Rings of the Gas Giants
LACC §11.1, 11.4
• Understand what conditions and processes
shaped the gas giant planets’ ring systems:
Roche limit, shepherding moons
• Know the following ring systems in some
detail: Jupiter (dust from moons?), Saturn
(recent break up of icy object?), Uranus (break
up of a moon?), Neptune (unknown)
• Bright = icy and young, Dark = dusty and old
An attempt to answer the “big questions”: what is
out there? Are we alone?
Jupiter's intricate, swirling
ring system is formed by
dust kicked up as
smash into the giant
planet's four small inner
moons, according to...
Most of the rings are only a few tens of meters
thick with a total mass equivalent to a medium
sized moon. The rings are made out of particles
ranging from microscopic dust to barnyard sized
boulders with perhaps a few kilometer-sized
objects as well. ...the rings are composed mostly
of ice crystals with some impurities.
Scientists once thought that the rings were
formed at the same time, as the planets when
they coalescing out of swirling clouds of
interstellar gas 4.8 billion years ago. Under this
model, remnants of material within the Roche
limit could not condense and would become
rings. However, in recent years this idea seems
to be ﬂawed. The rings appear to be young,
perhaps only hundreds of millions of years old.
One of the clues to this theory is that the rings
are bright. As Saturn travels though space, the
rings accumulate dust particles that have been
darkened from solar radiation. If the rings were
old, they should appear dark. Another theory
suggests that perhaps a comet few too close to
Saturn and tidal forces broke it into pieces....
Perhaps one of Saturn's moons was struck by
an asteroid smashing it into the bits and pieces
that form the rings.
This image shows Saturn's rings and the shadow of nearby Mimas.
They are now nearly edge-on toward the Sun, and long moon
shadows drape across them. Scientists are now studying the
clumpy, disturbed ring material, stretching up to two miles above
the ring plane - contrasted with an estimated normal ring
thickness of only six feet
This composite of two images shows Pan, left, and Prometheus, right, in
nearby rings. Pan is trailed by a series of edge waves in the outer boundary
of the gap. Prometheus just touches the inner edge of Saturn's F ring, and is
followed by a series of dark channels
Radio measurements showed
the outermost ring, the epsilon,
to be composed mostly of ice
boulders several feet across.
However, a very tenuous
distribution of ﬁne dust also
seems to be spread throughout
the ring system.
The particles that make up the
rings may be remnants of a
moon that was broken by a high-
velocity impact or torn up by
Shepherd moons work in pairs on the inner and outer edge of rings to
gravitational push and pull (accelerate and de-accelerate) ring particles.
The result is to conﬁne the ring particles to within the shepherd moons
None of Neptune’s rings were detected from
scattering effects on Voyager’s radio signal
propagating through the rings, which indicates
that they are nearly devoid of particles in the
centimetre size range or larger. The fact that the
rings were most visible in Voyager images when
backlit by sunlight implies that they are largely
populated by dust-sized particles, which scatter
light forward much better than back toward the
Sun and Earth.Their chemical makeup is not
known, but, like the rings of Uranus, the surfaces
of Neptune’s ring particles (and possibly the
particles in their entirety) may be composed of
radiation-darkened methane ices.
The present rings are narrow, and scientists have found it difﬁcult to explain how the orbits of the
known moons can effectively conﬁne the natural radial spreading of the rings. This has led many
to speculate that Neptune’s present rings may be much younger than the planet itself, perhaps
substantially less than a million years. The present ring system may be markedly different from any
that existed a million years ago. It is even possible that the next spacecraft to visit Neptune’s rings
will ﬁnd a system greatly evolved from the one Voyager 2 imaged in 1989.
Rhea’s (Rings!? 6 March ‘08)
They are not stable; they evolve and change over time.
Unless something replenishes them or keeps them from
dissipating, they will not last longer than a few 100
millions years; one of Neptune’s might not last a century.
They generally form inside a planet’s Roche limit.
Object’s that come closer than this distance to a planet
tend to be ripped apart by tidal forces. Since the gas
giants have strong gravitational ﬁelds, they have strong
Shepherding moons are moons that keep a ring system
nice an tidy, by not letting material drift out of a ring and/
or into gaps.
HW Ch 11: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
• Ch 11, pp. 263-264: 9.
• Ch 13: Image Analysis Quiz accessible from:
Due at the beginning of next class period.
Be working your Solar System project.