The Bringer of Old Age

   Saturn is the sixth planet from the Sun and the second largest:
        orbit:    1,429,400,000 km (9.54 AU) from Sun
        diameter: 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).

   Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini, now on its way, will arrive in 2004.

   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.

   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. 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 (red spot at center of image at right) 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).

   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 (right) 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.

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

   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

Saturn has 30 named satellites plus one discovered in 2003 and as yet unnamed:
           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

                  Radius   Radius             approx.   approx.
Name               inner    outer     width  position  mass (kg)
----              ------   ------     -----  --------  --------
D-Ring            67,000   74,500     7,500    (ring)
Guerin Division   
C-Ring            74,500   92,000    17,500    (ring)  1.1e18
Maxwell Division  87,500   88,000       500  (divide)
B-Ring            92,000  117,500    25,500    (ring)  2.8e19
Cassini Division 115,800  120,600     4,800  (divide)
Huygens Gap      117,680    (n/a)   285-440  (subdiv)
A-Ring           122,200  136,800    14,600    (ring)  6.2e18
Encke Minima     126,430  129,940     3,500   29%-53%
Encke Division   133,580                325       78%
F-Ring           140,210             30-500   (ring)
G-Ring           165,800  173,800     8,000    (ring)  1e7?
E-Ring           180,000  480,000   300,000    (ring)

  * 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.

More about Saturn and its Satellites

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Bill Arnett; last updated: 2003 Jul 28

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