In 1610 Galileo observed Saturn's rings as strange hoops seemingly attached to the edges of the planet. His telescope was not sufficiently developed to allow him to see that the rings were continuous (it only magnified by a factor of 20), and, as a result, they made no physical sense to him. He speculated that Saturn had twin moons orbiting it very closely. Much to his surprise, in 1612 the hoops had vanished: Saturn was aligned so that the edge of the ring plane pointed exactly at the Earth, and due to their extreme thinness the rings effectively disappeared. He wrote: "I do not know what to say in a case so surprising." In 1616 the rings were back, looking more like elliptical hoops than ever. Galileo knew a planet could not have handles like a cup, since planetary spin requires that the planet be largely symmetric around it axis of rotation.

In 1655 Christiaan Huygens, an astronomer from a prominent Dutch diplomatic family, saw the rings through the 50-power telescope that he had designed himself and surmised what they are: a thin flat ring around the planet, nowhere touching the planet itself. Huygens was able to have instruments and make connections unavailable to other scientists of his time from less prominent families. Huygens was tutored in mathematics by René Descartes, one of the greatest mathematicians and philosophers (he invented Cartesian geometry and made the famous statement "I think, therefore I am"). Along with the mathematical luminaries Pierre de Fermat and Blaise Pascal, Christiaan Huygens corresponded regularly with Marin Mer senne, the mathematician who effectively created the theory of prime numbers that is used today. Other scientists speculated in more wild ways about the rings, but Huygens quietly continued to study the planet. Though he had discovered the rings in 1655, he had been involved with publications concerning his invention of the pendulum clock and was not prepared to write a treatise on Saturn at that moment. In the strangely beautiful convention of the day, Huygens published his discovery as an anagram, which though virtually undecipherable at the time, would still prove the primacy of his discovery should someone else see the rings before he could publish properly. The anagram Huygens published was


aaaaaaa,ccccc,d,eeeee,g,h,iiiiiii,IIII,mm,nnnnnnnnn,oooo,pp,q, rr,s,ttttt,uuuuu

Unscrambled, it forms, "Annulo cingitur, tenui, plano, nuspua cohaerente, ad ecliptican inclinato," or in English: "He is surrounded by a thin flat ring, which does not touch him anywhere and is inclined to the ecliptic."

In 1659 Huygens finally published his book Systema Saturnium, containing his ring observations and theories that every 14 or 15 years the Earth passes through the plane of the rings, making them invisible from the Earth's vantage point. Huygens's detailed descriptions and explanations for ring observations gradually won over the

Jupiter Images Rings

When Saturn's rings are seen from the unlit side, the dense rings appear black because they block the light, while thin rings allow light to pass through. (NASA/JPL/Voyager)

scientists of the day; his preeminence in lens grinding had allowed him to precede his contemporaries because of better instruments. Still, in 1659 almost all scientists believed that Saturn's rings were solid. At one time, only Giovanni Cassini and Jean Chapelain (a minor poet oddly aligned with the French Académie des Sciences) were clearly in favor of a model in which the rings consisted of multitudes of tiny satellites.

In 1656 Cassini first saw a gap in the rings, now named Cassini's Division, forming the boundary between the A and B rings. In 1850 the C ring was found. In 1825 Henry Kater, an English physicist, reported seeing three gaps in the A ring, but for decades no one else could resolve these features. Finally, in 1837 Johann Franz Encke, a German astronomer who was trained by the Karl Friedrich Gauss, observed a dark band in the A ring that corresponded to one of Kater's gaps, and though Encke did not resolve this band as a gap, it came to be known as Encke's Division. Encke's Division was not clearly seen in a telescope until 1888, when James Keeler, an American astronomer working at the Lick Observatory, obtained a good view. In 1850 William and George Bond, father-and-son American astronomers who developed the technique of photography through a telescope, observed a narrow dark ring interior to the B ring. This new dark ring was originally named the Crepe Ring but became known as the C ring.

Cassini's Division can be seen clearly in the unusual image, shown above, of Saturn's rings taken from the unlit side. When seen with light shining on their far side, relatively opaque areas like the B ring turn black, while lightly populated zones, such as the C ring and the Cassini Division, allow diffuse sunlight to pass through and so appear

When Saturn's rings are seen from the unlit side, the dense rings appear black because they block the light, while thin rings allow light to pass through. (NASA/JPL/Voyager)

bright. The A ring, with intermediate opacity, is at an intermediate level of brightness.

