Saturn

Saturn is the second gas giant out, and the last planet out that can be seen with the naked eye. It is easily recognized by its prominent rings.

As can be seen above, Saturn's axis tilt is similar to both Earth and Mars. It is about 27 degrees. The fourth planet with a similar axis tilt is the last one out, Neptune. Saturn is 9.5 AU from the sun. It takes 29.5 Earth years to orbit the sun once, which makes one Saturn year. It is 9.5 times the diameter of Earth at 75,000 miles. Because it is a gas giant, what we can see is only the tops of the clouds. The temperatures up there are pretty cold, though, ranging from -292F to -170F, with an average of about -200F.

Given the size of Saturn, we would expect its density to be proportional, but it is not. Its density is so low that it is less than water. The density of water is assigned "1." The density of Saturn is 0.7.

The farther out from the sun we travel, the faster the winds on the planets are. The wind velocities at Saturn's equator are up to 1100 miles per hour. They do weaken some as they get toward the poles. Saturn's atmosphere is composed of 96.3% hydrogen, 3.3% helium, and only 0.4% is composed of methane, ammonia and water.

Like Jupiter, Saturn has colored belts extending across it, and, like Jupiter, they are caused by heated material rising from the core and then cooling and falling back down again. Its internal layers are also similar to Jupiter's.

Saturn's rotation on its axis is a bit of a mystery. First of all, like Jupiter, it is fast -- a little over 10 Earth hours makes up one Saturn day. This fast spin rate has caused Saturn to flatten out a little at the poles. This can be seen a bit in the uppermost photograph on this page. That is all quite understandable. What is a puzzle is that Saturn's rate of spin has been measured as slowing down slightly. In 1980-81, the Voyager 1 and 2 flybys measured a rotation rate of about 10 hours, 39 minutes. However, in 2004, when Cassini started orbiting the planet, the day on Saturn had lengthened to about 10 hours, 45minutes. Michael Desch, a Cassini team member, stated, "We all agree that the radio rotation period of Saturn is longer today than it was in during the Voyager flyby in 1980." But why? Is the rotation electrically controlled? Is there a slippage of the magnetic field? We simply don't know.

The Ring System

Saturn's ring system makes one of the most beautiful objects in the solar system. The rings are split into different parts, including the bright A and B rings and a fainter C ring and D ring. There are also two very faint outside rings, the F and G rings. The E ring is huge, extending beyond the F and G rings deep into space. There are gaps between the rings. The main gap is called the Casini Division; it separates the A and B rings. Space probes have shown that the main rings are really made up of a large number of narrow ringlets. The Encke Division is in the A ring.

The rings are composed of ice and ice covered rocks. They indicate a significant amount of water. The largest particles in the rings are only about 12 yards (36 feet) thick. It is thought some rings may only be one particle thick, or only 12 yards thick. The most abundant particles in the rings are about 4-5 inches across.

The elaborate structures of some of the rings are thought to be caused by the gravitational effects of nearby moons. In the F ring we can see two small moons that are called "shepherd moons," because they interact with the ring material. The outer of these two moons orbits a bit slower than the particles in the ring do, so as they pass it, the moon's slight gravitational tug slows them down a bit. As they slow, they drop into a slightly closer orbit to Saturn. But then they are passed by the faster, inner moon, which speeds them up again and they return to the slightly farther orbit. This interaction keeps the particles in a narrow ring but, at the same time, stops them from clumping together.

The movement of the two shepherd moons, Prometheus and Pandora, can be seen on a Youtube video.

You can see the tug of the inner moon below

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The Cassini Division is also supposed to be the result of the action of one of the moons, Mimas, on the particles in the rings. The Cassini division is 2980 miles across, and is about 73,400 miles from Saturn. Mimas is about 114,000 miles from Saturn and is 246 miles across. That means this little moon is about 40,600 miles away from the Cassini Division.

The rings were presumed to have been smoothly distributed at first. At the present time, however, there are a number of particles/rocks bordering the Cassini Division, and even a few in it, which orbit Saturn at a speed that appears to be relative to the moon Mimas. Some of the other, smaller divisions, are also thought to be the result of Mimas' gravitational pull, forcing particles out of orbit. The orbital periods of some of the particles bordering the rings appear to be specific fractions of Mimas' orbital period. Particles at the inner edge of the Cassini division, for instance, orbit Saturn twice for every time Mimas orbits the planet, which is written as a 1:2 resonance.

The question which must be asked is: can a 246 mile wide body exert enough of a pull over 40,600 miles (especially when in competition with the planet Saturn) to cause a large ring division which is 2980 miles across? To put this in a perspective more readily understood, if Mimas were one inch across, this theory is stating its gravitational tug would exist over 13.75 feet away.

If you look carefully above, you will see the Encke Gap. This gap we understand because the moon Pan circles there, keeping it clear of debris and therefore causing its own small division. Pan is only 17 miles across, and shaped kind of like a walnut. The gap is about 200 miles wide. Below is a photo taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 25, 2013.

