Saturn's Rings and the Roche Limit
- Planetary rings consist of a huge number of small particles all
independently following their orbits around the planet.
- Why don't the individual ring particles accumulate together
to form a single satellite?
- The reason lies entirely in how the gravitational attraction
between the planet and the ring particles compares with the
ring particle's gravitational attraction to each other.
- The planet tries to tug the ring particles apart, while
the particles themselves are trying to pull themselves together
via their own gravity.
- The distance the particles are from the planet determines
which process dominates.
- Far from the planet the mutual gravity of the particles
can pull them together.
- Close to the planet, the planet's gravity prevents the
particles from pulling themselves together.
- The borderline distance which separates the regions where
rings exist from the region where satellites form is called the
- Planetary rings are always found inside the Roche Limit.
- Sometimes satellites can be found inside the Roche limit.
- Their internal cohesive forces can resist tidal disruption.
- Rocks can resist tidal disruption. Rubble piles cannot.
- These moons inside the Roche limit may explain the origin of the rings. If a collision disrupts a satellite it produces the material for a new ring.
- Their gravitational influence can "shepherd" and maintain
- Much of the structure and gaps in the rings are induced by "orbital resonance" with the inner satellites.
- The "Cassini Division" is created by a 2:1 resonance with Mimas.
- Just like planets in orbit around the Sun, the more distant a ring particle is from Saturn the longer it takes to orbit.
- Ring particles at the location of the Cassini Division
orbit twice for every one orbit of Mimas.
- These particles feel a regular tug from Mimas, clearing them
from the region of the Cassini division.
- Many other gaps in Saturn's rings can be traced to this same effect.
- Other gaps can originate from satellites embedded in the rings.
- The same process may create gaps in the dust debris disks surrounding stars with planetary systems.
- The structure seen in these "debris disks" has been used to infer the presence of planets around such stars.
Revised December 1, 2006