The Solar System

F ring close up

Visual-wavelength image

PandoraPandora

PrometheusPrometheus

Because of collisions among ring particles, planetary rings should spread outward.
The sharp outer edge of the A ring and the narrow F ring are confined by
shepherd satellitesthat gravitationally usher straying particles back into the rings.

Some gaps in the rings, such as the Cassini Division, are caused by
resonances with moons. A particle in the Cassini Division orbits Saturn twice
for each orbit of the moon Mimas. On every other orbit, the particle feels a
gravitational tug from Mimas. These tugs always
occur at the same places in the orbit and force
the orbit to become slightly elliptical. Such an
orbit crosses the orbits of other particles,
which results in collisions, and that
removes the particle from the gap.

The F ring is clumpy and

sometimes appears braided

because of two shepherd

satellites.

The Encke Gap is not

empty. Note the ripples

at the inner edge. A

small moon orbits inside

the gap.

Encke Gap

Waves in the A ring

Visual-

wavelength

images

Saturn does not have enough

moons to produce all of its

ringlets by resonances. Many

are produced by tightly wound

waves, much like the spiral

arms found in disk galaxies.

How do moons happen to be at just the right

places to confine the rings? That puts the

cosmic cart before the horse. The ring particles

get caught in the most stable orbits among

Saturn’s innermost moons. The rings push

against the inner moons, but those moons are

locked in place by resonances with larger,

outer moons. Without the moons, the rings

would spread and dissipate.

Saturn’s rings are a very thin layer of particles

and nearly vanish when the rings turn edge-on to

Earth. Although ripples in the rings caused by waves

may be hundreds of meters high, the sheet of particles

may be only about ten meters thick.

This image was recorded by

the Cassini spacecraft looking

up at the rings as they were

illuminated by sunlight from

below. Saturn’s shadow falls across

the upper side

of the rings.

This combination of UV images has been

given false color to show the ratio of

mineral material to pure ice. Blue regions

such as the A ring are the purest ice, and

red regions such as the Cassini Division

are the dirtiest ice. How the particles

become sorted by composition is

unknown.

A ring

Cassini Division Encke Gap

This combination of UV images has been

given false color to show the ratio of

mineral material to pure ice. Blue regions

such as the A ring are the purest ice, and

red regions such as the Cassini Division

are the dirtiest ice. How the particles

become sorted by composition is

unknown.

F ring

A ring

Cassini Division Encke Gap

Ultraviolet imageUltraviolet image

NASA/JPL/Space Science Institute

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