George Bond concluded that three solid rings could not be stable in orbit around a planet, and decided the rings must be fluid. Other observers at about the same time were able to see Saturn through the C ring, certainly supporting the idea that the ring could not be solid. In 1849 Edouard Roche, a French physicist, had proposed that a satellite had approached Saturn too closely and had been torn apart by tidal forces, forming the rings from fluid droplets of this destroyed satellite (for more, see the sidebar "Why Are There Rings?" on page 128). Roche noted that the radius around Saturn interior at which large satellites would be torn apart by tidal forces was just external to the rings themselves. Roche's excellent calculations, which stand today, did not convince the majority of his contemporaries. Two centuries were required for the basic structure of the rings to be deciphered.

The solid ring theory prevailed, as sometimes an entrenched theory in science does when it is defended by established scientists in the face of newer researchers. In the mid-18th century, Pierre-Simon de Laplace, an exceptional French mathematician who specialized in celestial mechanics, and William Herschel, the German-born English discoverer of Uranus, both wrote that they believed Saturn was circled by two solid planar rings, continuing the majority view from the previous century. Finally, in 1859, James Clerk Maxwell, an eminent English mathematical physicist whose achievements are perhaps second only to Newton's, stated that Saturn's rings must consist of an "indefinite number of unconnected particles," definitively ushering in the now-proven view of ring structure. Maxwell's theory was proven in 1895, when James Keeler and William Campbell, then Keeler's assistant but later director of the Lick Observatory himself, were able to observe that the inner parts of the rings orbit more rapidly than the outer parts, demonstrating that they could not be solid.

The D and E rings were not seen until the 1960s, well into the era of giant institutional observatories. Spectra taken from Earth in 1970 showed the rings to consist mainly of water ice, a result that came even before Pioneer was launched in 1973.Thus, the A, B, C, D, E, and F rings were all discovered from ground-based observations, and all subsequent major ring discoveries to date have been made by space missions. Saturn's gorgeous outer B and inner C rings can be seen in the lower color insert on page C-6.

When Voyager 1 reached Saturn in 1980, its images revealed the tiny F ring that had been proposed in 1979 by Peter Goldreich, now a professor at the California Institute of Technology, and Scott Tremaine, now a professor at Princeton University, when they theorized that two moons could shepherd such a thin ring.The Voyager missions also revealed the existence of the G ring, as well as spokes in the B ring, braiding in the F ring, and the new ring gaps named Maxwell, Huygens, and Keeler.

Between 1996 and 2000, Saturn moved relative to the Earth such that astronomers could see the rings first edge-on, and then gradually more and more fully from their bottoms, as seen in the images below from the Hubble Space Telescope, captured as Saturn moved from autumn toward winter in its northern hemisphere. Saturn's obliquity, 27 degrees, is similar to the Earth's 23-degree tilt.The first image in this sequence, on the lower left, was taken soon after the autumnal equinox in Saturn's northern hemisphere. By the final image in the sequence, on the upper right, the tilt is nearing its extreme, or winter solstice in the northern hemisphere. Ring crossings are particularly fortuitous times to search for new satellites, since the view of the space around Saturn is obscured neither by the ring material itself nor by the brightness of reflected light.

The Hubble Space Telescope took a series of images showing Saturn's changing seasons. Between 1996 and 2000, Saturn moved relative to the Earth such that astronomers could see the rings first edge-on, and then gradually more and more fully from beneath. (NASA and The Hubble Heritage Team [STScI/AURA], R G. French [Wellesley College], J. Cuzzi [NASA/Ames], L. Dones [SwRI], and J. Lissauer [NASA/Ames])

Why Are There Rings?

^Galileo Galilei first saw Saturn's rings in 1610, though he thought of them as handles on the sides of the planet rather than planet-encircling rings. After this first half observation, there was a hiatus in the discovery of planetary ring systems that lasted for three and a half centuries, until 1977, when Jim Elliot, an astronomer at the Massachusetts Institute of Technology and the Lowell Observatory, saw the blinking of a star's light as Uranus passed in front of it and correctly theorized that Uranus had rings around it that were blocking the light of the star. Two years later, Voyager 1 took pictures of Jupiter's rings, and then in 1984, Earth-based observations found partial rings around Neptune. Now it is even hypothesized that Mars may have very tenuous rings, with an optical depth of more than 10-8 (meaning that almost all the light that shines on the ring goes straight through, without being scattered or reflected).

There are two basic categories of planetary rings. The first involves rings that are dense enough that only a small percentage of the light that shines on them passes through. These dense rings are made of large particles with radii that range from centimeters to meters. Examples of these dense rings are Saturn's main rings A and B and the Uranian rings. The second involves tenuous rings of particles the size of fine dust, just microns across. In these faint rings the particles are far apart, and almost all the light that strikes the ring passes through. Jupiter and Saturn's outermost rings are of this faint type. Neptune's rings, however, do not fall into either neat category.

