  This happens because solid objects in orbit around a planet have one side closer to the planet and one side farther away, and the two want to orbit differently: The Near Side wants to orbit faster and move ahead of the Far Side, so there is a shear tension.  Illustration of increasing tidal forces:

Tidal Forces

If you had a bunch of random material outside the Roche limit, it would gravitationally attract and clump into a large, significant moon, but inside the limit there is very limited opportunity for this to happen because the shear tensions of tidal forces are constantly overcoming gravitational attraction when objects meet.  Those that did manage to stick together would be small, structurally weak objects similar to "rubble pile" asteroids or comet nuclei, depending on what the material is made of.

In the case of Saturn, current theory holds that the rings are overwhelmingly primordial material that simply never had a chance to coalesce into larger objects due to being within Saturn's Roche limit.  The earlier theory, that a moon was at some point perturbed within the limit and broke up, is largely out of favor due to the apparently extreme age of the rings and the lack of strong evidence for a cataclysmic origin.  So the rings are thought to be "leftovers" of Saturn's formation rather than the wreckage of a cohesive object that was destroyed.  A relatively small amount of material is contributed on an ongoing basis to some rings by some of the closer moons, but the vast majority appears to be ancient.  However, cataclysmic origins have not been discounted, if the formative events happened very early in the system's history.

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III.  Properties

1.  Regions

Although there are thousands of distinct "ringlets," the visible plane is divided into five general regions with 2 large-scale "gap" regions among them: The innermost region is a faint, tenuous system called the D ring that is about 7,500 km wide and consists of only three distinct ringlets.  Next is the C ring: Very wide (17,500 km) and obvious, but darkish compared to the other main rings.  The B ring is the largest (25,000 km) and brightest, as well as densest, containing the vast majority of the mass in the ring plane.  Outermost is the A ring, which is also bright, dense, and thick (14,600 km).  Outermost of the visible rings is the tenuous F ring.  The two main gap regions are named the Cassini Division, and the Roche Division, with a number of smaller gaps both within them and within the ring regions.    

The thickest main ring regions are so solid-looking from some perspectives as to be surreal, and make the ring plane look textured like a vinyl record, while the tenuous inter-gap and outer rings associated with a few small moons are spooky and sinuous.  A large-scale map of the ring-plane (I've had to divide it vertically into multiple images):

Ring Map 1

Ring Map 2

Ring Map 3

Then there are very faint, non-visible rings that extend much farther outward, and in which the major moons orbit and contribute material:

PIA03550

2.  Orbital Dynamics

Every individual ringlet orbits at a different speed, but I haven't been able to find specific information about the spread of orbital periods that occur in the rings.  However, we can say that those near larger objects, such as the small asteroidal/cometary moons that orbit among the tenuous rings like F (known as shepherd moons for their role in stabilizing those rings), will orbit at a rate comparable to the moons while rings that are closer to Saturn will orbit faster and those farther away will be slower.  This is why the guess of early astronomers that Saturn had a solid ring is impossible in nature: The same shear tidal forces shown in the Roche limit diagram above would tear a solid ring plane apart.

Many of the gaps in the rings are caused by orbital resonances among the moons and the ring material itself: When a bit of material is perturbed into an orbit similar to one of the gaps, the combination of gravitational forces acting on it from the moons and other rings will tend to either speed it up or slow it down, sending it flying out of the rings either toward Saturn or out into a more distant part of the system.  So it isn't that there's nothing in the gaps, or that nothing goes into them - just that what does end up in them is routinely being tossed out.  This is the same way the gravitational influence of Jupiter causes Kirkwood gaps in the asteroid belt (see Asteroids (Vol. 1).  General views of the ring plane, with regions and gaps clearly visible:

PIA05421

PIA06422

PIA06424

PIA06485

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PIA11478

PIA11493

PIA11513

Rings1

Some ring gaps are caused by the presence of shepherd moons, of which there are five of significance: Pan, Daphnis, Atlas, Prometheus, and Pandora.  The motions of shepherd moons often cause disturbances in adjacent rings, such as spiral-like ripples, curves, or towering vertical features (up to 2 km tall) like cirrus clouds sticking up from the ring plane.  Images of shepherd moons and their effects:

PIA09782

PIA08969

PIA08997

PIA08972

PIA08947

PIA08926

PIA08920

PIA08910

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Shepherd Moon 5

Shepherd Moon 4

Shepherd Moon 3

PIA14618

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PIA12722

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PIA12703

PIA12622

PIA11676

PIA11668

PIA11581

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PIA11535

PIA10593

PIA10489

PIA10456

A few related videos showing ring motion:

