Saturn.
The sixth major planet of the solar system in order from the Sun. Saturn is one of the four Ôgas giants', second in size only to Jupiter. Its equatorial diameter is 9.4 times the Earth's and its mass 95 times greater. However, its average density is only 0.7 times that of water. Hydrogen and helium make up the bulk of the mass. There is a central core, ten or fifteen times the mass of the Earth, made of rock or a mixture of rock and ice. In the high-pressure region surrounding the core, the hydrogen takes on the form of a metal. The outer half of the planet consists of a deep atmosphere; the visible features of the planet are cloud bands at the top of this atmosphere.
The cloud patterns on Saturn do not normally show much colour contrast. However, in late September 1990, a large white spot developed, expanding over a period of weeks to encircle much of the planet's equatorial region. This apparent eruption of material from the lower atmosphere followed a pattern of such occurrences over a 30-year cycle, corresponding to the orbital period. Similar spots were seen in 1876, 1903, 1933 and 1960, close to Saturnian mid-summer in the northern hemisphere.
Computer processing of images obtained by Voyager 1 and Voyager 2 during their encounters in 1980 and 1981 reveal complex circulation currents, similar to those observed on Jupiter. Saturn rotates rapidly, spinning once every 10 hours 32 minutes on average, though the rate varies with latitude. The resulting flattening at the poles is significant; the polar and equatorial diameters differ by 11 per cent.
The most striking feature is the spectacular ring system. The rings lie in the planet's equatorial plane, which is tilted at an angle of 27¡ to its orbit round the Sun. They are easily visible in a small telescope. As the relative positions of the Earth and Saturn change, the rings are presented at differing angles, sometimes appearing open, at other times edge-on so that they disappear from view. The rings have the appearance of a series of zones of differing brightness, separated by dark divisions. The most marked divisions are Cassini's and Encke's. The Voyager images of the rings showed that they consist of many thousands of narrow concentric ringlets, resulting in a grooved appearance. They are only one kilometre thick and are made up of a huge number of separate rocks and particles, perhaps ranging in size from a hundred metres down to a micrometre.
Before 1980, ten satellites of Saturn were known. More have been discovered since, some telescopically in 1980 when the ring system was edge-on (thus removing the glare) and some by the Voyager 1 and 2 spacecraft in 1980 and 1981. Eighteen are now known for certain, and there are probably three others and possibly more, subject to confirmatory observations.The largest planet in the solar system and the fifth in order from the Sun. After Venus, it is the second-brightest planet as seen from Earth.
Jupiter is a giant ball of gas, ten times the size of the Earth and one-tenth of the Sun's diameter. Its mass is 0.1 per cent that of the Sun and its composition (by number of molecules) is very similar to the Sun's: 90 per cent hydrogen (in its molecular form in Jupiter) and 10 per cent helium. Of trace gases, the most significant are water vapour, methane and ammonia. There is no solid surface beneath the cloud layer. Instead, a gradual transition from gas to liquid takes place as the pressure increases with depth below the outermost layers, followed by an abrupt change to a metallic liquid, in which the atoms are stripped of their electrons. At the very centre there may be a small core of rock and perhaps ice.
A source of internal energy, heat generated when Jupiter formed by gravitational collapse, causes the planet to radiate between 1.5 times and twice as much heat as it absorbs from the Sun.
Observed visually, the disc of Jupiter is seen to be crossed by alternating light zones and dark belts. Results from four space probes that have passed by Jupiter between 1973 and 1981 (Pioneers 10 and 11, Voyagers 1 and 2) have revealed the full complexity of the flow patterns within these bands. There are five or six in each hemisphere, correlating with wind currents.
White or coloured ovals appear as relatively long-lived features. The best-known and most conspicuous is the Great Red Spot, which has been observed for around 300 years. The origin of this feature, which is as wide as the Earth, is uncertain; one popular theory is that it is essentially a huge anticyclone.
The coloured clouds are in the highest layers of Jupiter in a region with a depth of only 0.1-0.3 per cent of the total radius. The origin of their coloration remains a mystery, though it seems certain that it must have to do with trace constituents of the atmosphere, and is evidence of complex chemistry. Cloud colour correlates with altitude: blue features are the deepest, followed by brown, then white, with red being the highest.
The existence of a faint ring around Jupiter was first suggested by results from Pioneer 11 in 1974 and confirmed by direct Voyager images. The main part lies between 1.72 and 1.81 Jupiter radii from the centre of the planet. The nature of the ring is such that many of the particles must have dimensions measured in micrometres. A constant source of replenishment is required, which may be a population of orbiting boulder-sized objects, constantly bombarded by high-velocity particles.
There are sixteen known natural satellites orbiting Jupiter. They fall into four distinct groups. The four small inner satellites (Metis, Adrastea, Amalthea and Thebe) and the four large Galilean satellites (Io, Europa, Ganymede and Callisto) are in circular orbits in the equatorial plane. The third group (Leda, Himalia, Lysithea and Elara) are small satellites in circular orbits, inclined at angles between 25¡ and 29¡ to the equatorial plane and at distances between 11 and 12 million kilometres from Jupiter. The outermost group (Ananke, Carme, Pasiphae and Sinope) are small satellites in retrograde orbits that are relatively eccentric ellipses, inclined substantially to the equatorial plane. These orbits all lie between 21 and 24 million kilometres from Jupiter. The four Galilean satellites and their movements in orbit are easily visible with a small telescope or binoculars.
Radio emission from Jupiter was discovered in 1955. It was the first indication of the presence of the strong magnetic field, which is 4,000 times stronger than the Earth's. The ©magnetosphere is consequently 100 times larger. The radio emission is caused by the spiralling of electrons around the field lines. Trapped electrons near the planet give rise to synchrotron radiation at decimetre wavelengths. Decametric radiation, observed only from certain regions of the planet, is associated with the interaction between Jupiter's ionosphere and Io, whose orbit lies within a huge plasma torus: this interaction also creates aurorae. Radiation at kilometre wavelengths was discovered by the Voyager probes, originating at high latitudes near the planet and in the plasma torus.