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)
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
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
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
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
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.
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.
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
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
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.
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
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 :)
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