Rings of Saturn

The rings of Saturn are a system of planetary rings around the planet Saturn. They consist of countless small particles, ranging in size from microns to meters, that form clumps that in turn orbit about Saturn. The ring particles are made almost entirely of water ice, with some contamination from dust and other chemicals.

Although reflection from the rings increases Saturn's brightness, they are not visible from Earth with unaided vision. In 1610, the year he first turned a telescope to the sky, Galileo Galilei became the very first person to observe Saturn's rings, though he could not see them well enough to discern their true nature. In 1655, Christiaan Huygens was the first person to describe them as a disk surrounding Saturn.^[1]

Although many people think of Saturn's rings as being made up of "countless tiny ringlets" (a concept that goes back to Laplace), true gaps are few in number. It is more correct to think of the rings as an annular disk with concentric local maxima and minima in density and brightness. On the scale of the clumps within the rings there is a lot of empty space, but in general these empty spaces are discontinuous.

There are several gaps within the rings: two opened by known moons embedded within them, and many others at locations of known destabilizing orbital resonances with Saturn's moons. Other gaps remain unexplained. Stabilizing resonances, on the other hand, are responsible for the longevity of several rings, such as the Titan Ringlet and the G Ring.

Formation of Saturn's rings

Saturn's rings are very old, and may date to the formation of Saturn itself. It seems likely that they are composed of debris from the disruption of a moon about 300 km in diameter, bigger than Mimas. The last time there were collisions large enough to be likely to disrupt a moon that large was during the Late Heavy Bombardment some four billion years ago.[1]

Previous theories had posited that Saturn's rings must be a relatively recent phenomena as the infall of meteoric dust would have led to darkening of the rings. However, observations from the Cassini orbiter indicate that the rings are massive enough for them to have diluted infalling material and thus avoided substantial darkening over the age of the Solar system. Ring material may be recycled as clumps form within the rings and are then disrupted by impacts. This would explain the apparent youth of some of the material within the rings.[2]

Larry Esposito of the Cassini team used stellar occultation to discover 13 objects, ranging from 27 meters to 10 km across, within the F ring. They are translucent, suggesting they are temporary aggregates of ice boulders a few meters across. Esposito believes this to be the basic structure of the Saturnian rings, particles clumping together, then being blasted apart.

Subdivisions and structures within the rings

The Saturnian ring system comprises three major sections, labeled (from the outside) A, B, and C, of which B is the most substantial, as well as a tenuous D ring extending inward toward the atmosphere, and several narrow or diffuse rings, including F, E, G, beyond the A ring. These and several gaps are listed below.

Natural-color mosaic of Cassini narrow-angle camera images of the unilluminated side of Saturn's D, C, B, A and F rings (left to right) taken on May 9, 2007.

Name⁽⁴⁾	Distance from Saturn's center (km)⁽⁵⁾	Width (km)⁽⁵⁾	Named after
D Ring	66,900 - 74,510	7,500
C Ring	74,658 - 92,000	17,500
Colombo Gap	77,800 ? ⁽²⁾	100	Giuseppe "Bepi" Colombo
Titan Ringlet	77,800 ? ⁽²⁾	?	Titan moon of Saturn
Maxwell Gap	87,491 ⁽²⁾	270	James Clerk Maxwell
B Ring	92,000 - 117,580	25,500
Cassini Division	117,580 - 122,170	4,700	Giovanni Cassini
Huygens Gap	117,680 ? ⁽²⁾	285-440	Christiaan Huygens
A Ring	122,170 - 136,775	14,600
Encke Division	133,589 ⁽²⁾	325	Johann Encke
Keeler Gap	136,530 ⁽²⁾	35	James Keeler
R/2004 S 1⁽¹⁾	137,630 ⁽²⁾	?
R/2004 S 2⁽¹⁾	138,900 ⁽²⁾	?
F Ring	140,180 ⁽²⁾	30-500
Janus/Epimetheus Ring⁽³⁾	149,000 - 154,000	5,000	Janus and Epimetheus
G Ring	170,000 - 175,000	5,000
Pallene Ring⁽³⁾	211,000 - 213,500	2,500	Pallene
E Ring	181,000 - 483,000	302,000

