Saturn ... The Ringed Planet

Introduction to Saturn

Saturn is considered the most beautiful of the planets, with its graceful rings and soft yellow color. There is really nothing like it, and although the other three gas planets have ring systems, none is as significant as Saturn's. Saturn is gaseous, like Jupiter, but far less massive and less dense. In fact, if it were possible, Saturn would float on an ocean of water. Besides the wonderful rings, Saturn has 62 moons (latest count), one of which (Titan) holds a significantly thick atmosphere. Titan is one of the largest moons in the Solar System and offers some intriguing possibilities for hydrocarbons and potential biomolecules on its cold surface. Saturn has been visited by Pioneer 10 and 11, as well as by Voyager 1 and 2. Presently, the Cassini spacecraft is en route to Saturn for an extended orbital stay and will drop a probe into the atmosphere of Titan in July, 2004.

Planetary Data

Mass (kg), and mass relative to Earth

56.86x10^25 kg = 95.159 earths

Equatorial diameter (km) 120,536

Mean density (gm/cm^3) .69

Acceleration of gravity (m/s^2) 10.59

Velocity of escape (km/s) 35.6

Period of rotation 10.233 hoursPeriod of revolution 29.4577 yearsAphelion (AU) 10.042Aphelion (km) 1,502,300,000Perihelion (AU) 8.9900Perihelion (km) 1,344,900,000Mean orbital distance from the sun (AU)

9.5162

Mean orbital distance from the sun (km)

1,423,600,000

Orbital velocity (km/s)

9.64

Eccentricity .0553Oblateness 0.107Inclination to the ecliptic 2.488 degrees

Inclination of the equator to the orbit 26.73 degrees

Number of natural satellites 62

Names of natural satellites

Pan, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helena, Rhea, Titan, Hyperion, Iapetus, Phoebe, 13 recently discovered moons have recently been named, and 17 unnamed moons, and 13 more announced since 2006.

More Information on the Planet Saturn from the Nine Planets Website

Much of the information below is direct from the Nine Planets Website. Some material has been altered by me for this course, while other items and comments

are directly copied. I hope to maintain a continuous update of this material to keep up with the findings from space satellites and telescopes. Additionally, Saturn will be visited by the space satellite Cassini. This giant space craft was launched in 1997 and arrived in the summer of 2004. Please click on the Cassini website to learn more about its mission, design, and a great wealth of information about Saturn.

Saturn is the sixth planet from the Sun and the second largest: orbit: 1,429,400,000 km (9.54 AU) from Sundiameter: 120,536 km (equatorial)mass: 5.68e26 kg

In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday" (see Appendix 4).

Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).

MISSIONS TO SATURN

Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2 in 1980.

Cassini, now in orbit, arrived July 1, 2004. So much of our understanding of Saturn will be deepened greatly by this Cassini spacecraft. It is the last of the great planetary exploratory satellites, being nearly as large as a school bus. A diagram of this spacecraft is seen to your left. Toward the right side of the spacecraft is the Huygens probe that will release from the Cassini spacecraft and

plunge through the atmosphere of Saturn's largest moon Titan, and perhaps give us an indication of what lies beneath the orange clouds of this moon. The final inspection of Cassini is seen directly below, and an artist's depiction of this probe release is seen below and in the center.

http://www.youtube.com/watch?v=9PM-x7Qm7lg

SATURN'S GENERAL APPEARANCE

Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so. I talk about this in my college course. All of the planets in the solar system are widest at the equatorial regions due to the spinning of the planet. The faster a planet spins,

the more force will be generated outward, and this force expresses itself most strongly at the equator. Additionally, the more solid a planet is, the less likely that outward force will act to bulge the planet. Therefore, planets like Saturn and Jupiter which are made of gas and fluids, AND that also spin very rapidly will have a very pronounced flattening at the polar regions and bulging at the equatorial regions. Planets like Uranus and Neptune that are also made of gas and fluids, BUT that spin more slowly will have less noticeable flattening. Finally, rocky planets like Earth and Venus will be somewhat flattened in spite of their solid nature because the spin more quickly than Mercury and Venus that

spin so slowly as to have an imperceptible equatorial bulging. Oh, click on the image to your left for a larger version of this beautiful image.

