ScienceNOW - 10 May 2010
Jupiter and other gaseous planets are covered from pole to pole with stripes. But astronomers aren't exactly sure how they arise. Now a team of physicists reports that Jupiter's stripes may be produced in part by tides—a result of the gravitational tugging of its 60-odd moons—thanks to a very simple laboratory mock-up of the gaseous planet.
Jupiter is striped with so-called zonal winds: broad bands running parallel to the planet's equator in which the prevailing winds blow at different speeds. For decades, scientists have puzzled over exactly where these winds—and the striped pattern they produce—come from. “Even after 40 years, it’s still an active subject,” says Peter Rhines, an oceanographer at the University of Washington, Seattle. Researchers generally think that the zonal winds arise from convection, the tendency of hotter gases to rise and cooler gases to fall, he says, although they don’t agree whether the convection that produces the stripes reaches to the planet’s core or takes places only near the surface.
But perhaps convection has nothing to do with it, argue Andreas Tilgner, a geophysicist at the University of Göttingen, in Germany, and colleagues with France’s National Center for Scientific Research (CNRS) at Aix-Marseille University. Their idea is as follows: Jupiter or any other gaseous planet is essentially a sphere of fluid spinning on its axis. And the repeated tidal tugging of, say, an orbiting moon can cause the flowing fluid to organize itself in a particular way. Specifically, it forms cylindrical zones, or “columns,” one inside another, flowing around the cylinder's axis at different rates. Where these cylindrical zones intersect the surface of the sphere, they produce stripes running horizontal to the equator of the sphere, much like those seen in gaseous planets. Two years ago, Tilgner laid out the mathematics of the theory. Now, he and his colleagues have shown experimentally that it works.