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
5).
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 NASA's Pioneer
11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA
/ ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four
years.
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. They are prominent in the Cassini
images which also show some as yet unexplained wispy spiral
structures.
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 and S/2005
S1 is in the center of the Keeler 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, perhaps the breakup of larger
satellites. The current set of rings may be only a few hundred million years
old.
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.
Saturn's Satellites
Saturn has 53 named satellites (as of spring 2010):
- 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.
- See Scott Sheppard's site for the latest about recently discovered moons
(there are lots).
- There are 9 more that have been
discovered but as yet not named.
Major moons:
Distance Radius MassSatellite (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 DivisionC-Ring 74,500 92,000 17,500 (ring) 1.1e18Maxwell Division 87,500 88,000 500 (divide)B-Ring 92,000 117,500 25,500 (ring) 2.8e19Cassini 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) 6.2e18Encke Minima 126,430 129,940 3,500 29%-53%Encke Division 133,410 133,740Keeler Gap 136,510 136,550F-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) 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.
