Saturn

The Bringer of Old Age

Saturn Facts

• Saturn is the sixth planet from the Sun and the second largest:
  • distance from Sun: 1,429,400,000 km (9.54 AU)
  • equatorial diameter: 120,536 km; polar diameter: 108,728 km
  • mass: 5.688e26 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 later around Jupiter and Neptune).

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

• Saturn is visibly flattened when viewed through a small telescope (picture 10); its oblateness is almost 10%. This is the result of its rapid rotation and fluid state.

• Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.

• 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.

• The bands so prominent on Jupiter are much fainter on Saturn (picture 2). 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. Saturn also exhibits long-lived ovals and other features common on Jupiter. In 1990, HST observed an enormous white cloud has been observed near Saturn's equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed.

• 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 A ring is known as the Encke Gap (picture 13). 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 thousands of small particles ranging 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 no more than 1.5 kilometers 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 (picture 13). 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 two narrow, braided, bright rings along which "knots" are visible (picture 14). Scientists speculate that the knots may be clumps of ring material, or mini moons.

• 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 Gap. 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.

• Like the other jovian planets, Saturn has a significant magnetic field.

• When it is in the nighttime sky, Saturn is easily visible to the naked 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. Mike Harvey's planet finder charts show the current position of Saturn (and the other planets) in the sky.

Pictures

1. (above) Saturn with Rhea and Dione 162k gif; 34k jpg 298k gif

2. Saturn with Tethys and Dione 85k gif; 14k jpg

3. looking back at Saturn 316k gif

4. Large image of Saturn, B&W 358k gif
5. Planet Saturn, best of the "pocket sized" views 113k gif
6. Planet Saturn, full view of planet and rings 242k gif
7. Planet Saturn, closer full view of planet and rings 236k gif
8. Planet Saturn, with a moon passing over the surface 245k gif
9. Saturn from HST 71k gif; 39k jpg
10. Saturn from Nordic Optical Telescope 43k gif

11. Red Oval Cloud 103k gif; 16k jpg

12. Close up of Saturn, showing cloud formations on the planet. 353k gif

13. Ring spokes (false color) 204k gif

14. Braided Rings 5k jpg; 31k gif

15. Rings of Saturn, closeup of ring spokes 245k gif
16. Rings of Saturn, even closer view of ring spokes 287k gif
17. Spectacular false color image of the rings 130k jpg
18. Ring diagram 101k gif

19. Edge-on rings w/ two moons (HST) 64k gif

20. ... more Saturn images

Movies

1. Red Saturn rotating and two satellites 82k fle
2. Flyby of Saturn by Voyager 2 606k quicktime
3. Storm on Saturn 186k mpg
4. Saturn's Spokes in Rings 990k quicktime; 1300k AVI
5. some of Saturn's moons during the 1995 ring plane crossing 90k mpg

Saturn's Satellites

• Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously.
• 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's 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, at least a dozen more have been reported and given provisional designations.

           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Pan             134      10     ?     Showalter    1990
Atlas           138      14     ?     Terrile      1980
Prometheus      139      46  2.70e17  Collins      1980
Pandora         142      46  2.20e17  Collins      1980
Epimetheus      151      57  5.60e17  Walker       1980
Janus           151      89  2.01e18  Dollfus      1966
Mimas           186     196  3.80e19  Herschel     1789
Enceladus       238     260  8.40e19  Herschel     1789
Tethys          295     530  7.55e20  Cassini      1684
Telesto         295      15     ?     Reitsema     1980
Calypso         295      13     ?     Pascu        1980
Dione           377     560  1.05e21  Cassini      1684
Helene          377      16     ?     Laques       1980
Rhea            527     765  2.49e21  Cassini      1672
Titan          1222    2575  1.35e23  Huygens      1655
Hyperion       1481     143  1.77e19  Bond         1848
Iapetus        3561     730  1.88e21  Cassini      1671
Phoebe        12952     110  4.00e18  Pickering    1898

Saturn's Rings

      Distance   Width   Mass
Ring    (km)      (km)   (kg)
----  --------   -----  ------
 D       67000    7500     ?
 C       74500   17500  1.1e18
 B       92000   25500  2.8e19
Cassini division
 A      122200   14600  6.2e18
 F      140210     500     ?
 G      165800    8000    1e7?
 E      180000  300000     ?

More about Saturn and its Satellites

• from ASU
• from LANL
• from JPL
• from RPIF
• from RGO
• from NSSDC
• from NASA Spacelink
• Saturn Events from TAMU
• Saturn's Ring System
• Saturn Ring Plane Crossings of 1995-1996 includes lots of info about Saturn and its rings
• Voyager Saturn Science Summary
• Saturn Ring Plane Crossing info from JPL, info and images from STScI
• Sun Crossing of Saturn's Ring Plane from WIYN Observatory
• Historical Background of Saturn's Rings and satellites (from JPL)
• Saturnian System Nomenclature Tables

Open Issues

• How does Saturn generate its internal heat?
• What are the "spokes" in the rings?
• What is the origin of the rings? What does that tell us about the origin of the solar system as a whole? Why are Saturn's rings so much more dramatic than the others?
• If all goes well, the Cassini mission will be launched in 1997 and go into orbit around Saturn in 2004. In addition to an extensive survey of Saturn and its major moons, it will drop a probe (called Huygens, built by the European Space Agency) onto the surface of Titan.

Express to Titan