Tidal stresses are thought to be responsible for much of
the material found in planetary rings. If a moon (or other
planetesimal) orbits a large planet, the force of gravity will
be stronger on the side of the moon close to the planet, and
weaker on the side farther away (Chapter 4). This difference
in gravitational force stretches the moon out. If the tidal
stresses are greater than the self-gravity that holds the moon
together, the moon is torn apart. The distance at which the
tidal stresses exactly equal the self-gravity is known as the
Roche limit. If a moon or planetesimal comes within the
Roche limit of a planet, it is pulled apart by tidal stresses,
leaving many small particles to orbit the planet. The fragmented
pieces of the disrupted body are then distributed
around the planet in the form of a ring. The composition of
the ring will, of course, be the same as that of the fated body.