Download Planetary Rings

Planetary Rings
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All of the gas giants possess ring systems, a fact not realized prior to 1976.
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Planetary rings form when a moon or some other object ventures too close to the
planet, and is disrupted by the planet’s strong tidal forces. The “remains” form a
ring around the planet.
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Some rings, or portions of rings, may form when a large comet or asteroid impact
occurs on an outer planet moon, knocking pieces of the moon into orbit. The entire
moon may be disrupted if the impact is sufficiently powerful.
Saturn
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Viewed through a small telescope, Saturn possesses a spectacular ring system
surrounding the planet. First glimpsed by Galileo, the nature of Saturn’s rings was not
realized until 1659 by Huygens. Structure within the rings was discovered by Cassini
in 1675.
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The ring system extends ~270,000 kilometers in diameter, completely in Saturn’s
equatorial plane, and does not extend inward to the cloud tops.
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Saturn’s ring system contains several named rings, of differing width and density. The
B ring is the most massive and brightest of the rings.
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Saturn's rotation axis is tilted 26.7, affecting the brightness and appearance of the
ring system as seen from Earth. At times, the rings are viewed edge-on, and
essentially disappear from our view.
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The rings are extremely thin - a few hundred meters thick. Much thinner, compared
to its diameter, than a CD.
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The rings are NOT solid or gaseous. Infrared spectra reveal that they are composed
of billions of snowballs (!) - lumps of mostly water ice & rock, orbiting Saturn. The ring
particles range in size from sand grains to large boulders.
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The rings rotate according to Kepler’s laws. The inner ring particles orbit faster, with a
shorter period than the outer rings.
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Complex structures are found within the rings, first detected by the Voyager space
probes. These structures appear to arise from gravitational interactions between ring
particles and Saturn’s many small moons and, electromagnetic forces experienced
by small ice and dust particles. However, planetary scientists are still struggling to
explain many of these phenomena.
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Examples of some of the odd features found in Saturn’s rings:
 Gaps: relatively empty spaces between rings, for example the Cassini division.
These gaps are believed to be caused by the gravity of various small moons. The
Cassini division is caused by the influence of Mimas, Saturn’s innermost ice moon,
which “sweeps” ring particles from the gap.
 Ringlets: dense, narrow regions within the rings, where particles pile up, in some
sense, opposite to the gaps. Here the gravitational tugs from gap moons causes
“bunching” of ring particles.
 Spokes: apparently electrically-charged dust suspended just above the ring plane.
 The F-ring: a narrow ring, separated from the main ring system, which appears
“braided” and kinked.
 Shepherd satellites: tiny moons which apparently confine the narrow F-ring,
orbiting just inside and just outside the F-ring.
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Much of our knowledge of the ring system resulted from the imagery taken with the
Voyager I and II flybys in the early 1980s.
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The exact origin of the rings is uncertain; most likely Saturn’s ring system is young,
perhaps less than 50 million years old. Over time the rings should disperse.
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Saturn’s rings may have formed when a small moon approach the Roche limit, a
“boundary” around Saturn at which tidal forces are sufficient to break up a small, icy
moon.
Jupiter, Uranus & Neptune
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The rings surrounding these planets are narrow, dark and much less prominent than
those of Saturn and were only discovered in the recent past.
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Uranus’ rings were accidentally discovered in 1977 when they blocked the light of a
distant star as Uranus passed in front of the star, a phenomenon known as an
occultation.
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The Voyager probes discovered rings bracketing Jupiter (1979) and Neptune (1989).
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These rings are thin, dark, and composed apparently of mostly dust, or carbon-coated
ices.
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In the case of Jupiter, most of the ring particles appear to originate from dust sandblasted from the surfaces of small inner moons by charged atomic particles whipped
around by the jovian magnetic field.
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Many aspects of the gas giant ring systems remain unexplained.