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
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
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
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
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
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 has 30 named satellites plus one discovered in 2003 and as yet unnamed:
Of those moons for which rotation rates are known, all but Phoebe and Hyperion
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.
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
Radius Radius approx. approx.
Name inner outer width position mass (kg)
---- ------ ------ ----- -------- --------
D-Ring 67,000 74,500 7,500 (ring)
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
Bill Arnett; last updated: 2003 Jul 28