When a probe from the Galileo spacecraft took its death plunge into Jupiter’s atmosphere last December, it was buffeted by 400-mile-an- hour winds some 80 miles below the giant planet’s cloud tops. These winds aren’t easy to explain. Winds on Earth are generated by the heat of the sun. But solar heat can’t penetrate very far into Jupiter’s thick clouds. Even before Galileo reached Jupiter, astronomers wondered what was driving such turbulent Jovian weather systems as the huge storms we see as the Great Red Spot and the bands of clouds roiling through the planet’s atmosphere. Some suspected the existence of an internal heat source-- perhaps energy released by the gravitational compression of gases deep within the planet. That theory has just gotten a boost: two astronomers have simulated Jupiter’s distinctive bands using a scale model of the planet.
The model consists of a copper shell nested inside a foot-wide Plexiglas sphere. Chilled antifreeze fills the nine-inch-wide shell; water fills the one-and-half-inch gap between the copper and the Plexiglas. The apparatus is mounted on an electric motor and rapidly spun. You basically spin the sphere so that it becomes a centrifuge, in which the direction of the effective gravity is not pointing toward the sphere’s center, but radially outward, says Peter Olson of Johns Hopkins, one of the creators of the model. That makes a good model for Jupiter--except that gravity is pointing the wrong way.
Olson and his colleague Jean-Baptiste Manneville compensated for this reversed gravity in another aspect of their model. To simulate convection in Jupiter’s atmosphere, which on the planet itself would be driven by a heat source near the planet’s core, the astronomers cooled the center of their model with the antifreeze, so that the heat in their model flows in the opposite direction of the centrifugal force, just as heat on Jupiter flows opposite the pull of gravity.
After the model was set spinning and had begun to cool, Olson and Manneville injected a fluorescent dye into the layer of water between the Plexiglas and the copper shell. The idea was to see if the temperature difference between the antifreeze and the water would create convection currents that would produce Jupiter-like bands of turbulence. When the astronomers bathed the model in ultraviolet light, distinctive bands indeed appeared. They glowed brightly where convection was sluggish, which kept the dye at the outermost part of the watery atmosphere. But where convection was more vigorous, the dye was carried down away from the surface, creating a dark band.
The Galileo result indicates that the atmospheric circulation on Jupiter is not just due to solar heating, says Olson, and this experiment shows that the concept of bands produced by heat convection is plausible.