Saturn is the second largest planet of the solar system in mass and size and the sixth in distance from the Sun. It occu-pies almost 60 percent of Jupiter’s volume but has only about one-third of its mass and the lowest mean density—about 70 percent that of water—of any known object in the solar sys-them. Hypothetically, Saturn would fl oat in an ocean large enough to hold it. Both Saturn and Jupiter resemble stars in that their bulk chemical composition is dominated by hydro-gen. Also, as is the case for Jupiter, the tremendous pressurein Saturn’s deep interior maintains the hydrogen there in a fluid metallic state. Saturn’s structure and evolutionary history, however, diff er signifi cantly from those of its larger counterpart. Like the other giant, or Jovian, planets—Jupiter, Uranus, and Neptune—Saturn has exten-sive systems of moons (natural satellites) and rings, which may provide clues to its origin and evolution as well as to those of the solar system. Saturn’s moon Titan is distinguished from all other moons in the solar system by the presence of a sig-nifi cant atmosphere, one that is denser than that of any of the terrestrial planets except Venus.
Saturn’s name comes from the Roman god of agriculture, who is equated with the Greek deity Cronus, one of the Titans and the father of Zeus (the Roman god Jupiter). As the far-thest of the planets known to ancient observers, Saturn also was noted to be the slowest-moving. At a distance from the Sun that is 9.5 times as far as Earth’s, Saturn takes nearly 30 Earth years to make one solar revolution. The Italian astrono-mer Galileo in 1610 was the fi rst to observe Saturn with a telescope. Although he saw a strangeness in Saturn’s appear-ance, the low resolution of his instrument did not allow him to discern the true nature of the planet’s rings.
The greatest advances in knowledge of Saturn, as well as of most of the other planets, have come from deep-space probes. Four spacecraft have visited the Saturnian system—Pioneer 11 in 1979, Voyagers 1 and 2 in the two years follow-ing, and, after an almost quarter-century hiatus, Cassini-Huygens beginning in 2004. The fi rst three missions were short-term fl ybys, but Cassini went into orbit around Saturn for several years of investi-gations, while its Huygens probe parachuted through the atmosphere of Titan and reached its surface, becoming the fi rst spacecraft to land on a moon other than Earth’s.
BASIC ASTRONOMICAL DATA
Saturn orbits the Sun at a mean distance of 1,427,000,000 km (887 million miles). Its proximity to Earth is never less than about 1.2 billion km (746 million miles), and its phase angle—the angle that it makes with the Sun and Earth—never exceeds about 6°. Saturn seen from the vicinity of Earth thus always appears nearly fully illuminated, a limitation to observation fi nally overcome by the side- lit and backlit views enabled by deep- space probes .
Like Jupiter and most of the other planets, Saturn has a regular orbit—that is, its motion around the Sun is prograde (in the same direction that the Sun rotates) and has a small eccentricity (non- circularity) and inclination to the ecliptic, the plane of Earth’s orbit. Unlike Jupiter,
however, Saturn’s rotational axis is tilted substantially—by 26.7°—to its orbital plane. The tilt gives Saturn seasons, as on Earth, but each season lasts more than seven years. Another result is that Saturn’s rings, which lie in the plane of its equator, are presented to observers on Earth at opening angles ranging from 0° (edge on) to nearly 30°. The view of Saturn’s rings cycles over a 30-year period. Earth- based observers can see the rings’ sunlit northern side for about 15 years, then, in an analogous view, the sunlit southern side for the next 15 years. In the short intervals when Earth crosses the ring plane, the rings are all but invisible.
Saturn has no single rotation period. Cloud motions in its massive upper atmo- sphere trace out a variety of periods, which are as short as about 10 hours 10 minutes near the equator and increase with some oscillation to about 30 minutes longer at latitudes higher than 40°. Scientists have determined the rotation period of Saturn’s deep interior from that of its magnetic field, which is presumed to be rooted in the planet’s metallic-hydrogen outer core. Direct measurement of the field’s rotation is difficult because the field is highly symmetrical around the rotational axis. Radio outbursts from Saturn, which appear related to small irregularities in the magnetic field, show a period of 10 hours 39.4 minutes at the time of the Voyager encounters; this value was taken to be the magnetic field rotation period.
Viewed from Earth, Saturn has an overallhazy yellow-brown appearance. The sur-face that is seen through telescopes andin spacecraft images is actually a com-plex of cloud layers. Like the other giantplanets, Saturn’s atmospheric circulationis dominated by zonal (east-west) flow.This manifests itself as a pattern of lighterand darker cloud bands similar toJupiter’s, although Saturn’s bands are more subtly coloured and are wider nearthe equator. So low in contrast are the fea-tures in the cloud tops that it was notuntil the Voyager flyby encounters thatSaturn’s atmospheric circulation could be studied in any detail.
