Saturn's Rings

The rings of Saturn have puzzled astronomers ever since they were discovered by Galileo in 1610 using the first telescope. The puzzles have only increased since Voyagers 1 and 2 imaged the ring system extensively in 1980 and 1981. In addition to the images, several Voyager instruments observed occultations of the ring system with radial resolution as fine as 100 meters. The rings have been given letter names in the order of their discovery. The main rings are, working outward from the planet, known as C, B, and A. The Cassini Division is the largest gap in the rings and separates Rings B and A. In addition a number of fainter rings have been discovered more recently. The D Ring is exceedingly faint and closest to the planet. The F Ring is a narrow feature just outside the A Ring. Beyond that are two far fainter rings named G and E. The particles in Saturn's rings are composed primarily of water ice and range from microns to meters in size. The rings show a tremendous amount of structure on all scales; some of this structure is related to gravitational perturbations by Saturn's many moons, but much of it remains unexplained.

Giovani Domenico Cassini discovered a large gap in Saturn's rings in 1675. Later, an astronomer named Encke discovered a smaller gap near the edge of the rings. What causes these gaps? Well, the rings are not solid, but composed of a vast number of tiny particles, most of which are plain old water ice. These chunks of ice are very small; most are probably only centimeters across! Each chunk orbits Saturn individually, like a swarm of billions of moons. It was found long ago that an orbit in the Cassini Division, as the large gap is named, has a period almost exactly one-half that of one of Saturn's moons named Mimas. This means that every other time a chunk of ice orbits Saturn in the Cassini Division, it would see Mimas in the same position in the sky. Mimas has a pretty good mass, which means it has substantial gravity. Over millions of years, this periodic tug has yanked all the particles out of that region in Saturn's rings, leaving a gap present today.

The Encke division, on the other hand, is the result of a direct sweeping of a small satellite located inside the gap. It actually pulls particles out of its way as it orbits Saturn, leaving a gap in the rings!

False Color Image of Saturn's Rings
Possible variations in chemical composition from one part of Saturn's ring system to another are visible in this Voyager 2 picture as subtle color variations that can be recorded with special computer-processing techniques. This highly enhanced color view was assembled from clear, orange and ultraviolet frames obtained August 17, 1981 from a distance of 8.9 million kilometers (5.5 million miles). In addition to the previously known blue color of the C-ring and the Cassini Division, the picture shows additional color differences between the inner B-ring and and outer region (where the spokes form) and between these and the A-ring. (Courtesy NASA/JPL)

 

 

Voyager 2 image of Saturn's rings (upper left) taken on 19 August 1981, a little over 6 days before closest approach. The Cassini and Encke divisions are clearly visible, as are many other features in the rings. The image was taken from 6.5 million km.

 

 

 

 

I have included this large image of the details of Saturn rings, primarily because it is such a spectacular photograph, but also because you can see the fine structre of the rings and the mny concentric features contained within them.

Saturn's F-ring was discovered by Pioneer 11 in 1979. Photos of the F-ring taken by Voyager 1 showed three separate strands that appear twisted or braided (lower left). At higher resolution, Voyager 2 found five separate strands in a region that had no apparent braiding, and surprisingly revealed only one small region where the F-ring appeared twisted. The photopolarimeter found the brightest of the F-ring strands was subdivided into at least 10 strands. The twists are believed to originate in gravitational perturbations caused by one of two shepherding satellites, Prometheus (lower right), which is seenn across from Pan. Clumps in the F-ring appear uniformly distributed around the ring every 9,000 kilometers (5,600 miles), a spacing that very nearly coincides with the relative motion of F-ring particles and the interior shepherding satellite in one orbital period. By analogy, similar mechanisms might be operating for the kinky ringlets that exist in the Encke Gap.

SPOKES IN THE RINGS

Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called "spokes" which were first reported by amateur astronomers (above). Their nature remains a mystery, but may have something to do with Saturn's magnetic field. Additionally, Carolyn Porco (director of the Cassini mission and imaging team member of the Voyager team) mentioned to me that the spokes of Saturn's rings are tied to the orbital period of the magnetic field of Saturn, which is the method astronomers use to measure the rotation of the planet. The spokes do change their appearance over rotational time and are not completely understood. The image below is just another look at these mysterious structures, if they can be called so.

The Enke Gap is caused by the tiny moon Pan moving within the gap (seen in the Cassini image below and left), and its small gravity effect clears an area of the ring from debris, making it a "see-through" region. Pan also causes small pertubations in the neighboring section of the ring, resulting in waves of ring material, as seen in the image below.

