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The thermal profiles of Jupiter (left) and Saturn (right). Both show a stratospheric temperature inversion. Apart from minor diurnal differences in the outermost layers, the data from Voyager 2's signal as it passed behind Saturn and then reappeared on the far side are identical.

In bulk, measured in terms of mass, Saturn is about 80 per cent hydrogen, and most of the remainder is helium. All the other elements which condensed out of the solar nebula together contribute a little over 2 per cent. The heavy elements sank into the core. Much of the oxygen combined with hydrogen as water, much of the carbon as methane, and much of the nitrogen as ammonia, all of which are present in the envelope. However, the IRIS measured the helium-to-hydrogen molecular number ratio for the cloud tops as 6 per cent.82 The corresponding ratio for Jupiter had been 13 (±4) per cent.83 Jupiter was comparable to a solar value of 13 (±2) per cent.84 In terms of mass fraction, at Saturn's cloud tops helium is 11 (±3) per cent, as against 19 (±5) per cent on Jupiter. Because both planets would have started out with near-solar abundances, this implied that the helium within Saturn has, over time, migrated more deeply into its interior. The conditions for gravitational fractionation in a hydrogen-helium fluid had been computed by E.E. Salpeter, who had offered it as the source of Jupiter's excess energy.85 Further analysis suggested that conditions in Jupiter were probably not conducive, but Saturn might be susceptible.86 The depletion of helium implied that Saturn's interior is sufficiently cool for the helium to condense and fall as 'rain' towards the interior (which must be correspondingly enriched). In addition to the heat that it still derives from ongoing gravitational collapse, Saturn - unlike Jupiter - is also heated by the friction of the helium droplets falling through dense hydrogen. Saturn therefore not only radiates to space more energy than it receives as insolation, but at 1.8 as against 1.7 for Jupiter, it generates a proportionately greater 'infrared excess'. Evidently, Jupiter's interior is too hot for helium to condense.87,88 The model for Jupiter derived from its moment of inertia, as a result of the Voyager fly-bys, has three layers. It has a relatively small rocky core of iron, silicates and ices at 30,000K, the incorporated superheated 'ices' being in a superfluid state. This is enclosed by a thick shell of hydrogen which transitions from metallic to molecular phase at a radius of about 46,000 kilometres, at a pressure of 3 megabars and a temperature of 11,000K. Despite Saturn being less massive, and having a much lower density, it would seem to have a similar three-layer structure, except that its core is only 15,000K, half that of Jupiter's core, and the milder thermal profile enables the helium to settle. Although the mass of Saturn's core is about three times that of the Earth, it is denser and hence only slightly larger. But how did such giant planets form? Computer modelling conducted soon after the Pioneer 10 fly-by of Jupiter prompted the idea that they formed by a two-stage process. In each case the process began with the accretion from the nebula of a rocky core. Once this attained a mass several times that of the Earth, it induced instability in the gravitationally bound portion of the nebula, which underwent hydrodynamic collapse and promptly enveloped the rocky core 89,90,91,92,93,94,95,96,97,98,99,100,101

Synoptic imagery revealed that Saturn's zonal winds are different to Jupiter's. The fastest wind occurs in what, in 1891, A.S. Williams dubbed ''the great equatorial atmospheric current''. Indeed, at 1,750 kilometres per hour, this almost-supersonic wind was four times faster than any measured in Jupiter's violent atmosphere. The eastward flow forms a broad swath that extends some 35 degrees north and south of the equator. In the mid-latitudes, the flow first stagnates and then adopts a series of contra-rotating flows, as on Jupiter, except that the maximum and minimum velocities occur in the centres of the alternating features rather than at their boundaries. At that time, there was no certain knowledge of the process driving Jupiter's atmospheric circulation, but the probe that the Galileo spacecraft dropped into the atmosphere in December 1995 established that the circulation is deep and is driven by heat leaking from the interior, rather than being shallow and driven by differential absorption of insolation.102 Because Saturn radiates proportionately more energy than Jupiter, its winds are likely to be driven by the same process. The predominant easterly wind certainly suggests that the air flow is not confined to the outer atmosphere, and may well extend to a depth of several thousand kilometres. Voyager 1 documented considerable atmospheric structure at the surface. The fact that it is not as distinctive as on Jupiter could mean either that the condensates are chemically different and less colourful, or that the winds are so strong that the troposphere has been homogenised into a bland hue.103 As the atmosphere turns over, the 'exotic' chemicals will form strata (working outwards) of water droplets, water ice crystals, ammonium hydrosulphide crystals and ammonia crystals, above which, in view of the fact that it is not cold enough for methane to freeze, there is a haze of methane.104,105,1°6

