Saturn

Saturn (IPA: [ˈsæɾɚn], [ˈsætən]) is the sixth planet from the Sun. It is a gas giant (also known as a Jovian planet, after the planet Jupiter), the second-largest planet in the solar system after Jupiter. Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. It was named after the Roman god Saturn (the Greek equivalent is Cronos, father of Zeus). Its symbol is a stylized representation of the god's sickle (Unicode: ♄).

Physical characteristics

Saturn is an oblate spheroid, that is, it is flattened at the poles and bulges 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 the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably more dense than water, the average specific density of the planet is 0.69 due to the gaseous atmosphere.


Saturn's temperature emissions. The prominent hot spot at the bottom of the image is at Saturn's south pole.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 12,000 kelvins (11,700 °C) 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 atmosphere exhibits a banded pattern similar to Jupiter's (in fact, the nomenclature is the same), but Saturn's bands are much fainter and are 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). 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. If the periodicity is maintained, another storm will occur in about 2020.

In recent images from the Cassini spacecraft, Saturn's northern hemisphere appears a bright blue, similar to Uranus, as can be seen in the image below. This blue color cannot currently be observed from Earth, because Saturn's rings are currently blocking its northern hemisphere. The color is most likely caused by Rayleigh scattering.


Visual comparison of Saturn and EarthAstronomers 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. HST imaging of the south polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave. However, NASA reported in November of 2006 that the Cassini spacecraft observed a 'hurricane-like' storm locked to the south pole that had a clearly defined eyewall. This observation is particularly notable because eyewall clouds have not been seen on any planet other than Earth (including a failure to observe an eyewall in the Great Red Spot of Jupiter by the Galileo spacecraft).

Rotational behavior

Since Saturn does not rotate on its axis at a uniform rate, multiple rotation periods have been assigned to it (as in Jupiter's case): System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.

While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased slightly, to approximately 10 h 45 m 45 s (± 36 s). The cause of the change is unknown — however, it is thought that this is due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than an actual change in Saturn's rotation.

History

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he was unable to identify them as such. He wrote to the Duke of Tuscany that "The planet Saturn is not alone, but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one (Saturn itself) is about three times the size of the lateral ones ." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo.

In 1655, Christiaan Huygens became the first person to suggest that Saturn was surrounded by a ring. Using a telescope that was far superior to those available to Galileo, Huygens observed Saturn and wrote that "It is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic."

In 1675, Giovanni Domenico Cassini determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.

In 1859, James Clerk Maxwell demonstrated that the rings could not be solid or they would become unstable and break apart. He proposed that the rings must be composed of numerous small particles, all independently orbiting Saturn. Maxwell's theory was proven correct in 1895 through spectroscopic studies of the rings carried out by James Keeler of Lick Observatory.

Physical characteristics

The rings can be viewed using a quite modest modern telescope or with good binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, average close to one kilometer in thickness 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, can 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 discovered, 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) produced when ultraviolet light from the Sun disintegrates water ice in the rings. Chemical reactions between water molecule fragments and further ultraviolet stimulation create and eject, among other things O2. According to models of this atmosphere, H2 is also present. The O2 and H2 atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be on the order of one atom thick. The rings also have a similarly sparse OH (hydroxide) atmosphere. Like the O2, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic ions that bombard water molecules ejected by Saturn's moon Enceladus. This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope.

Saturn shows complex patterns in its brightness. Most of the variability is due to the changing aspect of the rings, and this goes through two cycles every orbit. However, superimposed on this is variability due to the eccentricity of the planet’s orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern. (Henshaw, C., 2003).

In 1980 Voyager I made a fly-by of Saturn that showed the F-ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.

Dark Rings

In 1974, NASA's Pioneer 11 spacecraft plunged through the rings of Jupiter.

And no one noticed.

Jupiter's dark rings--as wide as Saturn's yet nearly invisible--hadn't been discovered yet. Indeed, it wasn't until five years later that cameras onboard Voyager 1 caught sight of them for the first time. On Mar. 5, 1979, the spacecraft swung behind Jupiter, and from inside the planet's shadow the faintly sunlit rings were visible--but just barely.

Right: An artist's concept of Galileo, Amalthea, and Jupiter's rings.

Ever since, researchers have wished for another flyby like Pioneer 11's. NASA's Voyager, Cassini and Galileo spacecraft have photographed the rings many times, but always from a distance. No probe had actually entered the rings for 28 years.