In dense rings the constant collisions between particles act to spread out the rings. Particles near the planet lose speed when they collide with other particles, and thus fall closer to the planet. Particles at the outer edges of the ring tend to gain speed when they collide, and so move farther from the planet. In a complex way, the changes in velocity and redistributions of angular momentum act to make the ring thinner and thinner in depth while becoming more and more broadly spread from the planet.

Most dense rings exist within a certain distance from their planet, a distance called the Roche limit. Within the Roche limit, the tidal stresses from the planet's gravity overcome the tendency for particles to accrete into bodies: The gravitational stresses are stronger than the object's self-gravity, and the object is pulled to pieces. The Roche limit differs for every planet, since their gravities vary, and it also differs for each orbiting object, since their densities differ. The Roche limit (R ) can be calculated as follows:

* Psatellite '

where p , is the density of the planet, p ,, is the density of the object orbiting the

' planet J ' 'satellite planet, and R is the radius of the planet. Thus, moons that attempted to form within the Roche limit, or were thrown within the Roche limit by other forces, will be torn into rubble by the gravitational forces of the planet and form rings.

All the Uranian rings lie within the Roche limit, but Saturn's, Jupiter's, and Neptune's outer rings lie outside their Roche limits and orbit in the same regions as moons. The moons have important effects on the rings near which they orbit. First, if the moon and particles in the ring share an orbital resonance (when the ratio of the orbital times is an integer ratio, for example, the moon orbits once for every two times the particle orbits), they interact gravitationally in a strong way: The particle, moon, and planet line up regularly, exerting strong forces on the particle and warping its orbit. If the resonances are strong (low integer ratios), a gap can be created in the ring. In other cases the resonance results in a wavy ring.

Moons can also strongly affect ring particles that they orbit next to. Very thin rings that would otherwise be expected to widen with time can be kept thin by moons that orbit closely on either side. These are called shepherd moons, bumping any stray particles back into the rings or accreting them onto the moon's surface.

Intact moons outside the Roche limit may also shed material, forming the source of a ring. Small moons, with low gravity, may allow more material to escape than large moons do. Jupiter's small moons Adrastea, Metis, Amalthea, and Thebe are all thought to create their own rings. In the Saturnian system, by contrast, the large, 250-km radius moon Enceladus is thought to be the creator and sustainer of Saturn's E ring.

Once formed, the ring does not remain forever: Forces from radiation, meteoroid impacts, and drag from the outer parts of the planet's atmosphere (the exosphere) begin to erode the ring. It is estimated that even dense rings can only exist for a few hundreds of thousands or millions of years, and so even the gorgeous rings of Saturn are probably just a fleeting phenomenon in the age of the solar system. Faint rings may disappear within thousands of years, unless replenished by a moon.

A system of rings around a planet can be thought of as a miniature reenactment of the original solar nebula: The planet is the giant mass at the center of a rotating system, much as the early Sun was, and the rings are the material rotating around it. Material is taken from moons to make rings, and other material is swept up by moons, a sort of recycling between moons and rings. This may be the reason that the outer planets have rings and the inner planets do not; the outer planets have great inventories of moons to create and sweep up rings, while the inner planets do not have enough moons.

Saturn's bright rings are less than three miles (2 km) thick but 150,000 miles (250,000 km) in radius, equivalent to about two-thirds the distance from the Earth to the Moon. The thinnest rings are just 30 feet (10 m) thick.The seeming static calmness of the rings is simply a misleading consequence of viewing the rings from such an extreme distance.Though the large C, B, and A rings look continuous in most images, each ring is made up of dozens or hundreds of ringlets. Within each ring each particle collides with another once every few hours, probably occasionally accreting into short-lived fluffy clumps that are invisibly small from Earth.

Saturn's dark C and immense B rings are shown in the upper color insert on page C-8.The major rings, those most visible and massive, are the C, B, and A rings, moving radially out from Saturn.The B and A rings are separated by Cassini's Division.Though the rings are bright, they are not massive:The total mass of the C, B, and A rings is less than the total mass of the small moon Mimas, which has a radius of about 130 miles (209 km).The rings are bright because about 90 percent of the ring matter consists of pristine particles of water ice. The rings have some variation in albedo, implying that their composition also varies slightly. The main rings appear to consist of particles from about 0.4 inches (1 cm) to 16 feet (5 m) in diameter.The particle sizes for all the rings are inferred from their reflectivity and thermal emissions, since to date no single particle of the rings has been seen on its own.

