Many of the tiny dust grains dominating Jupiter's peculiar ring may linger for less than a day before drifting down toward the planet, according to a new study co-authored by a University of Colorado at Boulder astrophysicist.
Mihaly Horanyi of the Laboratory for Atmospheric and Space Physics said the surprising finding is part of a larger effort to explain the odd structure and behavior of the dusty ring. Jupiter's solitary ring has perplexed planetary scientists since its discovery 16 years ago by NASA's two Voyager spacecraft, he said.
The dust grains in the ring are believed to be created by the bombardment of large moonlets within the ring by much tinier "micro-meteors," he said. While the outer edge of Jupiter's ring is only about 200 miles thick, its inner edge balloons into a donut-shaped "torus" some 3,000 miles thick, making it unique in the solar system, he said.
The new study, which incorporates mathematical models and existing Voyager data, indicates the swelling of the inner ring is caused by positive electrical charges on the dust grains resulting from solar radiation, he said. The process causes the grains to oscillate inside the ring before losing momentum and spiraling toward Jupiter's surface, flaring the ring's inner edge like an inner tube.
A paper on Jupiter's ring dynamics and structure by Horanyi and University of Kansas researcher Thomas Cravens will be published in the May 23 issue of Nature, one of the world's leading weekly science journals.
"When these particles lose energy, they appear to be transported from the ring into Jupiter's atmosphere very rapidly," Horanyi said. "The lifetimes of the smaller particles appear to be on the order of days or even hours, which is unheard of in ring dynamics."
Jupiter's ring is about 150,000 miles in circumference and about 3,500 miles from the outer to inner edge. Its brightness is due to roughly equal parts of the tiny dust particles and larger boulders and moonlets, said Horanyi. The largest of the moonlets are Metis and Adrastea, each more than 10 miles across and located on the inner and outer edges of the ring.
The smallest dust grains in the ring have diameters even smaller than the thickness of a human hair, said Horanyi. The smoke-like dust in Jupiter's ring is so fine that if it was gathered up and compacted into a block of material, the block would be only about the size of a small office building on Earth.
Horanyi likened the dust-manufacturing process in Jupiter's ring to that of a dump truck speeding down the road. "Just as the truck would generate a continual cloud of dust as it sped over bumps, dust is generated continually from the main ring of Jupiter by constant collisions with micro-meteors," he said.
Scientists had thought the hot, gaseous environment of the ring was dominated by negatively charged oxygen and sulfur ions drifting inwards from Jupiter's volcanically active moon, Io, he said. The process was believed to result in negatively charged dust particles within the ring.
But the new modeling study indicates most of the charged atomic particles in the ring region probably arrived from Jupiter's ionosphere -- the region where atomic particles are trapped by Jupiter's magnetic field, he said. Since the density of the electrically charged gas in the ring is so low, ultraviolet radiation from the sun is likely providing the dust grains with a positive charge, according to calculations by the two researchers.
The researchers found the "brightness distribution" of the ring produced by the mathematical modeling effort closely matches the brightness distribution observed by the Voyager spacecraft, said Horanyi. "This suggests our model captures the most important processes that shape this ring," he said.
While Saturn's prominent rings were first observed more than 300 years ago, the fainter, less spectacular rings around Jupiter, Uranus and Neptune were only discovered following the Voyager tour of the solar system beginning in 1977.
The research effort by Horanyi and Cravens was supported by grants from NASA.