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Until this week.

On Nov. 5, 2002, Galileo took the plunge and flew through Jupiter's rings again. And this time scientists were ready.

"We've been looking forward to this flyby for a while," says Joe Burns, a planetary scientist at Cornell University and a member of the Galileo imaging team. "It's an opportunity to study the particles that make up these rings and to learn about their environment."

Galileo is nearing the end of its twice-extended 7-year mission to Jupiter. Risky maneuvers like flying through Io's volcanoes and Jupiter's rings were saved for last. This week's ring encounter and close-approach to Jupiter is one of the final things Galileo will do before it plunges into Jupiter itself next year.

Unlike Saturn's rings, which are made of bright, icy chunks as large as houses, Jupiter's rings consist of fine dust akin to the particles in cigarette smoke. The dust grains are dark (they reflect barely 5% of the sunlight that hits them) and they are spread so thin that the rings are almost transparent. This is what makes the rings so hard to study.

The origin of Jupiter's rings was revealed by Galileo's cameras more than five years ago. "The dust comes from small rocky moons orbiting Jupiter," says Burns. These moons are constantly pelted by meteoroids, which burrow into the ground and explode. Jupiter's rings are the debris from those impacts.

In fact, Jupiter has several rings: The main ring is the brightest. It's close to Jupiter and made of dust from the satellites Adrastea and Metis. Two wide gossamer rings encircle the main ring. These come from the satellites Thebe and Amalthea. There is also an extremely tenuous and distant outer ring that circles Jupiter backwards. No one is certain, but that ring might be made of captured interplanetary dust.

When Galileo approached Jupiter last Tuesday, it passed through one of the gossamer rings. The spacecraft's close approach to Amalthea on the same day was much-anticipated by scientists who will figure out the mass of that moon from its gravitational tug on Galileo.

Right: A photo of Jupiter's moon Amalthea, which is about as wide as Long Island.

Saturn's rings probably formed from the total breakup of an icy moon about the size of Amalthea (100 km wide). Jupiter's rings, on the other hand, are merely dust from the surface of such moons. "Saturn's rings are millions of times more massive than Jupiter's," notes Burns.

Meteoroids have been striking Jupiter's moons and kicking up dust for billions of years. So why isn't there more "stuff" in Jupiter's rings? Why are Jupiter's rings so much less massive than Saturn's?

Burns explains: "Dust grains ejected into Jupiter's rings don't stay in the rings forever. The grains spiral in toward Jupiter and eventually disappear."

They lose orbital energy for several reasons: Sunlight is one. Dust grains absorb and re-emit sunlight, losing momentum in the process. Scientists call this "Poynting-Robertson drag."

Plasma collisions are another reason. Jupiter's magnetosphere (a magnetic bubble that surrounds the planet) is filled with electrified clouds called plasmas. The dust grains in the rings are themselves charged--like the static-charged dust that accumulates on your computer screen. When charged grains collide with plasma clouds, the grains can lose orbital momentum.

Below: The life of a dust grain in Jupiter's rings. It begins as debris ejected from a rocky satellite and ends by spiraling in toward Jupiter. [more]

The "age" of Jupiter's rings depends on which of these mechanisms dominates. Plasma collisions might de-orbit ring particles in only a few years. Poynting-Robertson drag, which Burns favors, takes longer, perhaps 100,000 years. (The age of Saturn's rings is likewise controversial. Read Science@NASA's "The Real Lord of the Rings" for more information.)

Jupiter's rings are constantly replenished by meteoroid impacts, so they won't disappear any time soon. Next year's rings, however, might be made of different "stuff" than this year's. In that sense, Jupiter's rings might be younger than you are.

When Galileo flew through the rings this week, the spacecraft's suite of electromagnetic sensors and its Dust Detector were working well. (The spacecraft itself, bombarded by radiation from Jupiter, went into safe mode near the end of the ring encounter, but not before data had been collected.) Burns hopes the unprecedented in situ measurements will finally solve the puzzle.

Or they might reveal more surprises. Jupiter's dark rings remain, after all, unexplored territory.

Spokes of the rings

Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics. The spokes appear dark against the lit side of the rings, and light when seen against the unlit side. It is assumed that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism generating the spokes is still unknown.


Spokes imaged by Cassini in 2005.Twenty-five years later, Cassini observed the spokes again. They appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and the spokes reappeared in images taken September 5, 2005.