The tilt of the ring plane is the same as that of Saturn's axis (26.7°), so there are "lighting seasons" where the rings are lit differently and cast shadows differently on the planet.  You can tell the time of Saturnian year by the ring-plane shadows cast on to the cloudtops: If the shadows are in the North, it's Northern winter / Southern summer.  If they're in the South, Northern summer / Southern winter.  And if the shadow is a thin line along the equator, or invisible, it's close to one of the equinoxes.  As far as I know, we don't yet have any probe images of Northern summer / Southern winter because Saturn's year is 29.5 Earth years long, Cassini only arrived 9 years ago, and the Pioneer and Voyager flybys 33-31 years ago had been close to equinox, so they're all either Norther winter / Southern summer or equinoctial.  Ring season diagram, reposted from Saturn (Vol. 1):

Saturn Ring Plane Illumination

At or approaching equinox, the shadows are very thin and cast directly on to the equatorial zone, and the ring plane itself is much dimmer.  As a result, images taken at equinox may show the planet over-exposed in order to capture the rings more clearly.  Equinoctial images:

saturn_titanshadow_tr

PIA12708

PIA11667

PIA11593

PIA11583

PIA11570

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Northern winter / Southern summer - the rings are lit from beneath and shadows are cast into the North:

PIA08390

PIA08359

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3.  Mass, Composition, and Granularity

The mass of Saturn's rings is on order of 1019 kg, or about the same mass as the moon Mimas - an iceball the size of Jew Jersey - and is overwhelmingly composed of water ice with only tiny amounts of rock, accounting for far less than 1% of the mass.  Moreover, the material isn't evenly distributed within a given ring, but occurs in weakly-bound linear clumps that range in size from a few pebbles to aggregate features several meters in size.  An example illustration of ring granularity:

PIA03556 (Granularity)

4.  Temperatures

Being water ice in vacuum, the temperature of ring material has to be a lot colder than the freezing point of water to avoid sublimating (transitioning directly to gas) into space.  Observed temperatures in the ring plane range from 65 K (-208 °C / -343 °F) to 110 K (-163 °C / -262 °F).  Temperature chart:

PIA03562 (Temperature 2)

Relating temperature to lighting conditions:

PIA03561 (Temperatures)

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IV.  Miscellaneous Images

I recommend enjoying this soundtrack while viewing these images, and if you like it, maybe playing it while reviewing those above as well:

Some of the eeriest images are those of the ring plane seen edge-on, particularly with moons in the frame:

PIA07667

PIA08176

PIA12543

PIA09779

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PIA09806

Another interesting class of images are those where the rings visually intersect with Saturn's horizon:

PIA08265

PIA10442

PIA07555

PIA07552

PIA09822

PIA08946

Twilight conditions or near-perspective shots also create some interesting views:

PIA08929

PIA08911

PIA08853

PIA08850

PIA08340

PIA08297

PIA14936

PIA11564

PIA11464

PIA09820

Sometimes you can see stars through the rings:

PIA12612

PIA08825

PIA08287

PIA08285

PIA08283

PIA08281

PIA09829 (Antares through the rings)

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V.  Future Relevance to Humanity

In practical terms, Saturn's ring plane is a very productive antimatter generator because there's a lot of material with a huge amount of collective surface area exposed not only to solar but cosmic radiation, so there are constantly high-energy particle collisions going on that produce antiparticles.  About 250 micrograms of antimatter are produced in the rings per Earth year - which doesn't sound like a lot, but is actually monstrous.  At the expense of producing it in a laboratory today, Saturn's rings produce about $25 billion worth of antimatter every year, and that's before there's any actual market demand for it.  

A mere 10 micrograms of antimatter fuel would be enough to send 100 tons of payload from Earth to Jupiter in less than a year, and modestly greater amounts could reduce that duration to four months (and thus to Saturn in less than a year), so its future value as a commodity seems guaranteed to rise.  Moreover, antimatter, unlike peteroleum, is a renewable commodity generated by both solar and cosmic particle flux, so the main sustainability issue with mining the rings would be to avoid depleting the ring material that's reacting to create the antimatter.  That seems unlikely given that the amount of water involved could fill the Mediterranean, but it might be something for an advanced civilization to worry about.

Of course, any civilization advanced enough to deplete such masses would be capable of replenishing them, so I doubt it would be a big deal - and they'd have plenty of motivation to restore them if there was ever an impact on the way they look.  What's most amazing about all of this is that it's a real place, not a figment of some artist's imagination: You could look out a window and see this stuff.  So if people end up colonizing the Saturn system or Saturn itself via cloud cities, the rings will be as important and definitive a natural feature to those societies as the Moon has been to every generation of humans who has ever lived - probably more so, because they're likely to be a lot more prominent than the Moon ever is in Earth's sky.  

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VI.  Future of Saturn's Rings

Barring high-tech human stupidity, the rings are safe for another billion years at least, but as the Sun expands their temperature will gradually increase until they begin sublimating at an appreciable rate - which will increase further as solar expansion continues.  Ultimately they won't be there, but Saturn will be a very different planet by then anyway.

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VII.  Catalog of Exploration

1.  Past & current probes:

Pioneer 11 (USA - 1979 flyby)
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)

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