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. 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
Saturn was first visited by Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2.
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 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.
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 A ring is known as the Encke Gap.
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
Voyager confirmed the existence of puzzling radial inhomogeneities in the
rings called "spokes" which were
first reported by amateur astronomers. 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 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.
568.46 x (10^24)
Volume (km3) 82,713 x (10^10)
Radius (1 bar level) (km)
Volumetric mean radius (km) 58,232
Mean density (kg/m^3) 687
Gravity (eq., 1 bar) (m/s^2) 8.96
Escape velocity (km/s) 35.5
GM (km^3/s^2) 37.931 x (10^6)
Bond albedo 0.75
Visual geometric albedo 0.47
Visual magnitude V(1,0) -8.88
Solar irradiance (W/m^2) 15.0
Black-body temperature (K) 63.9
Moment of inertia (I/MR^2) 0.210
J2 16,298 x (10^-6)
Semimajor axis (km)
1,427.0 x (10^6)
Sidereal orbit period (days) 10,759.22
Tropical orbit period (days) 10,746.94
Perihelion (km) 1,347.6 x (10^6)
Aphelion (km) 1,506.4 x (10^6)
Synodic period (days) 378.09
Mean orbital velocity (km/s) 9.66
Orbit inclination (deg) 2.489
Orbit eccentricity 0.05565
Sidereal rotation period (hours) 10.500*
Obliquity to orbit (deg) 26.73
* Saturnian System III coordinates
Dipole field strength:
Dipole tilt to rotational axis: < 1 degree
Dipole offset (planet center to dipole center) distance: 0.04 to 0.05 Rs northward
Note: Saturn's magnetic field has significant quadrapole and octapole moments,
making approximation of
the central field as an offset dipole difficult.
Rs denotes Saturnian radii, 60,330 km
Surface Pressure: >>100 bars
Average temperature: ~97 K
Temperature at 1 bar: ~134 K
Density at 1 bar: ~0.19 kg/m3
Up to ~400 m/s (<30 degrees latitude)
Up to ~150 m/s (>30 degrees latitude)
Scale height: 59.5 km
Mean molecular weight: 2.07 g/mole
Major: Molecular hydrogen (H2) - 89%; Helium (He) - 11%
Minor (ppm): Methane (CH4) - ~3000; Ammonia (NH3) - ~200;
Ethane (C2H6) - ~2
Aerosols: Ammonia ice, water ice, ammonia hydrosulfide
Saturn has 18 named satellites, more than any other planet. There may very
well also be several small ones
yet to be discovered.
Of those moons for which rotation rates are known, all but Phoebe and Hyperion
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. In addition to the 18 named satellites, at least a dozen more have been reported and given
|C||74,500||17,500||1.1e^18||0.12 - 0.30||0.05 - 0.35|
|B||92,000||25,500||2.8e^19||0.4 - 0.6||0.4 - .25|
|A||122,200||14,600||6.2e^18||0.4 - 0.6||0.4 - 1.0|
|G||165,800||8000||1e^7||N/A||1.0 x (10^-6)|
|E||180,000||300,000||N/A||N/A||1.5 x (10^-5)|