SATURN'S PHYSICAL CHARACTERISTICS

Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water. As I mentioned above, this means that Saturn would float on water if such a large ocean could be found.

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

Saturn's interior is hot (12000 K at the core) and Saturn radiates more energy

into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not be sufficient to explain Saturn's luminosity; some additional mechanism may be at work, perhaps the "raining out" of helium deep in Saturn's interior. When I am talking about a planet's "luminosity" I am referring to the radiation of the planet of energy into space. While Saturn does not produce visible light or make gamma radiation as stars do, mechanisms within the planet cause electromagnetic radiation to be released, often in the form of heat or Infrared radiation. Luminosity is a measure of ALL of the wavelength energy emitted by an object. You will learn more about this in the Star Unit. For now, Saturn reflects light from the Sun, but emits heat from interior processes giving Infrared telescopes an opportunity to measure its total energy output, or "luminosity."

SATURN'S CLOUDS

The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the cloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of Saturn's atmospheric circulation could be studied. The image to the left is a "false color" image that allows variations in the bands of the planet to be more readily seen.

Saturn also exhibits long-lived ovals and other features

common on Jupiter. In 1990, HST observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed (left).

Original Caption Released with Image seen to your left: A large, bright and complex convective storm that appeared in Saturn's southern hemisphere in mid-September 2004 was the key in solving a long-standing mystery about the ringed planet.

Saturn's atmosphere and its rings are shown here in a false color composite made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Portions of the atmosphere with a large abundance of methane above the

clouds are red, indicating clouds that are deep in the atmosphere. Grey indicates high clouds, and brown indicates clouds at intermediate altitudes. The rings are bright blue because there is no methane gas between the ring particles and the camera.

The complex feature with arms and secondary extensions just above and to the right of center is called the Dragon Storm. It lies in a region of the southern hemisphere referred to as "storm alley" by imaging scientists because of the high level of storm activity observed there by Cassini in the last year.

The Dragon Storm was a powerful source of radio emissions during July and September of 2004. The radio waves from the storm resemble the short bursts of static generated by lightning on Earth. Cassini detected the bursts only when the storm was rising over the horizon on the night side of the planet as seen from the spacecraft; the bursts stopped when the storm moved into sunlight. This on/off pattern repeated for many Saturn rotations over a period of several weeks, and it was the clock-like repeatability that indicated the storm and the radio bursts are related. Scientists have concluded that the Dragon Storm is a giant thunderstorm whose precipitation generates electricity as it does on Earth. The storm may be deriving its energy from Saturn's deep atmosphere.

One mystery is why the radio bursts start while the Dragon Storm is below the horizon on the night side and end when the storm is on the day side, still in full view of the Cassini spacecraft. A possible explanation is that the lightning source lies to the east of the visible cloud, perhaps because it is deeper where the currents are eastward relative to those at cloud top levels. If this were the case, the lightning source would come up over the night side horizon and would sink down below the day side horizon before the visible cloud. This would explain the timing of the visible storm relative to the radio bursts.

The Dragon Storm is of great interest for another reason. In examining images taken of Saturn's atmosphere over many months, imaging scientists found that the Dragon Storm arose in the same part of Saturn's atmosphere that had earlier produced large bright convective storms. In other words, the Dragon Storm appears to be a long-lived storm deep in the atmosphere that periodically flares up to produce dramatic bright white plumes which subside over time. One earlier sighting, in July 2004, was also associated with strong radio bursts. And another, observed in March 2004 and captured in a movie created from images of the atmosphere (PIA06082 and PIA06083) spawned three little dark oval storms that broke off from the arms of the main storm. Two of these subsequently merged with each other; the current to the north carried the third one off to the west, and Cassini lost track of it. Small dark storms like these generally get stretched out until they merge with the opposing currents to the north and south.

These little storms are the food that sustains the larger atmospheric features, including the larger ovals and the eastward and westward currents. If the little storms come from the giant thunderstorms, then together they form a food chain that harvests the energy of the deep atmosphere and helps maintain the powerful currents.

