Saturn is the sixth planet from the Sun and the second
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,
was the son of Uranus and Gaia and the father of Zeus (Jupiter).
Saturn is the root of the English word "Saturday" (see
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
The bands so prominent on Jupiter are
much fainter on Saturn.
They are also much wider near the equator.
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
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
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 roughly parallel.
There are complex tidal resonance's 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 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 has a significant magnetic field.
When it is in the night time 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 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' 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 perfectly circular.
- 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
will go into orbit around Saturn on July 1st, 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
Express to Titan
... Sinope ... Saturn
; last updated:
08/21/14 07:53:32 PM
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