Notes:
⁽¹⁾ temporary designation
⁽²⁾ distance is to centre of gaps, rings and ringlets that are narrower than 1000 km
⁽³⁾ unofficial name
⁽⁴⁾ Names as designated by the International Astronomical Union, unless otherwise noted.
⁽⁵⁾ Data mostly from this NASA factsheet.

Oblique (4 degree angle) Cassini spacecraft images of Saturn's C, B, and A rings (left to right; the F ring is faintly visible in the upper image when viewed at full size). Upper image: natural color mosaic of Cassini narrow-angle camera images of the illuminated side of the rings taken on December 12, 2004. Lower image: simulated image constructed from a radio occultation observation conducted on May 3, 2005. Color in the lower image is used to represent information about ring particle sizes.

D Ring

The D Ring is the innermost ring, and is very faint. In 1980, Voyager 1 detected within this ring three ringlets designated D73, D72 and D68, with D68 being the discrete ringlet nearest to Saturn. Some 25 years later Cassini images showed that D72 had become significantly fainter and moved planetward by 200 kilometres. Present in the gap between the C ring and D73 is finescale structure with waves 30 kilometres apart.

C Ring

The C Ring is a wide but faint ring located inward of the B Ring. It was discovered in 1850 by William and George Bond, though William R. Dawes and Johann Galle also saw it independently. William Lassell termed it the "Crepe Ring" because it seemed to be composed of darker material than the brighter A and B Rings.^[2]

Its vertical thickness is estimated at 5 metres, its mass at around 1.1×10¹⁸ kilograms, and its optical depth varies from 0.05 to 0.12. [3]. That is, 5 and 12 percent of light shining through perpendicular to the ring is blocked, so that when seen from above or below, the ring is close to transparent.

Colombo Gap and Titan Ringlet

The Colombo Gap lies in the inner C Ring. Within the gap lies the bright but narrow Colombo Ringlet, centered at 77,883 kilometers from Saturn's center, which is slightly elliptical rather than circular. This ringlet is also called the Titan Ringlet as it is governed by an orbital resonance with the moon Titan. [4] At this location within the rings, the time period of a ring particle's apsidal precession is equal to the time period of Titan's orbital motion, so that the outer end of this eccentric ringlet always points towards Titan.

Maxwell Gap

Another gap within the C Ring is the Maxwell Gap. It also contains a dense non-circular ringlet, the Maxwell Ringlet.

B Ring

The B Ring is the largest, brightest, and most massive of the rings. Its thickness is estimated as 5 to 10 metres, its mass at 2.8×10¹⁹ kg, and its optical depth varies from 0.4 to 2.5, [5] meaning that well over 99% of the light passing through some parts of the B Ring is blocked. The B Ring contains a great deal of variation in its density and brightness, nearly all of it unexplained. These are concentric, appearing in the form of narrow ringlets, though the B Ring does not contain any gaps.

Spokes

During the Voyager encounters, extended short-lived spoke-like features were seen in the B ring under some viewing geometries. Their nature was the focus of lively scientific debate, and several hypotheses were proposed as to their cause and makeup. Spokes were seen by the Hubble Space Telescope in the late 1990s and early 2000s. When the Cassini spacecraft entered into orbit around Saturn, the spokes were mysteriously absent. Suggestions that spokes may be a seasonal effect, varying with Saturn's 29.7-year orbit, were supported by the gradual reappearance in the later years of the Cassini mission. The leading theory is that spokes are made of tiny dust particles suspended above the main ring by electrostatic repulsion.

