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UA astronomers uncover secret to Jupiter's rings

By Sarah Spivack
Arizona Daily Wildcat
September 21, 1998
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Jennifer Etsitty
Arizona Daily Wildcat

Planetary Sciences Professor Dr. Richard Greenberg stands next to Galileo images showing rings of Jupiter, Thursday in the lobby of the Kuiper Space Sciences Building. Greenberg was part of a 12-person team that began designing a digital camera for Galileo in the 1970s.

UA astronomers on the Galileo imaging team have solved the mystery of Jupiter's rings.

Drs. Richard Greenberg and Alfred McEwen, UA astronomers working with scientists from the National Optical Astronomy Observatories in Tucson, have discovered how Jupiter's rings are formed and sustained.

Particles that make up Jupiter's rings are falling into the planet, drawn by its gravitational force. At the rate that the rings are losing matter, it will take 100,000 years for them to be entirely gone - pulled to the surface of the giant, multi-colored planet they orbit.

UA astronomers discovered that formation of the rings depend on the rate at which particles impact Jupiter's moons.

The Galileo spacecraft has been circling Jupiter for two-and-a-half years. Greenberg and McEwen were part of a 12-person team that began designing a digital camera for the craft in the 1970s. The Galileo imaging team is finally enjoying the fruits of 20 years of labor and planning.

"There are literally new discoveries made every day," said Paul Geissler, a UA senior research associate who has studied some of Jupiter's larger moons.

The four small inner moons of Jupiter are bombarded by meteoroids - particles the size of grains of sand - which kick up puffs of dust when they hit the moons.

Jupiter's inner moons, Amalthea, Adrastea, Metis and Thebe, are just the right size for kicking things off the surface, Geissler said.

Amalthea, the largest of the inner moons, is 62 miles long - about the size of Long Island, N.Y. The moons do not have enough gravitational force to keep the dust on their surfaces, but are big enough to generate enough matter to keep the rings in existence.

The Voyager spacecraft photographed Jupiter's rings and moons in 1979, but the images from Galileo's camera are far more detailed.

"We knew they (the small moons) were there, but they only appeared as points of light," Greenberg said of the Voyager images.

The Galileo camera pro-vided detailed images of moons and rings orbiting Jupiter.

The main ring is thin, flat and bright when viewed edge-on. Inside this ring is a "halo," a much thicker ring of dust and gas. Extending beyond the main ring glows a fainter "gossamer" ring.

Using long-exposure photography, the Galileo camera has allowed astronomers to see that the gossamer ring is divided into a bright inner ring and fainter outer ring.

The structure of the rings correlates with the orbits of the inner moons. Amalthea orbits at the outer edge of the bright gossamer ring and maintains the ring with dust lost during cosmic impacts.

Thebe orbits at the edge of the fainter gossamer ring and Adrastea and Metis contribute dust to the main ring.

Jupiter's rings are a fluctuating system, Geissler said. Over the 4.5 billion years since the planet was formed, there have been periods when the rings entirely disappeared.

There is no predicting how long Jupiter's current rings will remain in existence, Geissler said.

Sarah Spivack can be reached via e-mail at Sarah.Spivack@wildcat.arizona.edu.