S A T U R N : Part 4

THE SATURNIAN RINGS

The wonderful Rings of Saturn are quite invisible to the naked-eye but become prominent in telescopes as small as 3cm. These bright rings surrounding its girth and provide the Saturn’s true wonder. Of all the celestial wonders visible in the night sky, it seems that the vast majority of the public are more attracted to Saturn with its Rings than any other object. Astronomical novices during all my days as teaching seemed to get far more excited to look at the rings in a telescope for themselves. The rings were first seen by Galileo in 1610, who did not know what they really were. He said all that he could see were two spherical balls near the main disk — an illusion created by the very poor quality of his telescope. Later he described them as ansae, which translates to mean literally handles.

“I have observed the highest planet to be tripled-bodied. This is to say that to my very great amazement Saturn was seen to me to be not a single star, but three together, which almost touch each other.”

Even more puzzling was that they had all but totally vanished during 1612 only to return again in 1615. It was decades later in 1655 that Hugyens first realised they were true rings encircling the planet. This result appeared in his “Systema Saturnium”, which was first published in 1659, saying;

“Saturn is surrounded by a thin flat ring not touching it anywhere, which is oblique to the ecliptic.”

Looking at the earlier observations he knew that the disappearance of the rings was due to the changing aspects of Saturn with the rings being tilted to the plane of the ecliptic. The rings are 272 000 km across being 2.2 times the size of the main disk and appear less yellowish than the main disk of the planet. These rings are not solid disk, as the first telescopic astronomers thought.

We can easily prove this transparency when on the rare occasion when a star occultates the rings, causing the star to dim and twinkle, but still remain visible. An example of one of the first was observed was by three amateurs from South Africa that occurred on 14th March. Here just before midnight on the 14th March 1920, an orangery-red 7.1 magnitude star SAO 99257 / HIP 52373 in Leo passed behind the rings. When the star passed behind the A ring, the light was only partly diminished. When the star reached the B-ring, the star dropped by about one magnitude then started to rapidly fluctuate in brightness. As the star passed deeper, about two-thirds into the inner B ring, the magnitude of the star further dropped to about 9.5 magnitude. After this it finally faded when it was occultated by Saturn’s shadow. Several other events have been predicted and observed — much with the similar effects.

Saturn’s two main bright A ring and B ring are now thought to consist of tiny fragments of ice and dust, ranging roughly in size between marbles and basketballs. Observations show the inner B ring must have more ice, and must have finer particles than the A ring. The B ring is certainly moderately brighter and whiter than the A ring.

Separating these two main rings is the darker ringlet of more rocky material that was first imagined to be just empty space. Called the Cassini Division — being first discovered by the Parisian Giovanni D. Cassini in 1675, and is visible in 7.5cm or 10.5cm apertures when the rings are at their widest.

Dividing the A ring is the Encke Division, which was discovered by Johans Encke in 1837. American astronomer George Bond in 1850 later discovered another inner greyish C ring, also known as the Crépe Ring or Dusty Ring. This is also about one-fourth to one-fifth the width of the B ring.

Fig. 4-1. SPOKES in the B Ring Courtesy NASA/JPL

Immediately inside the C ring, as first seen by French astronomer G. Fournier, and likely first seen by amateurs, is the darker D ring. Both the Voyager spacecrafts confirmed the D ring’s existence, however, Voyager found three new ones — the E, F and G rings.

Voyager also found that Saturnian rings were really countless small ringlets, and also found unexpectedly details in the ring structure such as spokes which radially appearing across the B ring and rotate with the planet. These spokes will appear and disappear over time whose nature has something to do with the inner moons and the effects of electrostatic charging from the particles in the rings. Upon arrival of the Cassini spacecraft, these spokes were no evident. They were later rediscovered in September 2005, though certainly less prominent, within the outer B Ring.

Most planetary scientists suggest that the rings are the remnants of material left over from the formation of the Solar System, and some still believe that it was a small moon that strayed too close to Saturn, at the so-called Roche Limit, and stripped apart the moon from the planet’s gravity. Clerk Maxwell in 1895 was the first to prove the rings were made up of small particles that are continually being ground down to small fragments by collisions within the rings. Spectroscopy confirmed that the inner part of the rings rotated faster than the outer rings. It was American astronomer Daniel Kirkwood who in 1867 proved beyond doubt that the rings were unstable because of this differential rotation. Kirkwood partly explained a solution by stating the instability could be improved by the rings producing gaps, called the Kirkwood Gaps, and this explained the general divided appearance of the rings. However, the number of ringlets was grossly underestimated, as Voyager discovered literally thousands of them. Another perplexing proposition that remained unsolved for a long time was why the rings have survived so long?. This was solved when Voyager discovered the shepherd moons within the rings that keep the rings in place and some astronomers think they have changed little since the formation of the Solar System. By mass, the rings are indeed equivalent to a very small moon that would have been only twenty to fifty kilometres in diameter. In thickness the rings are in some parts only one kilometre or so, though some parts may only be as thin as several hundred metres. For comparison, the rings thickness is something akin to a piece of paper compared to the size of any basketball.

Looking at the rings in the telescope, the appearance changes throughout the orbit of Saturn making the planet always an interesting target over the years. The ever changing tilt of the rings is caused by them being inclined 26.7° to the ecliptic. Combined with the slight inclination of Saturn’s orbit to the ecliptic, means the maximum that the rings can tilt is about 28° being either north or south of the planet. Twice the rings cycle between edgewise and full open during the Saturnian year. Now in the first decade of the 21st Century we are looking at the southern face of the rings. At the end of 2009, the rings for the next decade will show the northern face. During ring crossing the apparent visual magnitude is slightly lower than when the rings are open. The drop in brightness is because of the loss of light reflected by the planet minus the rings. Difference average about +1.2 to +1.4 magnitude.

Ring crossings occur successively every 13.25 years then 15.75 years, and always happen in the same region of sky. This occurs either in the constellation of Leo, when the ecliptic longitude is about 173°, or when in Pisces at 352°. The last crossings happened in February 1996, with the next series of crossings beginning on 4th September 2009, and then again in March and May 2025, then once again in October 2038. Maximum opening of the rings will be on the 5th April 2003 at −26.99°, and this always occurs either in Taurus or Ophiuchus at the respective ecliptic longitudes of 82° and 262°.

An image taken by the Hubble Space telescope (HST) was taken on the last ring crossing on the 24th April 1996 with shows the rings almost absent.

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Last Update : 14th November 2012

Southern Astronomical Delights © (2012)

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