Cassini Division

The Cassini Division is a 4,800 km (2,980 mile) wide region between the A Ring and B Ring. It was discovered in 1675 by Giovanni Cassini. From Earth it appears as a thin black gap in the rings. However, Voyager discovered that the gap is itself populated by ring material bearing much similarity to the C Ring.

The inner edge of the Cassini Division is governed by a strong orbital resonance. Ring particles at this location orbit twice for every orbit of the moon Mimas. The resonance causes Mimas' pulls on these ring particles to accumulate, destabilizing their orbits and leading to a sharp cutoff in ring density. Many of the other gaps between ringlets within the Cassini Division, however, are unexplained.

Huygens Gap

The Huygens Gap is at the inner edge of the Cassini Division. It contains the dense Huygens Ringlet, which is non-circular.

A Ring

The A Ring is the outermost of the large, bright rings. Its inner boundary is the Cassini Division and its sharp outer boundary is close to the orbit of the small moon Atlas. The A Ring is interrupted at a location 22% of the ring width from its outer edge by the Encke Division. A narrower division 2% of the ring width from the outer edge is called the Keeler Gap.

The thickness of the A Ring is estimated as 10 to 30 metres, its mass as 6.2×10¹⁸ kg (about the mass of Hyperion), and its optical depth varies from 0.4 to 1.0. [6]

Similarly to the B Ring, the A Ring's outer edge is maintained by an orbital resonance, in this case the 7:6 resonance with Janus and Epimetheus. Other orbital resonances also excite many spiral density waves in the A Ring (and, to a lesser extent, other rings as well), which account for most of its structure. These waves are described by the same physics that describes the spiral arms of galaxies. Spiral bending waves, also present in the A Ring and also described by the same theory, are vertical corrugations in the ring rather than compression waves.

Encke Division

The Encke Division, sometimes called the Encke Gap, is a gap within the A Ring. Johann Encke himself did not observe this division; it was named in honour of his ring observations. The division itself was discovered by James Edward Keeler in 1888.

The division is centered at a distance 133,580 kilometers from Saturn's center, and has a width of 325 kilometers. [7] It is caused by the presence of the small moon Pan, which orbits within it.

Images from the Cassini probe have shown that there are at least three thin, knotted ringlets within the gap. [8]

Keeler Gap

The Keeler Gap is a 42-kilometre-wide gap in the A Ring, approximately 250 kilometres from the ring's outer edge. It is named after the astronomer James Edward Keeler. The small moon Daphnis, discovered May 1 2005, orbits within it, keeping it clear.

R/2004 S 1

R/2004 S 1, also known as S/2004 1R, is the temporary designation of a newly discovered ring that lies between the A Ring and the F Ring, in the orbit of the moon Atlas. The faint, thin ring was discovered by the Cassini probe imaging team and announced on September 9, 2004.

R/2004 S 2

R/2004 S 2 is a temporary designation for a faint ring recently discovered by the Cassini probe imaging team and announced in 2005. The ring is located at 138,900 km from Saturn's center, between the orbits of Atlas and Prometheus.

F Ring

The F Ring is one of the outer rings of Saturn. It is located outside the larger rings, just 3000 km beyond the outer edge of the A Ring.^[3] It was discovered in 1979 by the Pioneer 11 imaging team.^[4] It is very thin, just a few hundred kilometers wide, and is held together by two shepherd moons, Prometheus and Pandora, which orbit inside and outside it.

Recent closeup images from the Cassini probe show that the F Ring consists of one core ring and a spiral strand around it. They also show that when Prometheus encounters the ring at its apoapsis, its gravitational attraction creates kinks and knots in the F Ring as the moon 'steals' material from it, leaving a dark channel in the inner part of the ring. Since Prometheus orbits Saturn more rapidly than the material in the F ring, each new channel is carved about 3.2 degrees in front of the previous one.

