With this review of Jupiter’s satellites complete we now consider SATURN’S RINGS. When Galileo turned his telescope towards Saturn in 1610 he saw the planet in three parts. Later, in 1612, it looked quite regular and he feared that his earlier observations may have been in error. In 1614 C.Scheiner drew Saturn as a planet with two handle-like extensions. G.B.Riccioli drew it as a triple body in 1640 but between 1647 and 1650 he too showed two handles.

The true explanation of this mysterious appearance was eventually discovered by Christiaan Huygens. In 1655 he saw a narrow arm on each side of the planetary disc. Early in 1656 these appeared to have completely disappeared and he saw the planet simply as a round disc, and by October of that year the narrow arms had reappeared. Huygens soon realized what was happening, and in 1656 he published a paper about his discovery in the previous year of the first-known satellite of Saturn. At the end he included an anagram of the Latin text: ‘Annulo cingitur tenui, plano, nusquam cohaerente, ad eclipticam inclinato’. Translated this is, ‘It is surrounded by a thin, flat ring not connected with the planet in any place and inclined at an angle to the ecliptic’. Huygens published a full account of this discovery in 1659.

Because of the inclination of the rings to the ecliptic, their aspect presented to the Earth varies during Saturn’s orbit around the Sun. Twice during the orbit, we on the Earth are able to see the rings open to the fullest extent; on one occasion the view is from the north and on the other from the south. At these times we are directly above latitude 28°. on Saturn. Halfway between two successive fully-open positions the rings are viewed edge-on and, except in powerful telescopes, they seem to disappear completely. Viewing from the Sun, one would, of course, always see the sunlit side of the rings. Because of the Earth’s own orbital motion it is sometimes possible for the other, unlit, side of the rings to’ be visible; they then appear as a dark narrow band across the planet’s disc. This can only happen when the Earth is passing through the plane of the rings. Again because of the Earth’s motion around the Sun, this passage through the plane of the rings may occur three times at fairly short intervals instead of just once. In 1966, for example, the Sun passed through the plane of the rings on June 15 but the Earth passed through on 2 April, 29 October and 18 December.

In 1675 G.D.Cassini realized that the ring around Saturn was double, with a dark gap between the inner and outer rings. This gap is now known as the CASSINI DIVISION. W.Struve named the outer ring KING A and the inner one RING B. Ring B is noticeably brighter than Ring A. A gap in Ring A was first noticed on 25 April 1837 by J. Encke and is now named ENCKE’S DIVISION after him. A third ring, inside Ring B, was seen by J.G.Galle in 1838. This is now called the CREPE RING, or simply RING c. It is tenuous and the planet can be plainly seen through it. Galle’s discovery was ignored and the ring was rediscovered in 1850 by G.P.Bond and, independently, by W.R.Dawes. There has been considerable controversy for many years about the presence of a fourth, extremely tenuous ring outside Ring A. This ring has been called RING D but its existence is by no means certain. Rather more certain is a ring inside Ring C. It is separated from Ring C by a dark division and reaches virtually down to the atmosphere of the planet. This ring has also been named Ring D, a logical choice, but one which can obviously lead to confusion.

The rings are undoubtedly very thin; this makes an actual measurement of their thickness very difficult. Certainly the thickness cannot be less than 500m, is unlikely to be more than 4km, and a value of 2 to 3 km seems most likely. The rings cannot be solid or liquid. James Clerk Maxwell showed this theoretically in 1857 and in 1895 J.E.Keeler found that the rings were in differential rotation (the inner parts take a shorter time to revolve around Saturn than do the outer parts) (figure 12.2). The rings consist of particles that move in individual orbits around Saturn according to Kepler’s laws. The particulate structure is confirmed by at least six observations of stars seen right through Ring A, even though they were of magnitude 7.2 or fainter. Two stars of eighth magnitude have also been seen through the brighter, more dense Ring B. Observations such as these show that for light falling perpendicularly onto the rings about 35 to 50 per cent passes straight through Ring B and about 60 to 70 per cent through Ring A. The visual surface brightness of Ring B and parts of Ring A are somewhat higher than the mean surface brightness of Saturn itself. This indicates that the particles hi the rings have a very high albedo. Spectroscopic studies of the light reflected from the ring8 have shown that the particles are largely ice or at least ice-covered. although the ice cannot be pure. Measurements of the infrared radiation from the rings have shown that the ring particles have a surprisingly high temperature, which implies that the particles must be fairly large and be warmer on the side facing the Sun and the Earth than on the other side

Saturn’s rings are the most distant objects to have been observed by radar ; the radio waves take about 2 hours 40 minutes to make the round trip to Saturn and back .In the early 1970’s R.M. Gold stein and G.A . Morris showed that the rings were efficient radar reflectors by transmitting and receiving strong echoes from them at a wavelength of 12.6 cm .The Doppler spread of the echoes matched that expected from the known range of orbital velocities of the particles in the bright inner Ring B.The radar reflectivity of the rings is five to ten times higher than for any of the other objects observed by radar: the Moon, Mercury ,Venus ,Mars and the asteroids Icarus ,Toro Eros.The radar observations require the reflecting objects to be centimeter-size or larger .

This minimum size of a few centimeters for the size of the ring particles is supported by several lines of evidence. Solar ultraviolet radiation and protons in the solar wind knock water molecules off the icy surface of the particles; this is technically known as SPUTTERING. During the lifetime of the Solar System several centimeters of ice would be eroded so that particles smaller than this would have completely disappeared by now unless they were of recent origin, which is thought to be unlikely. The POYNTING-ROBERTSON EFFECT would also clear the ring system of particles smaller than about 3cm. This effect arises because forces other than gravity – especially radiation pressure – cause the orbits of particles to become gradually smaller so that they spiral in to¬wards Saturn. The effect is greater for smaller particles than for large ones so that only those below a certain size could have spiralled all the way to Saturn’s atmosphere since the Solar System was formed.

The evidence as a whole suggests that the ring particles have sizes measured in centimeters but this is by no means certain. Other poorly-understood aspects of the rings are their dynamics and origin. Some astronomers believe that the particles rarely collide with one another whereas others believe that collisions are very frequent. The gaps in the rings are thought to be caused by dynamical interactions with the satellites of Saturn, similar to the Kirkwood gaps in the asteroid belt. The ring particles may have condensed where they are at the same time as Saturn and its other satellites were formed. Alternatively the rings may be the result of the break-up of a larger object (or objects) by tidal disruption or as the result of a collision.

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