Beyond Jupiter is Saturn, the most distant planet that was known to the ancients. With the obvious exception of the rings, it appears to be similar to Jupiter in many ways. Saturn’s rings effectively consist of an enormous number of minute satellites and are considered, together with the ten known satellites

Saturn is the least dense of all the planets. Its density of 704kgm-3 is less than that of water so that one might imagine Saturn being placed in a tank of water and floating! Like Jupiter, saturn must be composed predominantly of hydrogen and helium.We know much less about Saturn than Jupiter, since no spacecraft have yet passed in its vicinity. Our knowledge is confined to that which can be obtained by conventional Earth-based observations. We receive about 16 times less light from Saturn than from Jupiter and the geometric resolution of the surface as seen from Earth is only half as good. Consequently most studies of Saturn draw heavily on analogy with Jupiter.

Saturn’s mass is 95.147 times that of the Earth. The equatorial radius is 60000km. The polar radius is 11 per cent smaller, making Saturn by far the most flattened planet. The average density of 704kgnr3 follows from these figures; this is about half the average density of Jupiter and one-eighth that of the Earth.

Molecular hydrogen was first positively identified on Saturn in 1962, two years after the corresponding discovery on Jupiter; methane had been discovered earlier during the 1930s .Ammonia, the other major contribution to the spectrum of Jupiter, has not been definitely identified on Saturn. The observational evidence is conflicting; some observers have claimed to have seen spectral lines of ammonia while others have said that they cannot detect any. Ammonia may well be on the limit of what can presently be detected. It is also possible that the amount of gaseous ammonia in the atmosphere (which is what is detected spectroscopically) is variable since small changes in the atmospheric temperature would cause large changes in the amount of ammonia which is condensed into clouds. There can be little doubt, though, that ammonia is present. The same is true for helium, although as is the case of Jupiter, it cannot be detected from the Earth. Apart from phosphine (PH3), discovered in 1975, no other atoms or molecules have yet been definitely detected in Saturn’s atmosphere.

As with Jupiter, the visible surface of Saturn is the cloud layers of the upper atmosphere. There is a system of cloud belts with light zones between them which are sufficiently permanent to have received names . The intricate details of cloud structure seen on Jupiter are largely absent on Saturn. This makes the determination of the rotation period more difficult; the most satisfactory technique is to use the variation of the Doppler shift across the disc to measure the rotation speed and hence the period. The period at various latitudes is as follows:
Latitude Rotation Period
0° (equator) 10 hr 2min
27° l0hr38min
42° l0hr50min
57° 11 hr 8min

Each of these values has an uncertainty of several minutes.-

WHITE SPOTS occur on Saturn on rare occasions and persist for a few days or weeks. They never achieve the prominence or lifetime of the spots seen on Jupiter and nothing remotely like the great red spot has ever been seen. The longest lasting spot that has been observed was seen during a 490-day period between October 1969 and February 1971. Its average rotation period was l0hr 36min 27.9sec but the actual period varied somewhat. The spot itself measured 8000km from north to south and 6000km from east to west. As well as being the longest lasting, this spot set a record for the greatest southern latitude: 57.3°S. Another prominent spot was one discovered in August 1933 by Will Hay the famous comedian, who was also an amateur astronomer. The spot appeared in the equatorial zone and rapidly became very conspicuous. It gradually lengthened and the portion of the disc behind it darkened. The spot faded quickly and disappeared completely after a few months.

The rotation periods of the spots seem to be shorter nearer to the equator than further away. They do not appear to agree with the rotation periods measured by Doppler spectroscopy, but the measurements are not sufficiently precise for this result to be certain. In any case, it is well established that spots on Jupiter do not, in general, have the same rotation period as the surrounding clouds, so a similar result for Saturn is no surprise. Jupiter has a distinct equatorial jet with a sharp change in rotation period near latitudes 10° north and south. It is not yet known whether Saturn has a similar jet, but analogy strongly suggests that it does.

The colour of Saturn’s clouds varies over the disc from white through pale yellow to brownish yellow . Experienced observers have reported seeing colours such as orange and blue in places, but any such shades are extremely subtle. These colours presumably originate in the same way as those on Jupiter. The visible clouds are probably ammonia cirrus but somewhat more dense than those on Jupiter.

It is difficult to measure the albedo of Saturn because of the rings that generally obscure part of the disc and also contribute un¬wanted light of their own to the measurements. Measurements have been made though, and show that Saturn as a whole reflects less of the incident light than does Jupiter. In blue light the albedo of Saturn is about 60 per cent of Jupiter’s; in red light it is about 90 per cent. In the infrared the situation is the reverse and Saturn reflects more of the incident solar radiation than does Jupiter.

Measurements of the energy radiated by Saturn are seriously hampered by radiation from the rings and the results are less certain than they are for Jupiter. If Saturn radiated just the energy absorbed from the incident sunlight then its effective temperature would be 71K; in fact it is 97 K. It follows that Saturn is radiating three and a half times as much energy as it absorbs from the sun .Although this figure is rather uncertain there can be very little doubt that Saturn ,like Jupiter ,has an important internal energy source

The problems of understanding the structure of Saturn’s atmosphere arc similar to. but greater than those for Jupiter. Several important quantities are less well known: these include the details of the composition and the magnitude of the internal energy source. Saturn has a much lower surface gravity than does Jupiter: 9.05ms-2 compared to 22.88ms-2 at the equator. This causes a major difference in the structure of the two atmospheres; the SCALE HEIGHT of the atmosphere is much greater on Saturn than on .Jupiter. The scale height is a measure of the change in altitude over which the pressure or density changes by a factor of e = 2.71828. …. The results of one computation of the structure of Saturn’s atmosphere. Saturn’s clouds are expected to be denser than those of Jupiter. This is confirmed by optical observations which show that there is no penetration of radiation through the top cloud layer from below as does occur on Jupiter.

Studies of the INTERNAL STRUCTURE of Saturn proceed along the same lines as those for Jupiter. Some calculations suggest that to produce a model of the correct mass and volume and with an internal energy source of the observed strength, a high fraction of helium, about three quarters by mass, is necessary. This is generally considered unlikely as a composition roughly like the Sun with one quarter helium by mass is more plausible. As discussed above, the size of the internal energy source may be smaller than present measurements indicate because of confusion with radiation from the rings. If this is true then a smaller fraction of helium is necessary. A small high-density core would also reduce the proportion of helium required to explain the observations. Such a core might consist of a rocky centre with a layer of ice .The results of one model and prob¬ably gives the correct general trend of conditions inside Saturn.

RADIO EMISSION from Saturn has been detected over the wave¬length range from about 1 mm to 94cm. All this radiation appears to be due to thermal emission from the atmosphere. There is no evidence that there is any non-thermal radiation as there is from Jupiter. From the Earth a magnetic field on Saturn would only be detectable indirectly by observations of such non-thermal radio emission. Consequently there is at present no evidence that Saturn has a magnetic field.

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