Johns Hopkins Gazette: September 9, 1996

On Astronomy:
Scale Model
Recreates Jupiter's

Theories: At 1,000
rpms, model spins new
ideas about Jupiter's

Emil Venere
News and Information

Even before the Galileo spacecraft sent a probe into Jupiter's turbulent atmosphere last December, a pair of Hopkins scientists already were studying Jovian weather up close.

Using a fast-spinning, clear-plastic model, the scientists recreated Jupiter's distinctive atmospheric bands in the laboratory, supporting evidence from Galileo that the weather patterns are formed from an internal heat source, not the sun's radiation.

Up until now, two theories competed equally to propose how the speeding jets of wind might have been created. One theory said the patterns could be formed like earthly weather, by the sun's heat warming the atmosphere. But the other proposed that the winds are created by convection currents caused by an interior heat source--the planet's core radiating energy still remaining from Jupiter's birth four and a half billion years ago.

Recently the second theory received a major boost, as scientists analyzing data from the Jupiter probe learned that the wind speeds persist deep into the atmosphere; if the patterns were caused by solar radiation, high wind speeds would be a shallow phenomenon, with considerably lower velocities below the upper atmosphere.

At Hopkins, researchers designed a scale model of Jupiter that lends further support to the idea that the atmospheric bands have deep roots. The model, spinning at 1,000 revolutions per minute, simulates the banding pattern.

"We wanted to see whether we could generate a circulation that looked, at least in some respects, like the circulation in Jupiter's atmosphere," said Peter Olson, a professor in the Department of Earth and Planetary Sciences. The findings were detailed in a scientific paper, written by Olson and former Hopkins visiting researcher Jean-Baptiste Manneville, and published in the August issue of Icarus, a monthly planetary science journal.

The interior-source theory says the winds could be the surface expressions of huge concentric masses of rotating material; the temperature change, or gradient, from the planet's hot core to the cooler outer gases could create these atmospheric currents, which are speeding at roughly 200 miles per hour.

Olson's and Manneville's scale model was a fluid-filled shell surrounding a central core that could have its temperature adjusted. To simulate atmospheric motion and the acceleration of Jupiter's gravity, the sphere, which was about a foot in diameter, was spun with an electric motor at ultra-high speeds.

"It's like an amusement park ride taken to the limit," Olson said.

But centrifugal force pushes objects outward--just the opposite of gravity's inward-pulling direction.

"So, since we've reversed gravity, we've also reversed the temperature gradient in the experiment," he noted. The sphere's core was chilled, instead of heated, to recreate the proper conditions.

The plastic sphere was filled with water, colored with fluorescent dyes and then illuminated with ultraviolet light. As the model was spun at high speeds, the Jovian banding patterns emerged; the number of bands increased as the sphere was spun faster and the temperature gradient was increased.

Now that Olson has Galileo's observational data to back up the experimental results, the next challenge might be to use the model to study a mysterious feature in Jupiter's weather; the fastest-moving jet of wind is located over Jupiter's equator. For unknown reasons, that band has never been created in the laboratory model, and Olson would like to know why.

"We are intrigued now," Olson said. "The next question is, What do we need to do to this model to get this equatorial jet?"

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