Voyager 1 established that isolated storm systems are common at high latitudes. Their anticyclonic character, and the fact that they were concentrated in latitudes at which the winds were slow, implied that localised high-pressure system at a deeper level were forcing their way up through the ammonia cloud deck. Most of the spots lasted for only a few days, but a few survived long enough to be studied by both Voyagers. Telescopic observers had noticed that when 'white spots' appeared in the equatorial zone they were large, indicating that the ammonia cloud deck was rent by

The zonal banding of Saturn's atmosphere (right) is the result of rising and falling air masses being 'stretched' by the planet's rapid rotation. Windspeeds were measured by monitoring the motions of features over several planetary rotations. The wind speed varies with latitude symmetrically about the equator. The eastward equatorial flow is approximately 1,100 metres per second. The Jovian-style alternating jet streams (left) are confined to higher latitudes. The dark band on Saturn's equator is the near edge-on ring system.

The zonal banding of Saturn's atmosphere (right) is the result of rising and falling air masses being 'stretched' by the planet's rapid rotation. Windspeeds were measured by monitoring the motions of features over several planetary rotations. The wind speed varies with latitude symmetrically about the equator. The eastward equatorial flow is approximately 1,100 metres per second. The Jovian-style alternating jet streams (left) are confined to higher latitudes. The dark band on Saturn's equator is the near edge-on ring system.

a major updraft, and the emerging blobs were soon drawn out in longitude by the strong eastward wind.

The Saturn fly-by deflected Voyager 1 north of the ecliptic. Although this ended the planetary phase of the mission, it enabled the spacecraft to investigate the solar wind above the ecliptic as it searched for the heliopause. Its total success at Saturn cleared the way for Voyager 2 to adopt a trajectory for a slingshot that would set up the Grand Tour.

As Voyager 2 pursued its interplanetary cruise, it sensed filaments of the Jovian magnetotail washing over it several times,107'108'109 so this very likely does extend as far as Saturn's orbit, which is itself as far from Jupiter as that planet's orbit is from the Sun.110

On 21 June 1981, Voyager 2 began the 'observatory phase' of its programme. Saturn's atmosphere was much more active than in the previous year, with more prominent latitudinal banding and a multitude of spots, waves and storms. One early finding was that the banding was visible almost to the limb, indicating that there is very little haze, which confirmed that the very low contrast at visible wavelengths is the result of the chemicals in the clouds being either intrinsically less colourful, or thoroughly mixed.

As Saturn had moved around the Sun, the angle of illumination on the rings had

Epimetheus Janus

Mimas

Prometheus —Pandora

Cassmi's Division

Saturn

Erceladus

Tethys

A diagram of the inner part of the Saturnian system showing the rings and the newly discovered moonlets in relation to the innermost icy moons.

increased to 8 degrees; they now appeared rather brighter, which made it easier to determine their composition by the absorption features in the reflection spectrum. Filtered imagery showed that while the ring material is predominantly water ice, the composition of the 'C' ring is different from the 'B' ring, with a distinct transition - it is not simply a case of particles from the dense 'B' ring spiralling down through the tenuous 'C' ring towards the planet. The particles of the 'B' ring are mainly icy, but with a 'redder' spectrum than the 'C' and 'D' rings. However, the material in the ringlets within Cassini's Division is similar to that of the 'C' ring.111

The dark spokes on the 'B' ring were more prominent, too, so during the lengthy approach it was possible to monitor the long-term character of this phenomenon. A series of movies were made, in one case a 'tight shot' which followed the motion of a specific group as it emerged from the planet's shadow.

