Encounter With Saturn

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While JPL was determined to preserve the 'option' (as yet unfunded) of flying the Grand Tour, by sending Voyager 1 deep into the Jovian magnetosphere the resulting slingshot had accelerated it so much that it would reach Saturn too early for onward routeing. The two vehicles had been dispatched a few weeks apart - Voyager 2 first -and had arrived at Jupiter a few months apart - Voyager 1 leading - but by not approaching the giant planet so closely Voyager 2 had set off on a slower trajectory to Saturn which was timed to arrive when conditions were just right for the Grand Tour. However, at this stage in the overall mission Voyager 2 was officially backing up its mate. Only if the first spacecraft achieved all of its primary objectives at Saturn would the second be released for this 'extended' mission.

The overall scientific objectives at Saturn were:

• to investigate the dynamics of the planet's outer atmosphere;

• to profile the chemical composition of the outer atmosphere;

• to chart the temperature field at the cloud tops to infer the deeper thermal structure;

• to map the magnetic field;

• to chart the flows of charged particles in the magnetosphere;

• to determine Titan's key physical parameters, to investigate the composition of its atmosphere, and to discover how this interacts with the planet's magnetosphere;

• to determine the geological histories of the icy satellites;

• to determine the detailed structure of the ring system; and

• to search for new rings and moonlets.

Pioneer 11 had established that Saturn has an appreciable magnetosphere with charged particles circulating within it, but lacking the 'beating heart' of Io, it does not pump out decimetric and decametric radio energy. Nevertheless, it had been found to produce very weak long-wave emissions.

In February 1980, Voyager 1's Planetary Radio Astronomy instrument started to detect bursts with wavelengths of several kilometres.23,24,25,26 In closer to Saturn, a periodicity of 10 hours 39 minutes 24 (±7) seconds became evident in these very-low-frequency emissions.27 If the magnetic field was rotating with the core of the planet (as seemed to be the case for Jupiter) then this was a measure of the rotational period uncomplicated by the differential rotation of the atmosphere. The fact that this was slower than the measured rates for spots in the equatorial zone indicated that what A.S. Williams in 1891 had called the ''the great equatorial atmospheric current'' is 'super-rotating'.

By mid-1980, Voyager 1 had initiated long-range imaging of Saturn. At that time it was viewing the ring system virtually edge-on, but as it closed in the angle opened up. By the end of September, the narrow-angle camera's resolution exceeded that of the best terrestrial telescopes. Of course, terrestrial astronomers were unable to see the rings at that time as they were nearly edge-on. By early October, with the range down to 50 million kilometres, the resolution had improved to 1,000 kilometres per pixel. An astonishing amount of detail could be resolved, revealing some surprises.28 The 20,000-kilometre-wide 'C' ring showed several distinct gaps, including one 300 kilometres wide that had a narrow, dense, bright ringlet within it which was slightly eccentric, being 90 kilometres wide at its farthest point from the planet and only 35 kilometres wide at its closest. It had been expected that perturbations by the moons orbiting beyond the ring system would have swept the 5,000-kilometre-wide Cassini Division clean, but there was a thin strand of material present within it. How could there be material in the 'zone of avoidance'?

On 6 October, dark radial markings were spotted on the 'B' ring, rotating ''like spokes on a wheel'', as explained by R.J. Terrile, the member of the imaging team who discovered them. The increase in rotational period across the width of the 'B' ring exceeds 3 hours, hence a radial feature should become significantly distorted within a matter of minutes. However, the spokes emerged from where the planet's

The sequence (which runs from left to right and top to bottom) shows the rotation of the dark 'spokes' on Saturn's 'B' ring.

