Although rings around planets like Jupiter, Saturn, Uranus and
Neptune are relatively short-lived, new evidence implies that the
recycling of orbiting debris can lengthen the lifetime of such rings,
according to University of Colorado researchers.

Strong evidence now implies small moons near the giant planets like
Saturn and Jupiter are essentially piles of rubble, said Larry
Esposito, a professor at CU-Boulder’s Laboratory for Atmospheric and
Space Physics. These re-constituted small bodies are the source of
material for planetary rings.

Previous calculations by Esposito and LASP Research Associate Joshua
Colwell showed the short lifetimes for such moons imply that the
solar system is nearly at the end of the age of rings. “These
philosophically unappealing results may not truly describe our solar
system and the rings that may surround giant extra-solar planets,”
said Esposito. “Our new calculations of models explain how inclusion
of recycling can lengthen the lifetime of rings and moons.”

The observations from the Voyager and Galileo space missions showed a
variety of rings surrounding each of the giant planets, including
Jupiter, Saturn, Uranus and Neptune. The rings are mixed in each case
with small moons.

“It is clear that the small moons not only sculpt the rings through
their gravity, but are also the parents of the ring material,” said
Esposito. “In each ring system, destructive processes like grinding,
darkening and spreading are acting so rapidly that the rings must be
much younger than the planets they circle.”

Numerical models by Esposito and Colwell from the 1990’s showed a
“collisional cascade,” where a planet’s moons are broken into smaller
moons when struck by asteroids or comets. The fragments then are
shattered to form the particles in new rings. The rings themselves
are subsequently ground to dust, which is swept away.

But according to Colwell, “Some of the fragments that make up the
rings may be re-accreted instead of being ground to dust. New
evidence shows some debris has accumulated into moons or moonlets
rather than disappearing through collisional erosion.”

“This process has proceeded rapidly,” said Esposito. “The typical
ring is younger than a few hundred million years, the blink of an eye
compared to the planets, which are 4.5 billion years old. The
question naturally arises why rings still exist, to be photographed
in such glory by visiting human spacecraft that have arrived lately
on the scene,” he said.

“The answer now likely seems to be cosmic recycling,” said Esposito.
Each time a moon is destroyed by a cosmic impact, much of the
material released is captured by other nearby moons. These recycled
moons are essentially collections of rubble, but by recycling
material through a series of small moons, the lifetime of the ring
system may be longer than we initially thought.”

Esposito and former LASP Research Associate Robin Canup, now with the
Southwest Research Institute’s Boulder branch, showed through
computer modeling that smaller fragments can be recaptured by other
moons in the system. “Without this recycling, the rings and moons
are soon gone,” said Esposito.

But with more recycling, the lifetime is longer, Esposito said. With
most of the material recycled, as now appears to be the case in most
rings, the lifetime is extended by a large factor.

“Although the individual rings and moons we now see are ephemeral,
the phenomenon persists for billions of years around Saturn,” said
Esposito. “Previous calculations ignored the collective effects of
the other moons in extending the persistence of rings by recapturing
and recycling ring material.”

Esposito, the principal investigator on a $12 million spectrograph on
the Cassini spacecraft slated to arrive at Saturn in July 2004, will
look closely at the competing processes of destruction and re-capture
in Saturn’s F ring to confirm and quantify this explanation.
Esposito discovered the F Ring using data from NASA’s Voyager 2
mission to the outer planets launched in 1978.