The oddity in the rings of Jupiter

The Baltimore Sun

When it comes to planetary rings, Saturn gets all the glory.

But Douglas Hamilton belongs to a small fellowship of ring scientists fascinated by the Rodney Dangerfield of rings - the bands that circle Jupiter.

The University of Maryland astronomer says there's a reason people don't realize that Jupiter even has rings: They're made of particles no bigger than grains of sand - with 10 to 15 yards between the grains.

"They're poor excuses for rings, because there's not much in there," he concedes. But these days, Hamilton's work in this arcane corner of the solar system is paying off.

His analysis of Jupiter's rings, published today in the prestigious British scientific journal Nature, is drawing praise for resolving a 10-year-old mystery: why the dusty and barely visible bands around the gas giant behave so strangely.

"It brings together a lot of interesting concepts in a plausible way," said Joseph Burns, a Cornell astronomer who published authoritative findings on Jupiter's rings in 1999. "It's a very clever idea."

Jupiter's rings were first detected by the Voyager missions in 1979, but it wasn't till 1998 that the Galileo spacecraft captured enough detail to confirm theories about how they formed - when interplanetary dust smashed into four of Jupiter's inner moons.

Over thousands of years, the pounding produced lots of material, forming rings that generally follow the orbit of the parent moon. The Galileo spacecraft revealed two inner rings, as well as two outer "gossamer rings," formed when space debris collided with the more distant "moonlets" of Amalthea and Thebe.

But the spacecraft also detected an anomaly in the rings: a protrusion of dust outside Thebe. If the dust particles that formed the rings stayed in circular orbits, the dust - being pulled toward Jupiter by its gravity - should all be inside the orbit of Thebe, the most distant of the inner moons that formed the rings. "The material is drifting out from Thebe, and we couldn't explain that," Burns said.

The phenomenon, called the Thebe extension, has puzzled Hamilton ever since. "It's just been straight-up curiosity," Hamilton said. "We saw this happening and asked, 'Why does it look like that?' "

Hamilton and Harald Krueger, a German researcher at the Max Planck Institute in Heidelberg, came up with a computer model that shows why the dust acts so erratically.

Electromagnetic forces cause the dust to travel in an eccentric orbit - giving it a slightly elliptical path as speeds around Jupiter. As individual dust particles shoot from sunlight into Jupiter's shadow, their electromagnetic charge shifts from positive to negative, giving them a boost along the way - comparable to pushing a playground swing, Hamilton said.

In the paper, Hamilton and Krueger call the effect shadow resonance. They think it may explain also some of the more mysterious features of Saturn's lesser, dusty rings.

Hamilton said Nature published the report because the researchers' computer model matched up with real-world data that Galileo collected before its 14-year mission concluded in 2003 with a fiery suicide dive into Jupiter's crushing atmosphere. He plans to discuss the findings today at an American Astronomical Society gathering in Boulder, Colo.

So why bother studying Jupiter's or Saturn's rings?

Burns and Hamilton's answer: It adds to our understanding of how our solar system formed. The gravitational forces that accrete dust, debris and matter to form rings around planets are the same forces that formed the Earth and our neighboring planets billions of years ago. "The same sort of physics tend to go on in the galaxies, all over the place," Burns said.

Besides that, the work is just plain interesting. "It's a lot like what explorers were doing three or five centuries ago," Burns said.

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