Atmospheric Model Backs Galileo Jupiter Data

July 22, 1996

Scientists at The Johns Hopkins University have recreated Jupiter's distinctive atmospheric bands in a laboratory model, supporting evidence from the Galileo spacecraft that the patterns are formed from an internal heat source, not the sun's radiation.

Until Galileo sent a probe into the Jovian atmosphere in December 1995, two theories competed equally to propose how the jets of fast-moving winds might have been created. One theory said the patterns might 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 atmosphere's upper surface.

At Johns 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 are detailed in a scientific paper, written by Olson and former Hopkins visiting researcher Jean-Baptiste Manneville, to be 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. 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," Olson said. The sphere's core was chilled, instead of heated, to recreate the proper conditions.

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

The scientists had been working on the experiment for about three years. It is coincidental that their paper is being published shortly after the spacecraft data were discussed, during a recent meeting of the American Geophysical Union.

"The timing with Galileo was rather fortuitous," Olson said.

Now that he 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?"

The research was funded by the National Science Foundation.

Johns Hopkins University

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