Saturn is the sixth planet from the Sun and the second largest
In Roman mythology, Saturn is the god of agriculture.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.
ATMOSPHERE
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).
RINGS
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 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 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 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 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.