shadow fell on the rings and persisted for several hours as they rotated, becoming fainter and less well defined with time, finally losing their integrity only just before re-entering the shadow. A new command sequence was sent to the spacecraft to investigate this phenomenon, and so on 25 October, with the range now down to 24 million kilometres, a 500-kilometre-per-pixel frame was shot every five minutes over a 10-hour period - almost a complete planetary rotation. Over the next week, JPL's Image Processing Laboratory sequenced these frames to create a time-lapse movie showing the spokes rotating. The fact that the motion matched the planet rather than the ring system meant that the spokes were an artefact of the planet's magnetic field which in turn implied that the spokes were charged particles being swept along over the 'B' ring.29 The force that created the spokes was evidently at work where the planet's shadow fell onto the rings. Analysis suggested that the spokes were particles a few microns wide which were 'elevated' away from the plane by either magnetic or electrostatic forces that were effective in darkness. The spokes emerged well defined, and slowly diffused as they progressively migrated back down to the ring plane in sunlight, only to be elevated again upon re-entering the shadow.30 As Voyager 1 closed to within a few million kilometres of the planet, it detected lightning-like radio bursts. Some researchers suggested that these emanated from the ring system, the idea being that the material in the spokes was being 'charged up' in the shadow, and the bursts were electrostatic discharges between the clouds of dust as it settled towards the rings upon emerging into sunlight.31,32,33,34,35,36,37,38,39 Others suggested that the radio bursts were more likely due to atmospheric lightning in the super-rotating equatorial wind stream.40,41

In 1896, E.M. Antoniadi had seen radial features on the 'A' ring, but even the accomplished chronicler of Saturnian studies, A.F.O'D. Alexander, had dismissed them as being ''probably illusory''.42 Nevertheless, in 1977, Stephen O'Meara, an experienced visual planetary observer, reported radial structure on both main rings. Interestingly, the features seen by the Voyagers were confined to the 'B' ring.

Imagery on 14 October enabled the orbits of the moonlets Janus and Epimetheus to be refined. Their orbits are 30 kilometres either side of a planetocentric distance of 151,450 kilometres. In accordance with Kepler's laws of orbital motion, the lower one travels slightly faster and catches up with the higher one every 4 years or so, at which time they swap orbits. This occurs because the trailing moonlet, in the lower orbit, is accelerated and rises; at the same time, the leader is retarded and falls. Since the leader, which was in the higher orbit, accelerates away as it drops into the lower orbit, they do not actually pass. In fact, they probably never come closer than a few kilometres of one another.43

On 25 October, while checking the incoming imagery of the rings intended for the spokes movie, S.A. Collins discovered a moonlet just beyond the 'F' ring. The next day he found another one, just inside the ring. Several years previously, upon the discovery of the system of widely-spaced thin rings of Uranus,44,45 it had been suggested that this delicate structure was maintained by a number of moonlets which 'shepherded' the loose material, but these bodies were hypothetical.46 Now, it was apparent that the narrowness of the 'F' ring derived from the presence of this pair of moonlets. As the 13th and 14th satellites confirmed in Saturn's retinue, they were

On 16 February 1977, the noted planetary observer Stephen O'Meara drew radial structure on both the 'A' and 'B' rings.

later named Prometheus and Pandora. Prometheus, being 1,250 kilometres inside the ring, travels slightly faster and overtakes Pandora, which lies 1,100 kilometres beyond the ring, every 25 days.

As Voyager 1 closed to within 25 million kilometres, the other optical instruments began to make observations. The IRIS took spectra several times per day in order to determine the composition of Saturn's atmosphere and, in addition to hydrogen and helium, it detected traces of phosphene, methane, acetylene and ethane. As October drew to an end, with the range down to 17 million kilometres, the disk finally began to show dark belts and bright latitudinal zones, and there was evidence of plumes of material billowing up from below. The disk was of such low contrast, however, that the structure was barely discernible unless appropriate filters were used to penetrate the haze.47 By this time, the Ultraviolet Spectrometer had confirmed the presence of aurorae, so the solar wind was evidently able to force its way into the polar cusps of the magnetosphere.

