Saturnian equinox. Note the extremely narrow ring shadow. Credit: NASA/JPL/Space Science Institute.

 

Spring Has Sprung
Apr 26, 2010

On August 11, 2009, Saturn's rings turned edge-on to the Sun, something that happens once every 15 years as Saturn revolves. When the rings cross the equatorial plane, they cool down considerably—dropping more than 50 Kelvin in the last five years.

Many things about Saturn have changed in the 29 years since the two Voyager spacecraft passed by the giant gas planet. Saturn's magnetosphere grew larger by more than a million kilometers and then contracted, only to begin expanding again. The spokes in Saturn's B ring disappeared and then reappeared. The equatorial thunderstorm (known as the Dragon Storm) that raged continuously broke up, moved toward the poles, and then erupted again a short while ago in the lower latitudes.

The most likely explanation for the storms on Saturn is that they are equivalent to sunspots. As the Sun changes its behavior over the course of a 22 year cycle, the electrical output that connects it with its family of planets varies. If Saturn's Great White Spots, Dragon Storm, and ring spokes are driven by the same galactic Birkeland currents that drive the Sun, they should get stronger and closer to the equator as the sunspot cycle oscillates. It appears that that is just what has happened over the past three decades.

It is significant that the increases to the energetic phenomena on Saturn are occurring irrespective of whether the sunspot cycle is at maximum or minimum: the Sun is exhibiting large-scale electrical effects although it has just passed solar minimum. It was suggested in a previous Picture of the Day that it is the change in magnetic orientation that could be the key, rather than the strength of the solar magnetic field.

In 2008, scientists from the Max Planck Institute discovered partial rings around the gas planet in a nearly invisible collection of small arc segments. Using the Cassini-Equinox magnetospheric sensors, the team found two areas near Saturn where highly energetic electrons suddenly drop out, presumably because something is absorbing them.

The magnetospheric imager detected lower energy emissions from the ion torus that surrounds Saturn when the ring arc segments are oriented between Cassini and the electron flow. The region of absorption is almost 3000 kilometers wide, so Cassini mission analysts speculate that there could be unseen rings in several bands.

The invisible rings are most likely another manifestation of the electrical environment that exists near Saturn. What Cassini is detecting most likely supports the charged particle stream hypothesis originally proposed by Kristian Birkeland in 1913. He wrote: "It seems almost incredible that such a ring of cosmic dust should be able to exist for ever, so to speak, without other governing forces than gravitation..."

Seen from our planet, the view of Saturn's rings during equinox makes them invisible. However, Cassini is in orbit around Saturn 20 degrees above the ring plane. As NASA puts it, that "novel illumination geometry" causes objects above the rings to cast shadows across them. Cassini photographed the shadows from several of Saturn's moons, but also saw the shadows cast by something new: vertical structures embedded in the rings.

Planetary scientists already knew about material sticking out of the rings in a few places, but until Saturn reached its equinox it was not possible to measure how high these "ridges" are. Saturn's main rings are approximately 140,000 kilometers wide, but were only thought to be between 10 and 30 meters thick until the discovery of the vertical "walls" extending along the edges of some rings. Some of the other newly found formations are ripples over four kilometers high.

How did these strange clumps, undulations, and ridges form? According to theories presently under consideration, it is collisions and shock waves that were and are the culprits. The gravitational attraction of so-called "shepherd moons" is also said to be responsible.

As some moons, such as Daphnis, move up and down through the ring plane, they do disturb the motion of the ring particles. However, as was discussed in another Picture of the Day article, Saturn's rings and moons are electrically charged objects moving within its vast plasmasphere. The instabilities inherent in that system most likely contribute to the formation of unusual features.

As shepherd moons pass close to the rings, their effect is not like a turbulent wind, which would be expected when a gravitational field's torque effects act 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.

More data needs to be recovered, but Cassini continues to provide an embarrassment of riches for Electric Universe advocates, and a box of puzzles that consensus opinions cannot assemble.

Stephen Smith