Saturn is the sixth planet from the Sun. It is a gas giant, the
second-largest planet in the solar system after Jupiter. Saturn has
a prominent system of rings, consisting of mostly ice particles
with a smaller amount of rocky debris. It was named after the Roman
god Saturn. Its symbol is a stylized representation of the god's
sickle (Unicode: ♄
. The Chinese, Korean,
Japanese, and Vietnamese cultures refer to the planet as the earth
star (土星, based on the Five
Elements
Saturn is an oblate spheroid. It is flattened at the poles and
bulging at the equator; 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 to a lesser degree. Saturn is also the only one of the
Solar System's planets less dense than water, with an average
specific density of 0.69. This is only an average value, however;
Saturn's upper atmosphere is less dense and its core is
considerably more dense than water.
Saturn's interior is similar to Jupiter's, having a rocky core at
the center, a liquid metallic hydrogen layer above that, and a
molecular hydrogen layer above that. Traces of various ices are
also present. Saturn has a very hot interior, reaching 12000 K at
the core, and it radiates more energy into space than it receives
from the Sun. Most of the extra energy is generated by the
Kelvin-Helmholtz mechanism (slow gravitational compression), but
this alone may not be sufficient to explain Saturn's heat
production. An additional proposed mechanism by which Saturn may
generate some of its heat is the "raining out" of droplets of
helium deep in Saturn's interior, the droplets of helium releasing
heat by friction as they fall down through the lighter
hydrogen.
Saturn's temperature emissions, the prominent hot spot at the
bottom of the image is right at Saturn's south poleSaturn's
atmosphere exhibits a banded pattern similar to Jupiter's (in fact,
the nomenclature is the same), but Saturn's bands are much fainter
and they're also much wider near the equator. Saturn's winds are
among the Solar System's fastest; Voyager data indicates peak
easterly winds of 500 m/s (1116 mph)Solarviews. Saturn's finer
cloud patterns were not observed until the Voyager flybys. Since
then, however, Earth-based telescopy has improved to the point
where regular observations can be made.
Saturn's usually-bland atmosphere occasionally exhibits long-lived
ovals and other features common on Jupiter; in 1990 the Hubble
Space Telescope observed an enormous white cloud near Saturn's
equator which was not present during the Voyager encounters and in
1994 another, smaller storm was observed. The 1990 storm was an
example of a Great White Spot, a unique but short-lived Saturnian
phenomenon with a roughly 30-year periodicity. Previous Great White
Spots were observed in 1876, 1903, 1933, and 1960, with the 1933
storm being the most famous. The careful study of these episodes
reveals interesting patterns; if it holds another storm will occur
in ~2020.(Kidger 1992)
Astronomers using infrared imaging have shown that Saturn has a
warm polar vortex, and is the only planet in the solar system known
to do so.
An apparently permanent hexagonal wave pattern around the polar
vortex in the atmosphere at about 78°N was first noted in the
Voyager images[4] [5]. HST imaging of the south polar region
indicates the presence of a jet stream, but no strong polar vortex
nor any hexagonal standing wave
The rings can be viewed using a quite modest modern telescope or
with a good pair of binoculars. They extend from 6,630 km to
120,700 km above Saturn's equator, and are composed of silica rock,
iron oxide, and ice particles ranging in size from specks of dust
to the size of a small automobile. There are two main theories
regarding the origin of Saturn's rings. One theory, originally
proposed by Édouard Roche in the 19th century, is that the rings
were once a moon of Saturn whose orbit decayed until it came close
enough to be ripped apart by tidal forces (see Roche limit). A
variation of this theory is that the moon disintegrated after being
struck by a large comet or asteroid. The second theory is that the
rings were never part of a moon, but are instead left over from the
original nebular material that Saturn formed out of. This theory is
not widely accepted today, since Saturn's rings are thought to be
unstable over periods of millions of years and therefore of
relatively recent origin.
While the largest gaps in the rings, such as the Cassini division
and Encke division, could be seen from Earth, the Voyager
spacecrafts discovered the rings to have an intricate structure of
thousands of thin gaps and ringlets. This structure is thought to
arise from the gravitational pull of Saturn's many moons in several
different ways. Some gaps are cleared out by the passage of tiny
moonlets such as Pan, many more of which may yet be undiscovered,
and some ringlets seem to be maintained by the gravitational
effects of small shepherd satellites such as Prometheus and
Pandora. Other gaps arise from resonances between the orbital
period of particles in the gap and that of a more massive moon
further out; Mimas maintains the Cassini division in this manner.
Still more structure in the rings actually consists of spiral waves
raised by the moons' periodic gravitational perturbations.
Data from the Cassini space probe indicates that the rings of
Saturn possess their own atmosphere, independent of that of the
planet itself. The atmosphere is composed of molecular oxygen gas
(O2) and is thought to be a product of the disintegration of water
ice from the rings into its components, oxygen and hydrogen.
