11.5 Gravity and Ring Systems
Ring systems consist of countless numbers of small particles—
varying in size from tiny grains to house-sized
boulders—that orbit individually around the giant planets
like so many tiny moons. As is the case with the moons
discussed in Math Tools 11.1, Kepler’s laws dictate that
the speed and orbital periods of all ring particles vary
Gravitational interactions also occur between ring particles
themselves. These interactions determine the ring
shapes at the edges of gaps. Since the shapes of ring edges
are determined by gravity, scientists can use these shapes
to estimate the masses of the rings. Even though planetary
rings can be large and prominent, they account for only
the tiniest fraction of the mass of the material around a
giant planet. Saturn’s bright rings are by far the most massive
rings in the Solar System. They contain more material
than all other planetary
rings combined. Even so,
their total mass is estimated
to be less than that
of Mimas, Saturn’s small
icy moon that is less than 400 km in diameter. The amount
of material in the Solar System’s narrow rings is, of course,
much less. All of the particles in the largest ring of Uranus,
the Epsilon Ring, could be compressed into a single body no
more than 20 km across. All of the material in both the
Neptune rings and the Jupiter rings could fit into single
objects only a few kilometers in diameter.
Each time a moon nudges a ring particle inward, the
minute amount of energy lost by the particle is absorbed
by the more massive moon, causing the moon to move imperceptibly
farther from the ring and the planet. Over time,
a moon may move so far from the ring edge that it can no
longer provide stability to the ring. The ring is then free to
dissipate. In the case of Saturn’s bright rings, the situation
is more complicated because Janus itself is also in resonance
with other moons of Saturn. This means that all of
Saturn’s moons must be pushed away before its bright rings
can dissipate, and therefore the bright rings of Saturn may
be much more stable than most rings are.