INTRODUCTORY COMMENTS ABOUT SATURN'S RINGS

Two prominent rings (A and B) and one faint ring (C) can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Division (but this is somewhat of a misnomer since it was very likely never seen by Encke). The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright (albedo 0.2 - 0.6).

Though they look continuous from the Earth, the rings are actually composed of

innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely.

Saturn's rings are extraordinarily thin: though they're 250,000 km or more in diameter they're less than one kilometer thick. Despite their impressive appearance, there's really very little material in the rings -- if the rings were compressed into a single body it would be no more than 100 km across. The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called "spokes" which were first reported by amateur astronomers (left). Their nature remains a mystery, but may have something to do with Saturn's

magnetic field.

Saturn's outermost ring, the F-ring, is a complex structure made up of several smaller rings along which "knots" are

visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images (left) is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component rings are roughly parallel.

There are complex tidal resonances between some of Saturn's moons and the ring system: some of the moons, the so-called "shepherding satellites" (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping the rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is located inside the Encke Division. The whole system is very complex and as yet poorly understood.

The origin of the rings of Saturn (and the other jovian planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable and must be regenerated by ongoing processes, probably the breakup of larger satellites.

SATURN'S MAGNETIC FIELD

Like the other jovian planets, Saturn has a significant magnetic field, and thus it too exhibits auroral displays like Earth and Jupiter. Click on the image to your left for a larger version.

When it is in the nighttime sky, Saturn is easily visible to the unaided eye. Though it is not nearly as bright as Jupiter, it is easy to identify as a planet because it doesn't "twinkle" like the stars do. The rings and the larger satellites are visible with a small astronomical telescope. There are several Web sites that show the current position of Saturn (and the other

planets) in the sky. More detailed and customized charts can be created with a planetarium program such as Starry Night.

Saturn's Satellites/Moons

Saturn has 18 named satellites plus 42 newly announced moons, some of which have names and some which do not: Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously with the planet.The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact gravitationally in such a way as to maintain stable relationships between their orbits: the period of Mimas' orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance. In addition to the 18 named satellites, a 13 more have been reported and given provisional names, and 17 more were found in 2004 but have not yet been named, and 13 more were announced since 2006.

Saturn's Moons

Satellite Name Distance Radius Mass Discoverer DatePan 134,000 km 10 km ? Showalter 1990Atlas 138,000 km 14 km ? Terrile 1980Prometheus 139,000 km 46 km 2.70e17 kg Collins 1980Pandora 142,000 km 46 km 2.20e17 kg Collins 1980Epimetheus 151,000 km 57 km 5.60e17 kg Walker 1980Janus 151,000 km 89 km 2.01e18 kg Dollfus 1966Mimas 186,000 km 196 km 3.80e19 kg Herschel 1789Enceladus 238,000 km 260 km 8.40e19 kg Herschel 1789Tethys 295,000 km 530 km 7.55e20 kg Cassini 1684Telesto 295,000 k m 15 km ? Reitsma 1980Calypso 295,000 km 13 km ? Pascu 1980

Dione 377,000 km 560 km 1.05e21 kg Cassini 1684Helene 377,000 km 16 km ? Laques 1980Rhea 527,000 km 765 km 2.49e21 kg Cassini 1672Titan 1,222,000 km 2575 km 1.35e23 kg Huygens 1655Hyperion 1,481,000 km 143 km 1.77e19 kg Bond 1848Iapetus 3,561,000 km 730 km 1.88e21 kg Cassini 1671Phoebe 12,952,000 km 110 km 4.00e18 kg Pickering 189813 newly found

Here is a list of the recently discovered moons and their new moons

Satellite Name Distance Diameter Year

Kiviuq 11,365,000 km 16 km 2000

Ijiraq 11,440,000 km 12 km 2000

Paaliaq 15,199,000 km 23 km 2000

Skadi 15,647,000 km 3 km 2000

Albiorix 16,040,000 km 32 km 2000

Erriape 17,616,000 km 10 km 2000

Siarnaq 18,160,000 km 40 km 2000

Tarvos 18,247,000 km 15 km 2000

Mundilfari 18,709,000 km 7 km 2000

S/2003 S1 18,719,000 km 7 km 2003

Suttung 19,463,000 km 7 km 2000

Thrym 20,382,000 km 7 km 2000

Ymir 23,096,000 km 18 km 2000

To learn more about these new moons, go to Sam Sheppard's website at Saturn's New Moons.

