Rings Around The Planets: Recycling Of Material May Extend Ring Lifetimes(Forwarded)

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University of Colorado-Boulder
Boulder, Colorado

Contact:
Larry Esposito, (303) 492-5990,
Joshua Colwell, (303) 492-6805
Jim Scott, (303) 492-3114

Dec. 8, 2003

Rings Around The Planets: Recycling Of Material May Extend Ring Lifetimes

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.