|
| Cassini on its way! |
The Cassini
mission to explore Saturn and its moon
Titan
was launched successfully on 1997 Oct 15.
It is now on its way to Venus for the first of 4 gravity assists on its way to Saturn.
It will arrive at Saturn on 2004 July 1. |
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 Gap (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 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.
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 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, 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. Mike Harvey's planet finder charts 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.
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
Distance Width Mass
Ring (km) (km) (kg)
---- -------- ----- ------
D 67000 7500 ?
C 74500 17500 1.1e18
B 92000 25500 2.8e19
Cassini division
A 122200 14600 6.2e18
F 140210 500 ?
G 165800 8000 1e7?
E 180000 300000 ?
(distance is from Saturn's center to the ring's inner edge)
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
... 
