|
|
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".
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. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus and shortly afterwards 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 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.
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 atmosphere could be studied.
Saturn also has 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. The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright .
Though they look continuous from the Earth, the rings are actually composed of many 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 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.
Saturn's
outermost ring, the F-ring, is a complex structure made up of several
smaller rings along which "knots" are visible. Scientists think that
the knots may be 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 took pictures of regions where the rings
are roughly parallel.
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 renewed 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.
Saturn has 18 named satellites, more than any other
planet. There may very well also be several small ones yet to be
discovered.
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
Name Name Radius Radius width approx. approx.
Status inner outer position mass (kg)
---- ------ ------ ------ ----- -------- --------
D-Ring O 60,000 72,600 12,600 (ring)
Guerin Division O 72,600 73,800 1,200 (divide)
C-Ring O 73,800 91,800 18,000 (ring) 1.1e18
Maxwell Division O 91,800 92,300 500 (divide)
B-Ring O 92,300 115,800 23,500 (ring) 2.8e19
Cassini Division O 115,800 120,600 4,800 (divide)
Huygens Gap P 117,200 (n/a) 250-400 (subdiv)
A-Ring O 120,600 136,200 15,600 (ring) 6.2e18
Keeler Division O (n/a) (n/a) 230 25%
Encke Minima S (n/a) (n/a) 5,460 29%-53%
Encke Division O 132,600 (n/a) 325 78%
F-Ring O 141,000 (avg) (n/a) (ring)
G-Ring O 150,000 (avg) (n/a) (ring) 1e7?
E-Ring O 240,000 480,000 240,000 (ring)
Notes:
* distance is kilometers from Saturn's center
* status codes: O: official
P: provisional
S: slang
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.
... Sun ... Jupiter
... Sinope ... Saturn ...
Pan ... Uranus
... 
