Saturn Planet 

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.                       

The atmosphere

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

phenomena.

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-

chemical effects

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.

Moons

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

solar nebula.

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

their fragments.

SATURN’S RINGS

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.