When defined with respect to therotation of its magnetic field, virtually all of Saturn’s atmospheric flows, or winds,are to the east—in the direction of rota-tion. Measured against the slower magnetic rotation rate observed byCassini, the eastward flows are evenmore pronounced. The equatorial zone at latitudes below 20° shows a particularly active eastward flow havinga maximum velocity close to 500 metres per second (1,800 km [1,100 miles] perhour). This feature is analogous to oneon Jupiter but extends twice as wide inlatitude and moves four times faster. By contrast, the highest winds on Earthoccur in tropical cyclones, where inextreme cases sustained velocities may exceed 67 metres per second (240 km[150 miles] per hour).
The zonal flows are remarkably sym-metrical about Saturn’s equator; that is,each one at a given northern latitude usu-ally has a counterpart at a similar southernlatitude. Strong eastward flows—thosehaving eastward relative velocities in excess of 100 metres per second (360 km[225 miles] per hour)—are seen at 46° Nand S and at about 60° N and S. Westwardflows, which are nearly stationary in the magnetic field’s frame of reference, areseen at 40°, 55°, and 70° N and S. After theVoyager encounters, improvements inobservations of Saturn’s clouds at dis-Earth-based instrumentation allowedtance. Made over many years, these
tended to agree with the detailed Voyager
observations of the zonal flows and thus
corroborated their stability over time.
Some high-resolution observations of
Saturn’s atmosphere showed a large drop
in the velocity of the equatorial jet from
1996 to 2002. Analysis of data from the
Cassini orbiter, however, suggests that
any such velocity drop is confined to
superficial layers of the atmosphere.
The general north-south symmetry
suggests that the zonal flows may be connected in some fashion deep within
the interior. Theoretical modeling of a
deep-convecting fluid planet such as
Saturn indicates that differential rotation
tends to occur along cylinders aligned
about the planet’s mean rotation axis.
Saturn’s atmosphere thus may be built of
a series of coaxial cylinders aligned north-
south, each rotating at a unique rate,
which give rise to the zonal jets seen at
the surface. The continuity of the cylin-
ders may be broken at a point where they
intersect a major discontinuity within
Saturn, such as the core.
Saturn’s atmosphere shows many
smaller-scale features similar to those
found in Jupiter’s, such as red, brown,
and white spots, bands, eddies, and vorti-
ces, that vary over a fairly short time.
However, in addition to having a much
blander appearance, Saturn’s atmosphere
is less active than Jupiter’s on a small
scale. A spectacular exception occurred
during September–November 1990, when
a large, light-coloured storm system
appeared near the equator, expanded to a
size exceeding 20,000 km (12,400 miles),
and eventually spread around the equa-
tor before fading. Storms similar in
impressiveness to this “Great White
Spot” (so named in analogy with Jupiter’s
Great Red Spot) have been observed at
about 30-year intervals dating back to the
late 19th century. This is close to Saturn’s
orbital period of 29.4 years, which sug-
gests that these storms are seasonal
Initial analysis of data from the
Voyager spacecraft indicated that the
planet’s atmosphere is 91 percent molec-
ular hydrogen by mass and is thus the
most hydrogen-rich atmosphere in the
solar system. Helium, which is measured
indirectly, makes up another 6 percent
and is less abundant relative to hydrogen
compared with a gas having the composi-
tion of the Sun. If hydrogen, helium, and
other elements were present in the same
proportions as in the Sun’s atmosphere,
Saturn’s atmosphere would be about 71
percent hydrogen and 28 percent helium
by mass. According to some models,
helium may have settled out of Saturn’s
outer layers, but more-recent research
has suggested that the Voyager analysis
underestimated the helium fraction in
Saturn’s atmosphere, which may lie closer
to the value in the Sun.
Other major molecules observed in
Saturn’s atmosphere are methane and
ammonia, which are two to five times
more abundant relative to hydrogen than
in a gas of solar composition. Hydrogen
sulfide and water are suspected to be
major constituents of the deeper atmo-
sphere but have not yet been detected.
Minor molecules that have been detected
spectroscopically from Earth include
phosphine, carbon monoxide, and ger-
mane. Such molecules would not be
present in detectable amounts in a hydro-
gen-rich atmosphere in chemical
equilibrium. They may be nonequilib-
rium products of reactions at high
pressure and temperature in Saturn’s
deep atmosphere well below the observ-
able clouds. A number of nonequilibrium
hydrocarbons are observed in Saturn’s stratosphere: acetylene, ethane, and, pos-
sibly, propane and methyl acetylene. All
of the latter may be produced by photo-
Astronomers on Earth have analyzed
the refraction (bending) of starlight and
radio waves from spacecraft passing
through Saturn’s atmosphere to gain
information on atmospheric temperature
over depths corresponding to pressures
of one-millionth of a bar to 1.3 bars. At
pressures below 1 millibar, the tempera-
ture is roughly constant at about 140 to
150 K (−208 to −190 °F, −133 to −123 °C). A
stratosphere, where temperatures steadily
decline with increasing pressure, extends
downward from 1 to 60 millibars, at which
level the coldest temperature in Saturn’s
atmosphere, 82 K (−312 °F, −191 °C), is
reached. At higher pressures (deeper lev-
els) the temperature increases once again.
