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 may be very old, dating 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.[2]
The brightness and purity of the water ice in Saturn's rings has been cited as evidence that the rings are much younger than Saturn, perhaps 100 million years old, as the infall of meteoric dust would have led to darkening of the rings. However, new research indicates that the B Ring may be massive enough 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.[3]
The Cassini UVIS team, led by Larry Esposito, 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 densest parts of the Saturnian ring system are the A and B Rings, which are separated by the Cassini Division (discovered in 1675 by Giovanni Domenico Cassini). Along with the C Ring, which was discovered in 1850 and is similar in character to the Cassini Division, these regions comprise the Main Rings. The Main Rings are denser and contain larger particles than the tenuous Dusty Rings. The latter include the D Ring, extending inward to Saturn's cloud tops, the G and E Rings and others beyond the main ring system. The word "dusty" used to characterize these diffuse rings refers to the small size of the particles (often about a micron); their chemical composition is, like the main rings, almost entirely of water ice. The narrow F Ring, just off the outer edge of the A Ring, is more difficult to categorize; parts of it are very dense, but it also contains a great deal of dust-size particles.
Major subdivisions of the rings
| Name(3) | Distance from Saturn's center (km)(4) | Width (km)(4) | Named after |
|---|---|---|---|
| D Ring | 66,900 - 74,510 | 7,500 | |
| C Ring | 74,658 - 92,000 | 17,500 | |
| B Ring | 92,000 - 117,580 | 25,500 | |
| Cassini Division | 117,580 - 122,170 | 4,700 | Giovanni Cassini |
| A Ring | 122,170 - 136,775 | 14,600 | |
| Roche Division | 136,775 - 139,380 | 2,600 | Édouard Roche |
| F Ring | 140,180 (1) | 30-500 | |
| Janus/Epimetheus Ring(2) | 149,000 - 154,000 | 5,000 | Janus and Epimetheus |
| G Ring | 170,000 - 175,000 | 5,000 | |
| Pallene Ring(2) | 211,000 - 213,500 | 2,500 | Pallene |
| E Ring | 181,000 - 483,000 | 302,000 |
Structures within the C Ring
| Name(3) | Distance from Saturn's center (km)(4) | Width (km)(4) | Named after |
|---|---|---|---|
| Colombo Gap | 77,870 (1) | 150 | Giuseppe "Bepi" Colombo |
| Titan Ringlet | 77,870 (1) | 30 | Titan, moon of Saturn |
| Maxwell Gap | 87,491 (1) | 270 | James Clerk Maxwell |
Structures within the Cassini Division
| Name(3) | Distance from Saturn's center (km)(4) | Width (km)(4) | Named after |
|---|---|---|---|
| Huygens Gap | 117,680 (1) | 400 | Christiaan Huygens |
Structures within the A Ring
| Name(3) | Distance from Saturn's center (km)(4) | Width (km)(4) | Named after |
|---|---|---|---|
| Encke Gap | 133,589 (1) | 325 | Johann Encke |
| Keeler Gap | 136,530 (1) | 35 | James Keeler |
Notes:
(1) distance is to centre of gaps, rings and ringlets that are narrower than 1000 km
(2) unofficial name
(3) Names as designated by the International Astronomical Union, unless otherwise noted. Broader separations between named rings are termed divisions, while narrower separations within named rings are called gaps.
(4) Data mostly from the Gazetteer of Planetary Nomenclature and this NASA factsheet.
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.[4]
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. [1]. 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. [2] 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
The Maxwell Gap lies within the outer C Ring. 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, [3] 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 their 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. Suggestions for the origin of the electrical disturbances include lighting bolts in Saturn's atmosphere and micrometeoroid impacts on the rings. The spokes appear dark in backscattered light, and bright in forward-scattered light.
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 division may appear bright in views of the unlit side of the rings, since the relatively low density of material allows more light to be transmitted through the thickness of the rings.
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 Gap. A narrower gap 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. [4]
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 Gap
The Encke Gap is a 325-kilometre-wide gap within the A Ring, centered at a distance of 133,590 kilometers from Saturn's center.[5] 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.
Johann Encke himself did not observe this gap; it was named in honour of his ring observations. The gap itself was discovered by James Edward Keeler in 1888.
The Encke Gap is a gap because it is entirely within the A Ring. There was some ambiguity between the terms gap and division until the IAU clarified the definitions in 2008; prior to that, the separation was sometimes called the "Encke Division".
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.
-
The Encke Gap's central ringlet coincides with Pan’s orbit, implying its particles oscillate in horseshoe orbits. Spiral density waves (induced by resonances with nearby moons), which pervade much of the A Ring, are visible on both sides of the gap. -
The motion of Pan (not visible) through the Encke Gap induces edge waves and a set of spiraling wakes (which are not self-propagating) ahead of it in its orbit on the gap's inner side. The other more tightly wound bands are spiral density waves. -
The passage of Daphnis induces waves in the edges of the Keeler gap. Orbital motion is to the lower left; speed decreases going towards the A Ring edge.
