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How Jupiter Got Its Stripes
10 May 2010 5:59 pm
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.
To make a fake gaseous planet, the researchers hollowed out a spherical void in a cylinder of stretchy silicon and filled it with water, which mimics the gas that makes up gas giants. They seasoned the water with asymmetrical flakes of mica coated with titanium dioxide, which aligned with the flow and reflected laser light more in some directions than others, letting the researchers image the flows. Tilgner and colleagues next set the cylinder rotating and applied artificial tides. An effect of, perhaps, an orbiting moon, tides slightly deform a spherical planet, making it oval with one end pointed at the moon and the other pointed away. To reproduce that effect, the team squeezed the cylinder between a pair of rollers, which could circle the cylinder at a rate different from the one at which the cylinder spun on its axis.
And the team found what it expected. For certain ratios of the rate at which the cylinder spun and the rate at which the roller went around, the flow would break up into distinct cylindrical columns and give the “planet” a striped appearance, as the researchers report in a paper to be published in Physical Review Letters. In the case of Jupiter, the tides are small, notes lead-author Cyprien Morize, who is now with CNRS at Université Paris-Sud 11, Orsay, “so the tides are not the whole story, they’re just one part of it.” But they could play a more important role on Jupiter’s moons, such as Io or Europa.
“I think it’s a cool idea and it’s definitely worth checking it out in more detail,” says Adam Showman, a planetary scientist at the University of Arizona, Tucson. However, to determine whether it applies to Jupiter, researchers would have to extrapolate to the precise hydrodynamic conditions there, he says. And Rhines agrees with Morize that the idea is likely to be more applicable to bodies like Jupiter’s moons. He cautions, however, that in a simple experiment like this one, almost any sort of stirring will suffice to produce such cylindrical flows. So the result hardly rules out other explanations for the stripes.
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*This article has been corrected. An earlier version stated that the flakes used in the experiment, conducted at the Institute for Research on Non-Equilibrium Studies at Aix-Marseille University, were made of plastic. They were made out of mica coated with titanium dioxide.