Saturn's Rings

Distance to Inner Distance to Outer Width Type MassD-Ring 67,000 km 74,500 km 7,500 km ringGuerin DivisionC-Ring 74,500 km 92,000 km 17,500 km ring 1.1e18Maxwell Division 87,500 km 88,000 km 500 km divideB-Ring 92,000 km 117,500 km 25,500 km ring 2.8e19Cassini Division 115,800 km 120,600 km 4,800 km divideHuygens Gap 117,680 km n/a 285-440 km subdivideA-Ring 122,000 km 136,800 km 14,600 km ring 6.2e18Encke Minima 126,430 km 129,940 km 3,500 km 29%-53%Encke Division 133,580 km 325 km 78%F-Ring 140,210 km 30-500 km ringG-Ring 165,800 km 173,800 km 8,000 km ring 1e7?E-Ring 180,000 km 480,000 km 300,000 km ring

Notes:* distance is kilometers from Saturn's center* the "Encke Minima" is a slang term used by amateur astronomers, not an official IAU designation

This categorization is actually somewhat misleading as the density of particles varies in a complex way not indicated by a division into neat regions: there are variations within the rings; the gaps are not entirely empty; the rings are not perfectly circular.

To learn more about Saturn and to see more incredible pictures, go to the JPL webpage devoted to Saturn.

VOYAGER 1 VISITED SATURN AND THEN LEFT THE SOLAR SYSTEM

After Voyager 1 flew by up through the rings of Saturn, it continued in a northerly trajectory relative to the ecliptic, and is continuing to travel out of our solar system. There are two things

which control the future of the Voyager spacecraft ... the amount of nuclear fuel as well as the amount of thruster engine fuel. The spacecraft uses the decay of Plutonium to generate heat, which in turn is converted into electricity, that can be used to send signals to the JPL facility in Pasadena, as well as receive signals from Earth. The thruster fuel is used to point the Voyager receiver/transmitter antenna at the Earth. Scientists believe that there is enough of both fuels to keep Voyager 1 operational until about 2015-2020. It is hoped by that time, that Voyager will have escaped the heliosphere (the bubble or gas blown out by the Sun), cross the heliopause (where the Sun's wind-blown bubble ends and interstellar space begins), and enter interstellar space. The mission has been renamed Voyager Interstellar Mission, and scientists are continuing to maintain contact with both Voyager 1 and 2.

This information in the last paragraph of this page is found in several other places within my course because I believe it to be incredibly important, interesting, and relevant to a potential creative writing assignment of yours :)

Before you move forward to the Saturn quiz, I just wanted to include this absolutely fantastic picture of Saturn taken by the Cassini spacecraft and borrowed from the APOD (Astronomy Picture of the Day) website.

There are three webpages you are asked to look through in the Saturn lesson: Saturn's Moons, Saturn's Rings, and Titan. After working through these three pages, there are 10 questions I would like you to respond to:

1) Give a brief description of the physical appearance of the planet Saturn?

2) What is the most distinguishing feature of the planet Saturn?

3) What is the name of the satellite that is presently orbiting Saturn, when was it launched, how long did it take to arrive at Saturn, and what is the name of the probe that dropped to the surface of Titan?

4) What are the wind speeds of Saturn's clouds?

5) Why is there so much scientific interest in the moon Titan?

6) What are Saturn's rings made of?

7) What is the effect of Mimas on the rings of Saturn?

8) Why is Saturn flattened at the poles in such a pronounced manner?

9) What do astronomers believe the high albedo of Enceladus is caused by?

10) Can you think of a reason why a manned mission to Saturn would be challenging that is different from any other challenge to the other eight planets?

When you believe you know the answers to these questions, please go forward to the Saturn Quiz page and submit them to me.

Once you have completed that reading, then move forward to Uranus, back to the Gas Giant Introduction, or the Syllabus.

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