Moonlets
In 2006, four tiny "moonlets" were found in Cassini images of the A Ring.[5] The moonlets themselves are only about a hundred meters in diameter, too small to be seen directly; what Cassini sees are the "propeller"-shaped disturbances the moonlets create, which are several km across. It is estimated that the A Ring contains thousands of such objects. In 2007, the discovery of eight more moonlets revealed that they are largely confined to a 3000-km belt, about 130,000 km from Saturn's center.[6] Over 150 "propeller" moonlets have now been detected.[7]
Location of the first four moonlets detected.
Roche Division
The separation between the A Ring and the F Ring has been named the Roche Division in honor of the French physicist Édouard Roche.[6] The Roche Division should not be confused with the Roche limit, a physical concept that describes when a large object gets so close to a planet (such as Saturn) that the planet's tidal forces will pull it apart. Lying at the outer edge of the main ring system, the Roche Division is in fact close to Saturn's Roche limit, which is why the rings have been unable to accrete into a moon.
Like the Cassini Division, the Roche Division is not empty but contains a sheet of material. The character of this material is similar to the tenuous and dusty D, E, and G Rings. Two locations in the Roche Division have a higher concentration of dust than the rest of the region. These were discovered by the Cassini probe imaging team and were given temporary designations: R/2004 S 1, which lies along the orbit of the moon Atlas; and R/2004 S 2, centered at 138,900 km from Saturn's center, inward of the orbit of 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.[8] It was discovered in 1979 by the Pioneer 11 imaging team.[9] 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. -
-
Prometheus (at center) and Pandora are the inner and outer F Ring shepherds.
Outer Rings
"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 [10]. Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.[11]
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.[12] 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.[12] 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.[13]
"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.[10] 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.[11]
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.
-
The outer rings seen back-illuminated by the Sun. -
-
The backlit E ring, with Enceladus silhouetted against it at center. The moon's south polar jets erupt brightly below it, while tendrils of the E Ring wrap around it.
Possible ring system around Rhea
Saturn's second largest moon Rhea may have a tenuous ring system of its own consisting of three narrow bands embedded in a disk of solid particles.[14][15] These rings have not been imaged, but their existence has been inferred from Cassini observations in November of 2005 of a depletion of energetic electrons in Saturn's magnetosphere near Rhea. The Magnetospheric Imaging Instrument (MIMI) observed a gentle gradient punctuated by three sharp drops in plasma flow on each side of the moon in a nearly symmetric pattern. This could be explained if they were absorbed by solid material in the form of an equatorial disk containing denser rings or arcs, with particles perhaps several decimeters to approximately a meter in diameter. However, not all scientists are convinced that the observations were caused by a ring system.
References
- ^ "Historical Background of Saturn's Rings". Retrieved 2006-03-08.
- ^ Kerr, Richard A. 2008. "Saturn's Rings Look Ancient Again", Science 319 (5859), 21.
- ^ "Saturn's Rings May Be Old Timers". NASA/JPL and University of Colorado. 2007-12-12. Retrieved 2008-01-24.
- ^ David M. Harland, Mission to Saturn: Cassini and the Huygens Probe, Chichester: Praxis Publishing, 2002.
- ^ Matthew S. Tiscareno; et al. (2006). "100-metre-diameter moonlets in Saturn's A ring from observations of 'propeller' structures". Nature. 440: 648–650.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ Miodrag Sremčević; et al. (2007). "A belt of moonlets in Saturn's A ring". Nature. 449: 1019–1021.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ Matthew S. Tiscareno; et al. (2008). "The population of propellers in Saturn's A Ring". Astronomical Journal. 135: 1083–1091. doi:10.1088/0004-6256/135/3/1083.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ 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) - ^ Jones, Geraint H. (2008 March 07). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science. 319 (5868). AAAS: 1380–1384. doi:10.1126/science.1151524.
{{cite journal}}: Check date values in:|date=(help); Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Lakdawalla, E. (2008-03-06). "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea". The Planetary Society web site. Planetary Society. Retrieved 2008-03-09.
{{cite web}}: External link in|work=
External links
See also
- Rings of Rhea
- Rings of Uranus
- Rings of Neptune
- Rings of Jupiter
- Édouard Roche - French astronomer who described how the breakup of a satellite could form the rings, when it comes within the Roche limit of a celestial body.
- Galileo Galilei - the first person to observe Saturn's rings, in 1610
- Christian Huygens - the first person to propose that there was a ring surrounding Saturn, in 1655
- Giovanni Cassini - discovered the separation between the A and B rings, in 1675 - (Cassini Division)
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