University of Colorado-Boulder

July 28, 1997


A University of Colorado at Boulder planetary scientist has developed a new model to explain the structure of an odd ring around Neptune resembling a string of beads that was discovered by NASA's Voyager 2 fly-by of Neptune in 1989.

According to Larry Esposito of the University of Colorado's Laboratory for Atmospheric and Space Physics, the ring, known as the Adams ring, was formed when a comet collided with a tiny moon. The impact of the crash dispersed matter from the two bodies into a ring of particles orbiting Neptune.

Esposito estimates that the splintered moonlet was less than 6 miles in diameter and at least 300 times smaller than Neptune's largest moon, Triton.

The Adams ring lies roughly 37,000 miles from Neptune's center and is comprised of material varying in size from small moons to specks of dust, he said. The largest amount of material resides in four arcs of densely packed matter that are connected by a sparse ring of finer dust particles.

Esposito's model shows that the arc structure seen in the Adams ring results from such factors as the collision and aggregation of ring fragments, the pull of Neptune's tidal forces, gravitational effects caused by one of the planet's moons, Galetea, and the chaotic motions of the particles in the ring.

Esposito presented his latest findings at the American Astronomical Society's annual Division of Planetary Sciences Meeting July 28 to Aug. 1 in Cambridge, Mass.

Soon after the Adams ring was formed, its fragments began to reassemble, said Esposito, a professor in the astrophysical and planetary sciences department. This clustering of material is countered by Neptune's tidal forces and the gravity of Galetea, which lies just inside the Adams ring's orbit.

Galetea's gravity causes the ring's particles to "resonate," or synchronize with the motion of Galetea, he said. Particles resonating with Galetea interact only with fragments in their own arc, slowing the aggregation process.

If the motion of the ring particles were purely random, the ring would be uniform around the planet, Esposito said. Instead, the ring is "clumpy," a result of the chaotic diffusion of particles within the ring that allows them to jump unpredictably from one arc to another.

Esposito's model predicts the ring material will come together to reform the original moonlet within the next 10,000 years, and the moonlet will likely be struck by another comet within a million years. The ring images from Voyager 2 probably represent an intermediate stage in the process of the moonlet's reassembly.

"The likelihood of Voyager 2 having the good fortune to view this arc structure is similar to the chance of seeing Old Faithful erupt while driving through Yellowstone Park at 60 miles per hour," he said. Recent results by CU's planetary rings and moons research group "emphasize the major role that chance events played in the history of the solar system," said Esposito, who has spent more than 15 years working on planetary rings.

"This is slow science," he said. "The pictures sent to Earth by Voyager created instant excitement, but understanding the complicated story behind the images takes considerably more time."

Thorough testing of the new Adams ring model will require another NASA mission to Neptune, Esposito said. In the meantime, he will continue his computer simulations of the Adams ring.

Esposito was an investigator on two CU-Boulder instruments that flew on Voyager and is an investigator on two CU instruments on Galileo. He also is the chief scientist on an ultraviolet spectrometer slated for launch on NASA's Cassini Mission to Saturn in October 1997.

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