Saturn's Secret in a Teacup


The wonderful hexagonal structure at Saturn's north pole has persisted since first viewed by Voyager, 26 years ago. The south pole has an equally unusual structure.

The plasma field surrounding Saturn is interesting, it is precisely aligned with the planet's rotational axis, due to the fact that the magnetic field is precisely aligned thus. The magnetic field rotates at the same speed as the planet.

Saturn's moon Enceladus shoots out water that is ionized by the sun. The heavy oxygen ions generated are captured by Saturn's magnetic field which turns it into a tube ring of energized plasma. As Saturn's magnetic field rotates, it drags the ions around the planet in a counter-clockwise direction.

My friend Vik Sohal suggested that perhaps the persistent storms were similarly formed by electrodynamic forces. This recent Cassini image showing auroral activity over the hexagon lends credence to that notion.

I noticed that the turbulent structure seems to be quite pronounced near the surface of Saturn's clouds. That made me think of this paper by Kinney and McWilliams:

The role of coherent structures in magentohydrodynamic turbulence

R. Kinney, and J.C. McWilliams, Small-Scale Structures in Three-Dimensional Hydrodynamic and Magnetohydrodynamic Turbulance, Springer Berlin/Heidelberg, 1995

Long-term self-similar behavior is exhibited in high-resolution numerical solutions of decaying turbulent two-dimensional magnetohydrodynamics. Random initial conditions produce coherent structures which dominate the fluid dynamics. During the self-similar evolution, current monopoles (magnetic vortices) with an accompanying vorticity distribution (which is axisymmetric but not necessarily monotonic) emerge spontaneously. Thin current and vorticity sheets are created as a result of close encounters between the vortices. The sheets are sites of current enstrophy production, conversion into kinetic enstrophy, and dissipation, all of which maintain constant ratios but which produce no net change in the total enstrophy. The chief mechanism for removal of enstrophy is the disappearance of magnetic vortices during coalescence.

I contacted Dr. McWilliams, and he agreed that perhaps Saturn's atmosphere could be undergoing a process similar to that in his simulations, although he suggested that the prevailing view is that the effect is more likely due to friction. That led me further, to Einstein's explanation of the tea leaf paradox.

Tempest in a teacup

In the tea leap paradox, tea leafs settle in the center of a cup when stirred, rather than at the edges, as one might expect by analogy to a centrifuge. Stirring the tea makes the water spin about the cup's central axis, and spiral out from the center. The water below is slowed by friction with the cup's bottom, and its spin is weakened - this sets up a circulation in the cup where water at the top is spun out, travels down the walls of the cup, and then flow back up the middle, forming a circulating donut.It seems to me that Saturn's spinning ring current is the spoon stirring the cup, lower layers of atmosphere are the bottom of the cup providing the friction to set up the donut shaped convective flow, and persistent coherent structures in magnetohydrodynamic turbulence provide the fascinating detail observed in the tea leaves.

Nick Porcino, Nov. 2008


Content by Nick Porcino (c) 1990-2011