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Stern Saturn - By Nick Kollerstrom
Diameter: 120,536 km
Distance to Sun: 10 AU
Solar Orbit: 29.5 years
No of Moons: 17
When the spacecraft Voyager flew past in 1981 it relayed back the first close-up images of Saturn:
'Then again, the pictures coming across the
monitors speak directly to the imagination.
Not fiery, chaotic and psychedelic like those of Jupiter,
they look cool, ethereal and from a distance
orderly enough to have been drawn with a celestial compass' (1)
The images of this grey planet (2) were of measure and precision. One report spoke of its 'translucent, marble-like surface graced with diaphanous rings.' Its rings formed exact circles, the only such in the solar system, level as if on a glass plate, and detached from one another. Multi-hued, they all seemed to 'know their places', and some were even 'plaited' together. With its exquisite rings, dead level and circular, there could be no greater contrast than with Jupiter. It seemed to be a more introvert world.
As Voyager flew past the rings of Saturn, it viewed their strangely regular patterning, seemingly defying the laws of physics. Astronomers had hypothesised that 'shepherd moons' must exist nearby, which could somehow aid the rings to sustain their entropy‑defying symmetry: especially programmed to detect these, Voyager could not find any more. The rings were made of thin, separate lines which could be just a few metres deep: 'when the Voyager cameras zoomed in on the main rings, they appeared to break up into countless rings - almost without limit. Thousands of tiny ringlets appeared...some of them making a complete circle around Saturn' (3)
The Rings of Saturn:-
In the immensity of space, what established these rings, so ordered and symmetrical, and what sustained them? What astronomers call 'Shepherd moons' were noted, which somehow helped the rings to stay in place, but even so the astronomers were baffled. The 'spoke' patterns continued to appear as an out‑radiating pattern through the various ring‑layers, as if superimposed upon them. The rings were moving round at different speeds, but the 'spokes' of light and dark rays just passed through them. (4)
Harmony in the Saturn-rings
Let's hear Dava Sobell's view, she seems to have really got the hang of how Saturn's rings work:
"When the Cassini spacecraft reached Saturn in the summer of 2004, it trumpeted its arrival by soaring up through the gap between the F and G rings, skimming across the broad expanse of the ring plane, and then diving back down through the far side of the same gap, where it emerged unscathed."
"The most notable resonance effect in the rings of Saturn is the Cassini division - the three-thousand mile-wide separation between the A and B rings. The Division derives from its 2:1 resonance with the moon Mimas, orbiting more than 40,000 miles away. Ring particles within the Cassini division travel twice around Saturn to Mimas's once, and so they repeatedly overtake the slower-moving moon at precisely the same two points in their orbit. There they gravitate towards it. Eventually the pull of the moon, boosted by rhythmic repetition, boots the particles out of the resonant orbit, clearing the gap. A similar but narrower gap near the outer margin of the A ring, called the Encke division (for Johann Encke. a former director of the Berlin Observatory), shares a 5:3 resonance with Mimas and a 6:5 resonance with another moon. Also, the decorative scalloped border on the outer edge of the a ring owes its six petal-like lobes to a 7:6 resonance with two small satellites that occupy a single orbit and may once have been a single object." (5)
The magnetic axis coincided with its axis or rotation. This emphasised the strong right angle which Saturn forms between these two axes and the level rings on the equator plane. But, it was a problem because the magnetic field of a planet is supposed to be generated as a result of the angle between these two. Saturn's rings vanish from view as they become edge-on, emphasising its thirty-year period. Until 2008 the 'underside' of the rings is visible through a telescope, then they disappear for a while and a top view emerges.
The Cassini-Huygens mission to Saturn discovered a large hexagon pattern around its north pole.
The above diagram shows the retrograde loops of Saturn, Earth being at the centre of this big circle. While moving retrograde, approximately 4 and a half months each cycle, Saturn draws nearest at its solar opposition - this happens yearly, as the Sun opposes Saturn. It then grows brightest in the sky, because it's reflecting the Sun's light, so it is easiest to see.
Moving further out, the planets spend more of their time being retrograde, until Neptune's retrograde loops overlap.
Music: 'Shepherd moons' song by Enya
(1) - Voyager 1 at Saturn, National Geographic, July 1981
(3) - Lang & Whitney 1991, p.203.
(4) - Supposedly, these dark spokes came from "Saturn's magnetic field, which levitates microscopic ice crystals out of the main ring plane, causing their shadows to fall on the rings below." Booth, 1995 p.116.
(5) - Dava Sobell, The Planets, 2005 p182