This region is analogous to the lowest
layer of Earth’s atmosphere, the tropo-
sphere, in which the increase of
temperature with pressure follows the
thermodynamic relation for compression
of a gas without gain or loss of heat. The
temperature is 135 K (−217 °F, −138 °C) at a
pressure of 1 bar, and it continues to
increase at higher pressures.
Saturn possesses more than 60 knownmoons. (A table summarizing data forSaturn’s moons appears in Appendix B,“Moons of Saturn.”) Of the first 18 dis-covered, all but the much more distantmoon Phoebe orbit within about 3.6 mil-lion km (2.2 million miles) of Saturn.
Nine are more than 100 km (60 miles) inradius and were discovered telescopi-cally before the 20th century; the others were found in an analysis of Voyagerimages in the early 1980s. Several addi-
tional inner moons—tiny bodies with
radii of 3–4 km (1.9–2.5 miles)—were dis-
covered in Cassini spacecraft images
beginning in 2004. All of the inner
moons are regular, having prograde,
low-inclination, and low-eccentricity
orbits with respect to the planet. The
eight largest are thought to have formed
along Saturn’s equatorial plane from a
protoplanetary disk of material, in much
the same way as the planets formed
around the Sun from the primordial
A second, outer group of moons lies
beyond about 11 million km (6.8 million
miles). They are irregular in that all of
their orbits have large eccentricities and
inclinations; about two-thirds revolve
they move opposite to the planet’s rotation. Except for Phoebe, they are less
around Saturn in a retrograde fashion—
than about 20 km (12 miles) in radius.
Some were discovered from Earth begin-
ning in 2000 as the result of efforts to
apply new electronic detection methods
to the search for fainter—and hence
smaller—objects in the solar system; oth-
ers were found by Cassini. These outer
bodies appear to be not primordial
moons but rather captured objects or
SATURN IS ENCIRCLED BY THE MOST SPECTACULAR RING SYSTEM IN THE SOLAR SYSTEM. THE BRIGHT PLATTERS VISIBLE FROM EARTH CONSIST ALMOST ENTIRELY OF ICE FRAGMENTS THAT WHIRL AROUND THE PLANET IN CONCENTRIC RINGLETS.
Saturn’s rings contain billions of pieces of ice, varying from house-sized boulders to minute crystals. Jostling together, these particles are constrained by the planet’s gravity to orbit in a flat plane above Saturn’s equator. The system is complex, with each large ring being made up of many narrow ringlets. Several distinct gaps between the rings are created by the gravitational pull of Saturn’s more distant moons and the clumping together of material within the rings themselves. The particles consist predominantly of water ice, which makes them naturally reflective. Although their surfaces become dust-coated over time, constant collisions within the rings cause them to fracture, exposing bright new facets. The origin of the rings is something of a mystery. They may be the remains of a small, icy moon that was either torn apart by Saturn’s powerful gravity or destroyed in a collision with another body.
DESTINATION SATURN’S RINGS
THE B RING IS THE LARGEST, BRIGHTEST, AND MOST DENSELY PACKED OF SATURN’S RINGS. HERE, GIANT BOULDERS OF SPARKLING ICE FLOAT ALONGSIDE ONE ANOTHER IN A SEEMINGLY IMPOSSIBLE ORBITAL BALLET. THE DENSE DISK OF DEBRIS SPELLS DOOM FOR ANYTHING THAT ATTEMPTS TO CROSS ITS PATH.
While the plane of particles orbiting Saturn extends to many times the planet’s own diameter, and contains trillions of objects, the particles’ individual paths are remarkably uniform—each follows a near-perfect circular orbit in a plane directly above Saturn’s equator. Objects straying into more elliptical orbits or attempting to cross the plane soon collide with their neighbors and are nudged back into more orderly paths. Fragments produced by recent collisions are everywhere, gleaming brightly in the sunlight as they slowly attempt to reassemble under their own gravitational attraction.
MISSIONS TO SATURN
SATURN AND ITS MOONS HAVE BEEN VISITED BY SEVERAL SPACECRAFT SINCE THE 1970S. THE FIRST MISSIONS WERE FLYBYS, BUT MORE RECENTLY A DECADE-LONG INVESTIGATION WAS UNDERTAKEN BY NASA’S CASSINI ORBITER.
Saturn was a key destination for the Pioneer missions that paved the way for the exploration of the outer solar system. While Pioneer 10 merely flew past Jupiter, Pioneer 11 used a gravitational slingshot from the giant planet to propel itself to Saturn in September 1979. The twin Voyager probes arrived in November 1980 and August 1981 and gave the first detailed views of Saturn’s intriguing family of moons. Saturn was not revisited until 2004, when Cassini (and its companion, the Huygens Titan probe) became the first craft to orbit the ringed planet.