Strange vertical structures in Saturn's B ring. Credit: NASA/JPL/Space Science Institute

 

Where the Long Shadows Fall
Apr 27, 2011

A recent press release argues that Saturn's rings could behave like spiral galaxies. The same forces that create galactic "arms" might be responsible for the unusual features that have been seen rising vertically from the gas giant's ring plane, as well as for the oscillations in the B ring.

According to Carolyn Porco of NASA's Cassini-Equinox mission team: "We have found what we hoped we'd find when we set out on this journey with Cassini nearly 13 years ago: visibility into the mechanisms that have sculpted not only Saturn's rings, but celestial disks of a far grander scale, from solar systems, like our own, all the way to the giant spiral galaxies."

Cassini has been in orbit around Saturn since July 1, 2004. On August 11, 2009 the spacecraft was in position to observe the giant planet's equinox, when its rings turned edge-on to the Sun, something that happens every 15 years. During that phase, several complex configurations were seen within the rings: so-called "propellors," ridges, and waves rising up as high as four kilometers. Since the rings had been long thought to be about twenty meters thick, anomalous meta-stable shapes of such dimension were a complete surprise to mission specialists.

How do clumps, undulations, and ridges form? Researchers suggest collisions and shock waves initiate the resonant vibrations. Gravitational attraction from so-called "shepherd moons" is said to be an additional source of influence. Small moons, such as Daphnis, do move up and down through the ring plane, affecting the motion of ring particles.

However, a far stronger force than gravity is neglected in their speculations: Saturn's rings and moons are electrically charged objects moving within its vast plasmasphere. Instabilities inherent in that system probably contribute to the formation of the perpendicular features.

The effect of shepherd moons is not like a wind. Gravitational torque is not seen acting on a cloud of fine particles. Instead, sine waves, perpendicular "braids," and cylindrical arcs are seen. Some are multiply woven, like those in the remote F ring. In fact, NASA scientists now think that the observed oscillations in the B ring are not caused by moons or any other body. Instead, "unforced 'free' waves grow on their own and then reflect back again at the edge."

Those waveforms are also thought to exist within spiral galaxies. So-called "density wave theory" was designed to explain how mass variations within a galaxy, along with tidal forces from other galaxies, can induce ordered structure like spiral arms. Although those motions can never be observed on the galactic scale, only in computer simulations, it is assumed that a small-scale version is occurring in Saturn's rings.

Important factors are not considered in this theory. Bodies immersed in plasma are not isolated, they are connected by circuits. Most of the time they are not in equilibrium because they are in unstable conditions. The majority of them are moving across the plasma filaments that exist in the Solar System, in the plasmaspheres around planets, or in interstellar and intergalactic space. Currents in plasma contract into those filaments and the force between filaments is linear, so the electromagnetic fields created by them are the most powerful long-range attractors in the Universe, as well as short-range repulsors.

Those magnetic fields also trace out the spiral arms in galaxies because electric current flows through them, both from the intergalactic circuit feeding the galaxy and from the homopolar action of the galaxy itself. The magnetic fields exist because the spiral arms behave as large Birkeland current filaments.

Since, like a galaxy, Saturn's ring plane possesses a magnetic field, diocotron instabilities might occur in the same fashion as seen in NGC 3646, for example. It is in this way that Saturn and galaxies relate.

Stephen Smith