When Voyager 1 revealed the complexity of the ring system, it was realised that, with the exception of a gap as striking as Cassini's Division, Daniel Kirkwood's idea that the fine divisions were induced by resonances with the moons orbiting beyond was untenable. As an alternative, it was suggested that there were a large number of moonlets ranging up to several dozen kilometres wide embedded in the system, each of which was responsible for forming a fine division. J.N. Cuzzi, a specialist in the rings from the Ames Research Center, had studied the 500-kilometre-wide clear zone near the inner edge of Cassini's Division, and concluded that it might be being swept by an object about 25 kilometres in diameter. As Voyager 2's resolution improved, it secured imagery of almost the entire circuit of two broad gaps in Cassini's Division, but no bodies as large as 10 kilometres wide were evident within them.112 A detailed search of 'B' ring imagery failed to find any embedded moonlets. If they exist, they are smaller than the 10-kilometre-per-pixel resolution. Despite R.S. Ball's yearning a century ago ''to see the actual texture of the rings'', the cause of their fine structure defied even a spacecraft's close-up inspection.

Although taken from 34 million kilometres, this image of Saturn on 21 July 1981 by Voyager 2 shows considerably more atmospheric structure than its predecessor had been able to see from this range.

A Voyager 2 shot of the dark radial 'spokes' on the 'B' ring.

On 15 August 1981, Voyager 2 snapped this image of the 'F' ring showing the two shepherds: Prometheus orbits 1,250 kilometres inside and Pandora 1,100 kilometres outside the ring. Abiding by Kepler's laws of orbital motion, Prometheus is moving slightly faster, so we were lucky to catch them in the same vicinity.

On 19 August, at a distance of 10 million kilometres, Voyager 2 refined its trajectory to move its point of closest approach to the planet 900 kilometres closer to optimise the slingshot for the Grand Tour. Its trajectory complemented that of its predecessor. Voyager 1 had inspected Titan, Mimas, Dione and Rhea, and Voyager 2 would investigate Iapetus, Hyperion, Enceladus and Tethys. In addition to the search for new moonlets, it was to snap the co-orbital moonlets and - now that their orbits had been determined with sufficient accuracy to aim the scan platform - the ring shepherds, so its workload deep within the Saturnian system would be significantly greater than that of its predecessor.

On 21 August, Voyager 2 took its first look at Iapetus. Although the moon was on the opposite side of its orbit when inspected by Voyager 1, the view was of the same cratered region of the illuminated leading hemisphere near the north pole. The best imagery, taken late on 22 August from a range of 1 million kilometres, had a resolution of about 20 kilometres per pixel. The contrast between the light and dark terrains was striking. David Morrison of the University of Hawaii, and a member of the imaging team, explained to the reporters that the contrast was ''equivalent to that between snow and asphalt''. In fact, the albedo of the dark side is less than one-tenth that of the bright side. The cratered bright terrain was reminiscent of Rhea, Iapetus's near-twin in size. Some detail was apparent along the fringe of the dark terrain, but since nothing could be discerned on that terrain (subsequently named Cassini Regio) it was impossible to determine whether this is cratered terrain which had been coated black or a smooth plain which was intrinsically black. The imagery enabled the moon's diameter to be measured

As Voyager 2 flew past Iapetus, it took a sequence of images. The main image shows the north polar region, with the dark patch of Cassini Regio on the limb (lower left).

accurately as 1,436 kilometres, and the degree to which it deflected the trajectory enabled its mass to be refined to within 10 per cent, so the bulk density of 1.1 g/cm3 means that there is almost no rock and it is virtually all ice. The bright terrain is therefore almost certainly 'native', and typical of the other icy moons. The dark area is the anomalous surface. However, it was not immediately evident whether it is of endogenic or exogenic origin.

Voyager 2 switched its attention to Hyperion on 23 August. The distant imagery by its predecessor had determined only that this small moon is irregularly shaped. The first image, from 1.2 million kilometres, showed an oblong shape with squared-off edges, reminiscent of a brick. Surprisingly, the primary axis was not aimed at the planet, implying it did not rotate synchronously. An image from 700,000 kilometres from a different perspective revealed it to be roughly 360 by 280 by 236 kilometres, like a fat hamburger. The final image, taken on 24 August from 500,000 kilometres, showed that the surface is cratered. An arcuate ridge suggestive of an impact crater comparable in size to the moon's longest dimension hinted that Hyperion might be a fragment of a much larger body.

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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