Turbulence Ho!

From the American Institute of Physics News No. 603.

HOW JUPITER GOT ITS STRIPES. A new study of turbulence in the atmosphere around a rotating sphere is helping to explain the dramatic stripes on Jupiter, Saturn, and the other giant planets. On Earth, turbulence caused by solar heating and friction with the ground disrupts atmospheric flows and dissipates the energy provided by the sun that might otherwise lead to the formation of circulating, global cloud bands. In the thin atmospheres of gas giants, however, energy dissipation is small, and some of the sun's energy is gradually collected in stable, global jets that trap clouds and form planetary stripes.

Researchers at the University of South Florida and Ben-Gurion University of the Negev (Israel) have now developed a model that shows how planetary rotation and nearly two-dimensional atmospheric turbulence may combine to create large scale structures. Scientists have long suspected that the interaction between planetary rotation and large-scale turbulence governs the banded circulations on giant planets. The new research has quantified the phenomenon, leading to an equation that characterizes the distribution of energy among different scales of motion, and to simple formulae that describe basic energetic features of giant planets' circulations. The model helps explain the paradoxical observation that the outer planets have stronger atmospheric flows, even though the energy provided by the sun to maintain such flows decreases with increasing distance from the sun. The researchers (B. Galperin) have found that the atmospheres of distant planets dissipate even less energy than their warmer sisters. Although the outer planets receive less energy from the sun, they keep more of the energy they receive. As a result, the model shows why Neptune has the strongest atmospheric circulation of all the gas giants even though it is the farthest of the bunch from the sun. (S. Sukoriansky, B. Galperin, N. Dikovskaya, Physical Review Letters, 16 September 2002)