How the planets got their stripes. Stripes observed on the disks of gaseous giant planets (Jupiter, Saturn, Uranus and Neptune) are formed by trains of clouds transported by organized, steady atmospheric currents. Typically, atmospheric turbulence would break the clouds down into smaller and smaller features until no large features were visible. However, in thin atmospheres the effect is opposite. The rotation of the planets combines with the turbulence to create the large-scale structures we see as rings, stripes and spots.
This paper is due to appear in Physical Review Letters in the September 16 print issue.
(From my Physics Tip Sheet)
The semi-technical version
This text was written for us at the American Physical Society by the authors of the research paper.
Stripes observed on the disks of gaseous giant planets (Jupiter, Saturn, Uranus and Neptune) are formed by trains of clouds transported by organized, steady atmospheric currents. There are no obvious energy sources sustaining such circulation. Kinetic energy introduced on smaller scales is expected to be dissipated by chaotic turbulent motion rather than contributing to the highly organized large-scale circulation.
What factors sustain that circulation? Typically, turbulent motion features large vortices that break down to smaller and smaller ones (direct energy cascade) until the smallest of them dissipate by the action of molecular viscosity. In thin planetary atmospheres, however, the dynamics is completely different. Turbulence gains very peculiar properties; instead of dissipating energy, it transfers it to large-scale flow configurations giving rise to inverse energy cascade.
The rate of vertical rotation on spherical planets changes with latitude (the so-called beta-effect) giving rise to planetary (or Rossby) waves and rendering the inverse cascade even more peculiar. Due to the intricacy of interaction between turbulence and Rossby waves, the inverse energy cascade becomes redirected into alternating, very steady latitudinal (zonal) jets that contain most of the energy of the large-scale circulation. These jets trap the clouds and appear as the stripes on the planetary disks.
The physical mechanisms that cause the emergence of the jets and regulate their behavior have long remained a mystery. This research has identified the physical law that governs the behavior of two-dimensional turbulence with a variable rate of rotation and has given it mathematical quantification. This law relates the energy level in every spatial component to such basic planetary parameters as the planet's radius and the angular velocity of rotation. Using this law, we have developed simple relationships between various flow characteristics. Surprisingly, we found that the total kinetic energy of the planetary circulation depends not on the rate of energy supply to the atmosphere but on the rate of energy dissipation. If the dissipation is small, even a minuscule forcing can spin up a very strong circulation over a long time.
Assuming that colder planets (those farther away from the Sun) have smaller dissipation due to weaker thermal activity, our research explains why the giant planets' atmospheres reveal increasing intensity of circulation the farther they are from the Sun. Neptune, the farthest giant planet away from the Sun, proves this point by having the strongest circulation in the Solar system. The new law is of fundamental importance not only in the theory of two-dimensional turbulence but also in atmospheric sciences and planetology. The results explain some of the fascinating features of atmospheric circulations on gaseous giant planets. The importance of these results is further underscored by their universality. When appropriate data becomes available, the new theory can be applied to gain insight into the atmospheric circulation on extra-solar giant planets and obtain quantitative information on the basic features and energetics of these circulations. [David Harris: Science news]
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