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Saturn, Queen of the solar system, reigns in majesty over a 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, countless scientific wonders, 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, because while planet Saturn is a picture in grace from a distance, below its clouds a 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
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3.  Internal Structure

Interior of Saturn 2

Like Jupiter, the interior of Saturn is divided into roughly five layers (although the above graphic only shows 4): The inner core is rock and metal, like a terrestrial planet, and is thought to be about 3-4 times the diameter of Earth.  The outer core consists of high-temperature, high-pressure compounds like methane and ammonia in a fluid state, although they are often referred to as "ices" despite being liquid.  Surrounding the outer core is an envelope of "metallic" hydrogen and helium, consisting of single-atom H and He under such pressure that it is electrically conductive.  Compared to Jupiter's, Saturn's metallic H layer is both smaller and a smaller proportion of the planet's interior.  The convections of this layer are responsible for Saturn's magnetic field.  

Above that, the hydrogen atoms are able to form stable bonds to make molecular hydrogen (H2), but are still under such pressure at high temperature that they are not a gas, but rather a supercritical fluid - an exotic state that is often misidentified as "liquid," and is a thick layer common to all gas giants while also being possible in terrestrial planets like Venus with thick atmospheres.  Supercritical states behave somewhat like both gases and liquids, in that they have no surface tension, so they fill up a space like a gas, but can also act as a solvent like a liquid.  The longest that any space probe has survived in a supercritical environment was the Venera 13 Soviet probe to Venus, that lasted 127 minutes.  

The topmost and thinnest layer (not shown in the graphic) is the atmosphere proper, where H2 enters a fully gaseous state along with much smaller amounts of other gaseous and vapor compounds to form the haze and cloud layer that we see.  This layer is about a few hundred kilometers in depth, although there is no distinct boundary between the atmosphere and the supercritical layer because the phrase-transition from gas to supercritical occurs smoothly as pressure and temperature increase.

As mentioned in Vol. 1, Saturn radiates more than twice as much heat as it receives from the Sun, and most of that comes from the ongoing process of gravitational contraction that began with its formation.  In other words, it's still ever-so-slowly releasing heat and getting smaller.  Part of that process is the slow raining down of helium droplets to lower layers, which causes friction and heat along the way.  The colder and smaller Saturn becomes, the smoother and faster its winds will move due to lack of turbulence, and the more featureless its clouds will become.  So we can guess that its clouds were much more to look at in the distant past.

4.  Atmosphere

Like most gas giants, Saturn's atmosphere is overwhelmingly hydrogen (96%) with a minority of helium (3%), and trace amounts of a few compounds like methane (CH4 - 0.4%), ammonia (NH3 - 0.01%), ethane (C2H6 - 0.0007%), as well as small concentrations of water and ammonia ices and ammonium hydrosulfide (NH4SH) particularly in the clouds.  However, this doesn't describe the full interior of the planet: Compared to Jupiter, where more than 10% of the atmosphere is helium, Saturn's is relatively depleted of He because of the process mentioned before of slow settling of helium into lower layers, so the proportion of helium is higher at greater depths.  Also, because Saturn is much less turbulent than Jupiter, it has evolved on a more rapid timescale into a more advanced state despite being a little younger.

The Saturnine cloud layer ranges between temperatures of 100K to 330K (-173 °C / -280 °F to 60 °C / 134 °F) and pressures between half to about a dozen times Earth sea level.  As such, and given other factors like the relatively modest gravity experienced at these depths, it would be ideal for balloon exploration, both by unmanned probes and, in the far future, possibly manned dirigible stations (we will explore these possibilities in Vol. 3).  The highest layer consists of ammonia ice crystal clouds, and the middle consists of two overlapping layers of different thicknesses - one water ice, the other ammonium hydrosulfide.  And the lowest cloud layer is water and ammonia vapor.  A temperature/pressure/altitude diagram illustrating the cloud layer:

saturn_atmosphere

Cloud layers are all that can be directly seen of a gas giant, because the increasingly thick, dense, and hot atmospheric layers below are thought to be relatively simple and visually featureless - although we don't know what kind of convection patterns occur in them.  As such, when we think of a gas giant like Jupiter or Saturn, what we think of are its clouds.

5.  Cloud Formations

Like most gas giants, Saturn's clouds shear into bands because different latitudes rotate around the planet at different speeds.  However, because its winds are much faster and less turbulent than those of Jupiter (reaching speeds up to 1,800 km/h in the fastest regions), there are many more bands, they tend to be thinner, and the boundaries between them are less visually distinct.  Consider this image and try to figure out how many there are:

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The above is a true-color photo, so it is exactly what you would see if you were at this location relative to the planet with the Sun at the same position.  However, it should be understood that this isn't always what it looks like - the appearance changes seasonally, with different weather, and from different perspectives relative to the sunlight.  

We can see 8-10 large-scale regions with distinct colors, but within the color bands there are also a number of thinner, less-obvious bands.  Similar terms to those used for Jupiter also apply here: Lighter-colored regions are called zones, corresponding to upward-moving air that propels highly reflective ammonia clouds to high altitudes.  They alternate with darker belts, which have descending air with darker cloudtops occurring at warmer, lower altitudes.  Rapid perpetual winds called jets occur at zonal boundaries, and the fastest are at the boundary between the Equatorial Zone (the wide yellow band) and the tropical belts to the North and South (the greenish and reddish regions above and below the Equatorial Zone).

As with Jupiter, finer cyclonic (and anticyclonic) cloud structures become more prevalent at higher latitudes because the shear forces of planetary rotation become less significant, allowing clouds to be more cohesive, with the largest storms occurring in the temperate mid-latitude bands.  However, nothing on the order of Jupiter's Great Red Spot has ever been observed in the clouds of Saturn.  Some Southern hemisphere views showing good examples of fine-banding and storms:

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The next two are limb-shots where the horizon visually intersects with the ring plane:

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As we can already see from some of the above images, the South polar region is very finely-structured with a substantial number of storms, and converges concentrically toward a relatively small, circular polar vortex:

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The South polar vortex itself:

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Mid-latitude and equatorial cloud formations:

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The shadow of Enceladus on the clouds:

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Mimas over the cloud tops:

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The Nothern hemisphere is even more intricate, and has a distinct blue coloration in the polar region not seen in the South:

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The North polar region is one of the most fascinating weather systems in the solar system, both because of its Tolkien-esque swirling complexity, its strange coloration, and the fact that the polar vortex is surrounded by a unique hexagonal cloud system whose causes are unclear.  It should be noted that the hexagon is larger than Earth:

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The North polar vortex:

North Polar Vortex

Closer shots of cloud formations, unstated/unclear latitude:

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The North and South polar vortices in motion:

Motion of the North polar hexagon:

Closing image: Saturn's limb against the stars:

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(Continued in Volume 3)

I leave you with the perfect theme song for this gloomy but beautiful wonder of a planet:

Originally posted to Troubadour on Sat Apr 13, 2013 at 07:20 PM PDT.

Also republished by SciTech and Astro Kos.

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