The shepherd moons Pandora (left) and Prometheus orbit on either side of the F ring; Prometheus is followed by dark channels that it has carved into the inner strands of the ring.
Close-up view of Prometheus and the F Ring. A movie of Prometheus at its apoapsis drawing a streamer of material out of the ring, leaving a dark channel, may be viewed here or here.
Prometheus (at center) and Pandora are the inner and outer F Ring shepherds.

Prometheus (at center) and Pandora are the inner and outer F Ring shepherds.

"Janus/Epimetheus" Ring

A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006. The ring has a radial extent of about 5,000 km ^[5]. Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths ^[6].

G Ring

The G Ring is a very thin, faint ring about halfway between the F Ring and the beginning of the E Ring, with its inner edge about 15000 km inside the orbit of Mimas. It contains a single distinctly brighter "arc" near its inner edge (similar to the arcs in the rings of Neptune) that extends about one sixth of its circumference, which is held in place by a 7:6 orbital resonance with Mimas.^[7] The arc is believed to be composed of icy particles up to a few meters in diameter, with the rest of the G Ring consisting of dust released by collisions within the arc. The radial width of the arc is about 250 km, compared to a width of 6000 km for the G Ring as a whole.^[7] The arc is thought to be the remains of a small icy moonlet about a hundred meters in diameter that broke up relatively recently. Dust released from the larger chunks by micrometeoroid impacts drifts outward from the arc due to interaction with Saturn's magnetosphere (whose plasma corotates with Saturn's magnetic field, which rotates much more rapidly than the orbital motion of the G Ring). These tiny particles are steadily eroded away by further impacts and dispersed by plasma drag. Over the course of thousands of years the ring will gradually lose mass and eventually disappear.^[8]

"Pallene" Ring

A faint dust ring shares Pallene's orbit, as revealed by images taken in forward-scattered light by the Cassini spacecraft in 2006 ^[5]. The ring has a radial extent of about 2,500 km. Its source is particles blasted off Pallene's surface by meteoroid impacts, which then form a diffuse ring around its orbital path ^[6].

E Ring

The E Ring is the outermost ring, and is extremely wide, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea. It is a diffuse disk of icy or dusty material. Unlike the other rings, it is composed of microscopic rather than macroscopic particles. In 2006, cryovolcanism on the moon Enceladus was determined to be the source of the E Ring's material.

References

^ "Historical Background of Saturn's Rings". Retrieved 2006-03-08.
^ David M. Harland, Mission to Saturn: Cassini and the Huygens Probe, Chichester: Praxis Publishing, 2002.
^ H. Karttunen, P. Kröger, ed al. (2000). Springer (ed.). Fundamental Astronomy. p. 221. {{cite book}}: Unknown parameter |address= ignored (|location= suggested) (help); Unknown parameter |ed= ignored (help)CS1 maint: multiple names: authors list (link)
^ T.G. Gehrels et al, "Imaging Photopolarimeter on Pioneer Saturn", Science 207, 434-439 (1980)
^ ^a ^b NASA Planetary Photojournal PIA08328: Moon-Made Rings
^ ^a ^b Cassini-Huygens press release NASA Finds Saturn's Moons May Be Creating New Rings, 11 October, 2006.
^ ^a ^b M. M. Hedman, J. A. Burns, M. S. Tiscareno, C. C. Porco, G. H. Jones, E. Roussos, N. Krupp, C. Paranicas, S. Kempf (2007). "The Source of Saturn's G Ring". Science. 317: 653–656. doi:10.1126/science.1143964.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Davison, Anna (02 August 2007). "Saturn ring created by remains of long-dead moon". NewScientist.com news service. {{cite news}}: Check date values in: |date= (help)

v t e Ring systems
Planets	Rings of Jupiter Rings of Saturn Rings of Uranus Rings of Neptune Rings of Earth (space debris) Rings of exoplanets
Minor planets	Rings of Chariklo Rings of Chiron Ring of Haumea Rings of Quaoar
Moons	Rings of Rhea (unconfirmed)
Related topics	Gas torus Circumstellar disc Circumplanetary disc