Pan is that tiny little white dot in the black gap.

from NASA: The rings show a tremendous amount of structure on all scales; some of this structure is related to gravitational perturbations by Saturn's many moons, but much of it remains unexplained.

When Voyager flew by Saturn, it saw spokes in the B ring. They are composed of fine, dust-size particles and are elevated up to 50 miles above and below the B ring.

We are not sure why the spokes are formed. But we do know a few things about them. They form and disappear in synchrony with the rotating magnetic field of the planet. We also know that electrostatic charges are responsible for levitating the dust particles above and below the ring. These spokes were viewed by astronomers from Earth since the 1800's, but were dismissed as being some kind of illusions produced by light contrasts between Saturn and its rings. When these spokes were then seen by Voyager, it came as a surprise.

Saturn's Moons

...and it has a few -- Saturn has 62 moons with confirmed orbits, 53 of which have names and only 13 of which have diameters larger than 50 kilometers. An excellent page to get information on these moons is presented by NASA. Click on any of the moons for information about it. We strongly suggest doing that -- especially taking the 'virtual tour' of Titan, the largest moon.

A little extra here:

Saturn's moons fall into three groups

1. “Small” moons – irregular chunks of ice, all less than 190 miles wide.  They have many different shapes and sizes.  Many orbit at high angles to Saturn as well as in retrograde orbits.  This indicates they are captured moons, almost certainly from the Kuiper Belt.
2. Six “medium-size” moons, 250-1000 miles across.  These moons are spherical and were probably formed along with Saturn.
3. Titan, the big moon.  It is 3220 miles across, which makes it larger than our moon or Mercury.  It has a dense atmosphere, and is the only moon that does.  Its atmosphere is 1.6 times as dense as Earth’s.

The orbits of the first group of small captured moons look like this:

Both the second group of moons and Titan, however, orbit on the same plane as the rings.

Titan

Again, for a virtual tour of Titan please go to the NASA website linked above. However, here is a little more information:

Its atmosphere: 90% nitrogen, 8% argon, and 2% methane. It is only the haze of this atmosphere that we see through telescopes. This haze is due to reactions powered by UV radiation from the sun. But, like Venus, below the cloud layer (almost 200 miles thick), the atmosphere is clear. The cloud layer ends about 60 miles above the surface. The clouds themselves are composed primarily of methane and ethane, two hydrocarbons. Below the clouds, the atmosphere is nitrogen, some hydrocarbons, carbon monoxide, and some trace amounts of hydrogen.

The surface of Titan has rivers of methane forming lakes, and even sand dunes. A closer look at the sand dunes with good explanations can be found at "Cassini Takes a Closer Look at Titan’s Sand Dunes."

This colorized flyover movie from NASA's Cassini mission takes viewers over the two largest seas on Saturn's moon Titan and nearby lakes. Titan is the only world in our solar system other than Earth that has stable liquid on its surface. The liquid in Titan's lakes and seas is mostly methane and ethane.

Iapetus

 Iapetus is the 17th of Saturn’s 33 known moons and the third largest.  The astronomer Cassini first viewed Iapetus with a telescope. The moon seemed to disappear every forty days, however (for half of its 79 day orbit time). Then Cassini noticed that one side of the moon was much darker than the other side,and this was causing the 'winking' effect.  He also noticed that this moon seemed to be locked into a sychrony with Saturn so that, like our moon with us, the same side of Iapetus faced Saturn at all times. Three hundred ten years later, on 11/14/80, NASA’s Voyager 1 transmitted the first clear images of Iapetus back to Earth.  Cassini was correct.  One side of Iapetus was ‘ten times darker’ than the moon’s other side. 

This is Iapetus as seen by the Cassini Probe. The picture is not distorted; the moon really is squashed a bit.

Another remarkable feature of Iapetus is a ridge that runs about 3/4 of the way around its equator. This ridge is about 10 miles high (our passenger jets only fly a little over 5 miles high). From a distance, it makes this moon look a bit like a walnut!

As can be seen in all the above photographs, Iapetusis heavily cratered. Even its ridge system is heavily cratered, indicating it is very old. There is a very dramatic crater in its southern hemisphere which can be seen in the first photograph. This crater is Engelier, 315 miles wide, and has a prominent central peak.

Iapetus appears to be mostly ice with less than 20% of rocky materials. More information on Iapetus can be found in "Explanations and Interpretations"

Two other of Saturn's moons of note are Mimas and Hyperion. Neither is large, but each has something notable about it. Mimas is about 250 miles across. It has a major impact feature named Herschel. And, just like we see on Mercury, Earth, and Mars, there are fracture marks on the antipodal, or opposite side of it. Hyperion is only 174 miles across, but it is the largest known irregularly-shaped moon in the solar system. The spongy appearance occurs because craters have been deepened by dark material on the crater floors, absorbing sunlight and melting the icy crater floor. Below, Mimas is on the left and Hyperion on the right.

Saturn, like Jupiter, has an enormous plasma sphere:

Like Earth, it has 'weak spots' at its northern and southern magnetic poles. That means it, too, has auroras.