Planetary Rings Are Ephemeral
Solar System objects. Ring particles are constantly colliding
with one another in their tightly packed environment, either
gaining or losing orbital energy. This redistribution of
orbital energy can cause particles at the ring edges to leave
the rings and drift away,
aided by nongravitational
influences such as the
pressure of sunlight. Although various orbital resonances
with moons may help guide the orbits of ring particles and
delay the dissipation of the rings themselves, at best this
condition can be only temporary. Most planetary rings
eventually face their inevitable fate: total dissipation. At
least one ring, however, seems immune from this eventual
demise (Math Tools 11.3).
Ring systems have come and gone over the history of the
Solar System. Some astronomers have argued that Saturn’s
bright rings are geologically ancient; others, that the rings
are relatively recent. Even Earth may have had several
short-lived rings at various times during its long history.
Any number of comets or asteroids must have passed within
Earth’s Roche limit (about 25,000 km for rocky bodies
and more than twice that for icy bodies) and disintegrated
catastrophically into a swarm of small fragments, creating
temporary rings in the
past. But Earth lacks shepherd
moons to provide
orbital stability to rings.
Interactions between ring
particles would have caused such a ring to spread out and
dissipate. In addition, the inner parts of a ring around Earth
would feel the drag of Earth’s extended atmosphere and
spiral inward, creating spectacular meteor displays as they
fell. A similar absence of small inner moons also prevents
Venus and Mercury from keeping rings over geological
timescales. Although no ring around Mars is known, its
two tiny moons—Phobos and Deimos—could, in principle,
shepherd a collection of orbiting debris.
Planetary rings in the outer Solar System will continue to
form and dissipate as long as the giant planets maintain the
small moons that provide temporary ring stability. Whether
any will rival the splendor of Saturn’s bright-ring system that
is seen today is, of course, unknown. We may be living