In closing, these rings are exceedingly thin. In most places, the ring system is less than 200 meters in thickness, and near the gaps may be less than 10 meters this. On those occasions when the orientation of Saturn relative to Earth is edge-on, the rings will disappear from sight.

SATURN'S RINGS DISAPPEAR WHEN VIEWED EDGE-ON

This sequence of images from NASA's Hubble Space Telescope documents a rare astronomical alignment -- Saturn's magnificent ring system turned edge-on. This occurs when the Earth passes through Saturn's ring plane, as it does approximately every 15 years.
These pictures were taken with Hubble's Wide Field Planetary Camera 2 on 22 May 1995, when Saturn was at a distance of 919 million miles (1.5 billion kilometers) from Earth. At Saturn, Hubble can see details as small as 450 miles (725 km) across. In each image, the dark band across Saturn is the ring shadow cast by the Sun which is still 2.7 degrees above Saturn's ring plane. The box around the western portion of the rings (to the right of Saturn) in each image indicates the area in which the faint light from the rings has been multiplied through image processing (by a factor of 25) to make the rings more visible.

 

 

 

 

 

 

[Top] -
This image was taken while the Earth was above the lit face of the rings. The moons Tethys and Dione are visible to the east (left) of Saturn; Janus is the bright spot near the center of the ring portion in the box, and Pandora is faintly visible just inside the left edge of this box. Saturn's atmosphere shows remarkable detail: multiple banding in both the northern and southern hemispheres, wispy structure at the north edge of the equatorial zone, and a bright area above the ring shadow that is caused by sunlight scattered off the rings onto the atmosphere. There is evidence of a faint polar haze over the north pole of Saturn and a fainter haze over the south.

[Center] -
This image was taken close to the time of ring-plane crossing. The rings are 75% fainter than in the top image, though they do not disappear completely because the vertical face of the rings still reflects sunlight when the rings are edge-on. Rhea is visible to the east of Saturn, Enceladus is the bright satellite in the rings to the west, and Janus is the fainter blip to its right. Pandora is just to the left of Enceladus, but is not visible because Enceladus is too bright. An oval-shaped atmospheric feature has just rotated into view (near the eastern limb, at the northern edge of the equatorial zone), and appears to be a local circulation pattern that is not penetrated by the bright clouds that are deflected around it.

[Bottom] -
This image was taken approximately 96 minutes (one Hubble orbit) after the center image. The rings are 10% brighter than they were in that image. Rhea is visible just off the eastern limb of Saturn, and casts a shadow on the south face of Saturn. During this exposure, the Earth and Sun were on opposite sides of Saturn's ring plane (they remain in this configuration until 10 August 1995). The atmospheric circulation pattern has rotated to just past the center of the planet's disk, and is followed by more wispy structure in the bright band of clouds, reminiscent of the structure seen during the Saturn storm observed in 1990.

These images will be used to determine the time of ring-plane crossing and the thickness of the main rings and to search for as yet undiscovered satellites. Knowledge of the exact time of ring-plane crossing will lead to an improved determination of the rate at which Saturn "wobbles" about its axis (polar precession).

Technical Notes
Each of these images is a 7-second exposure at 8922 Angstroms in a methane absorption band. North is up and east is to the left.

Credit: Amanda S. Bosh (Lowell Observatory), Andrew S. Rivkin (Univ. of Arizona/LPL), the HST High Speed Photometer Instrument Team (R.C. Bless, PI), and NASA.

 

After Voyager 1 flew by up through the rings of Saturn, it continued in a northerly trajectory relative to the ecliptic, and is continuing to travel out of our solar system. There are two things which control the future of the Voyager spacecraft ... the amount of nuclear fuel as well as the amount of thruster engine fuel. The spacecraft uses the decay of Plutonium to generate heat, which in turn is converted into electricity, that can be used to send signals to the JPL facility in Pasadena, as well as receive signals from Earth. The thruster fuel is used to point the Voyager receiver/transmitter antenna at the Earth. Scientists believe that there is enough of both fuels to keep Voyager 1 operational until about 2015-2020. It is hoped by that time, that Voyager will have escaped the heliosphere (the bubble or gas blown out by the Sun), cross the heliopause (where the Sun's wind-blown bubble ends and interstellar space begins), and enter interstellar space. The mission has been renamed Voyager Interstellar Mission, and scientists are continuing to maintain contact with both Voyager 1 and 2.

This information in the last paragraph of this page is found in several other places within my course because I believe it to be incredibly important, interesting, and relevant to a potential creative writing assignment of yours :)

Return Saturn or to the Planet Introduction, or the Syllabus.


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