Archive for Thursday, May 23, 1996


May 23, 1996


A KU scientist and a colleague study Jupiter's inner halo to explain where it comes from and how it works.

Go no farther than the kitchen sink to get a sense of a long-awaited theory about the workings of Jupiter's peculiar environment.

Researchers Tom Cravens of Kansas University and Mihaly Horanyi of the University of Colorado collaborated on a project to answer questions that puzzled scientists since the planet's dusty ring was discovered 16 years ago by two NASA Voyager space probes.

The duo focused on Jupiter's ring structure -- especially a faint doughnut-shaped halo extending from the ring toward the planet.

"That halo continually drains out and fills, as if a faucet came from the big ring," said Cravens, professor of physics and astronomy.

Cravens and Horanyi believe that dust particles linger in the ring only hours or days before drifting toward the planet. Older estimates had yielded dust lifetimes of 100 years or more.

Draining Jupiter's 200-mile-thick ring creates a halo, or torus, 3,000 miles thick, the researchers said. This process is thought to be unique in the solar system.

A paper on their work was published today in an issue of Nature, one of the world's leading weekly science journals.

Thomas Armstrong, KU professor of physics and astronomy, said the new study would generate more than academic intrigue.

"It's important to understand Jupiter. It's the largest planet in the solar system," said Armstrong, who had no part in the Nature article. "In many ways, its presence probably has determined much of Earth's early evolution."

The NASA-sponsored study incorporating sophisticated mathematical models and Voyager data indicated to Horanyi and Cravens the source of dust wasn't Jupiter's volcanically active moon, Io.

Instead, they said, dust grains come from the bombardment of tiny rock moonlets within the ring. The largest moonlets are Metis and Adrastea, each about 10 miles across. The smallest dust grains have diameters smaller than the thickness of a human hair.

"Just as a 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 micrometeors," Horanyi said.

The researchers concluded the halo forms when solar radiation gives the dust grains a positive electrical charge. That charge causes grains to oscillate inside the ring before losing momentum and spiraling rapidly toward Jupiter's surface, flaring the ring's inner edge.

Scientists had thought the planet's hot, gaseous environment was dominated by negatively charged Io dust.

Jupiter's ring is 150,000 miles long and 3,500 miles from the outer to inner edge. The ring is invisible from Earth. The planet's diameter is 89,000 miles and it has a mass 317 times that of Earth. While Saturn's prominent rings were observed more than 300 years ago, rings around Jupiter, Uranus and Neptune were confirmed following the Voyager tour of the solar system beginning in 1977.

Cravens, a KU faculty member since 1988, expects some researchers to reject this new theory about Jupiter.

However, he hopes NASA's Galileo spacecraft can relay more information about Jupiter's environment to Earth to enhance study of the halo. The 2 1/2-ton Galileo reached Jupiter Dec. 7 after a six-year journey and is slowly relaying data.

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