In contrast to the bright and relatively massive C, B, and A rings, the E ring is thin and consists of particles in the micron range, a thousand times smaller than sand. The G ring particles are not so well sorted, and range from dust-sized up to kilometer-sized. The F ring is intermediate between the main rings and the E ring in terms of its density and particle size, but it is outstanding for its bizarre configurations.The F ring is narrow, multistranded, and inclined to the plane of the other rings. It contains kinks, clumps, and parts that appear braided; these structures continue to be the subject of study, and shocked scientists when first discovered by the Pioneer 11 mission.The F-ring kinks are caused by two "shepherd" moons, Prometheus and Pandora, each about 60 miles (100 km) in diameter (for more on shepherd moons, see the sidebar "Why Are There Rings?" on page 128).

The more that is learned about the rings and the better the images become, the more complex the rings' structures appear. Though Cassini's Division and Encke's Division appear empty from Earth,


Ring, (moon),

Radial extent

or division

(miles [km])


cloud tops

37,470 (60,300)

D ring

41,570 to 46,392

contains at least two narrow ringlets

(66,900 to 74,658)

C ring

46,392 to 57,152

contains at least four narrow ringlets,

(74,658 to 91,975)

including the Titan and Maxwell ringlets

B ring

57,152 to 73,017

most opaque of rings

(91,975 to 117,507)

Cassini's Division

73,017 to 76,021

contains the Huygens ringlet

(117,507 to 122,340)

A ring

76,021 to 84,993

12.1 to 14.3

(122,340 to 136,780)

Encke's Division

70,472 to 83,104

gap in A ring kept open by Pan

(113,410 to 133,740)

Keeler Gap

84,826 to 84,851

empty gap near edge of A ring

(136,510 to 136,550)

(Atlas, Prometheus)

a ring 190 miles (300 km) wide is associated

with Atlas

F ring

centered on 87,752

eccentric ringlet with kinks, clumps, and braid


(Pandora, Epimetheus, Janus)

G ring

103,150 to 107,624

faint and isolated

(166,000 to 173,200)

E ring

111,800 to 298,000

broad faint dust ring including the orbits of

(180,000 to 480,000)

Mimas, Enceladus, Tethys, Telesto, Calypso,

Dione, and Helene

they are actually filled with very faint ringlets. Cassini's Division is maintained by the moon Mimas, and Encke's Division contains the orbit of the moon Pan.The larger rings contain waves and clumps and even smaller ringlets within their overall structures, in addition to the hundreds of tiny regular rings that spread evenly to make up the bulk of the major rings.

In the table on page 131, the rings are listed in order from closest to Saturn outward. Gaps and moons are also included in their proper order. Note that even though the Cassini and Encke Divisions appear empty from Earth, they are actually filled with particles and even faint rings. They are simply much less dense areas than their surrounding rings.

Another surprising discovery from the Voyager 1 mission was dark radial structures in the rings. These dark structures are about 5,000 miles (8,000 km) long and 1,200 miles (2,000 km) in width. They develop rapidly, over a matter of minutes, and disappear after several hours.While they last, the spokes travel with the rotation of the rings. Though they look dark in reflected light, they appear bright in scattered light, indicating that they are made of micrometer-sized particles. Electromagnetic fields may be responsible for these patterns of denser and thinner rings, since tiny particles are most easily moved by electromagnetic forces. There are a number of complex theories involving the planetary magnetic field to explain the formation of these spokes, but no one theory predominates at the moment.

Saturn's rings are very bright, which means the material in them is fresh and has not been covered by space-weathering products, indicating that the rings are geologically young. Uranus's and Neptune's, by comparison, are dark and red. Because the processes that destroy rings work so quickly, the rings cannot have existed for the age of the solar system and therefore cannot have formed at the same time as the other moons. It is thought that Saturn's rings are about 100 million years old, the result of an icy moon about 60 miles (100 km) in diameter being ripped apart by gravitational forces and scattered into rings, or perhaps they are the result of a comet capture and breakup. The ring material will eventually darken and redden, like those of the other planets.The rate of darkening by meteoroid impacts and plasma can be calculated, and this calculation results in the estimate that the rings are now about 100 million years old.

Rings consisting of tiny particles, such as the D, E, F, and G rings, will have the shortest lifetimes. Dust-sized particles are easily moved by drag, magnetic forces, and meteoroid impacts. These processes efficiently remove dust from the rings, and so dust must be replen ished in the rings from a nearby source. Some scientists think that larger bodies, on the order of one kilometer in diameter, exist inside these dusty rings and continually shed dust into the rings to replenish them. Other scientists believe that by coincidence with the time of human space exploration, the rings are at the fleeting moment of their peak development. Dusty rings are only expected to exist for tens of millions of years, and denser rings for not significantly longer than 100 million years, and so ring systems are expected to form and dissipate many times over the age of the solar system.

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