The amazing complexity of the ring system was becoming more apparent each day. Even with Pioneer 11's 'taster', the scientists were incredulous as the ring imagery streamed in. When the resolution improved to 150 kilometres per pixel, an enhanced image revealed 95 individual ringlets. Furthermore, five strands of material were visible inside Cassini's Division, separated by a few hundred kilometres. A few days later, one of these mysterious ringlets was found to be elliptical. From farther out, Cassini's Division had appeared to be sharply defined, but it became difficult to identify its edges as more structure became evident. By the time the resolution had improved to 10 kilometres per pixel, several dozen narrow ringlets could be seen in Cassini's Division. Daniel Kirkwood's hypothesis that the gap was swept clean by Mimas's presence clearly had to be reconsidered. One idea suggested that a density wave effect was at work, such that in perturbing a ring particle at the resonant radius Mimas makes the particle's orbit elliptical. This then induces 'bunching' which, in turn, sets up a spiral density wave within which particles collide, lose energy and spiral in, forming a gap just outside the resonant radius.48 This was a good start, but - as always - the proof would be in the fine detail, and the evidence would be the final arbiter. For example, the outer edge of the 'B' ring was not only found to be elliptical, meaning that the orbits of the particles there are elliptical, but the elliptical form 'rotates' to maintain its short axis aimed at Mimas as the moon travels around its orbit.

As Pioneer 11 had passed Titan at a range of 363,000 kilometres, it had returned a few spin-scan images showing not only that the atmosphere was optically thick but that the moon was completely enshrouded by an obscuring haze. Titan was one of Voyager 1's primary targets. The spacecraft's inbound trajectory to the Saturnian system had been calculated specifically to provide a fly-by of the moon. It started to take pictures of it in early November, in the hope of catching a glimpse of the surface through a gap in the overcast, but the prospects did not look promising. With the resolution now 200 kilometres per pixel, it was realised that the 'F' ring was slightly eccentric, extending 400 kilometres farther from the planet on its semi-major axis. On the planet itself, a profusion of zonal structure was now apparent. Indeed, 24 narrow belts and zones were counted in the southern hemisphere, with abundant evidence of isolated storms. Very little of this atmospheric structure was apparent to telescopic observers, however. On 6 November, the spacecraft performed a small manoeuvre to nudge its closest point of approach 650 kilometres nearer to Titan, onto a trajectory that would later result in an occultation by the ring system after passing Saturn. Imaging showed hints of a banded structure and a darker polar zone on Titan. By now, all the remote-sensing instruments were observing the moon on a regular basis. The Ultraviolet Spectrometer's detection of emission from both neutral atomic and molecular nitrogen from the moon was the first proof that there was nitrogen in the moon's atmosphere. The 'small atmosphere' model developed by J.J. Caldwell's group predicted a methane-dominated atmosphere at a low (20 millibars) pressure, a cold (86K) surface and an upper atmospheric inversion. The 'massive atmosphere' of D.M. Hunten's group predicted a nitrogen-dominated atmosphere with methane as a minority constituent at a very high (20 bars) pressure, sufficient for a methane-, hydrogen- and nitrogen-induced 'greenhouse' in the lower atmosphere which would produce a warm (200K) surface. Observations by the Very Large Array in 1980 had indicated a surface temperature of 87K, in which case if molecular nitrogen was present it contributed no more than 2 bars to the atmospheric pressure.49 Voyager 1's positive detection of nitrogen therefore ruled against the thin methane-based atmosphere. The fact that

Although Voyager 1's imaging system was far superior to Pioneer 11's 'spin-scan', there were no gaps in Titan's optically-thick atmosphere to reveal the nature of the surface.

the haze layer was evidently both global and permanent lent credibility to a denser atmosphere.

With the improving resolution, the 'F' ring that Pioneer had discovered but had been barely able to resolve, was found to be no more than 100 kilometres wide, with some sections being 'thicker' than others. Some of the concentrations stretched for thousands of kilometres around arcs. ''We have no explanation of what causes them, what holds them together, or how long they last,'' admitted B.A. Smith, leader of the imaging team. As R.J. Terrile was examining one of the new images of the 'F' ring, he discovered another new moonlet about 100 kilometres in diameter, which was later named Atlas. The fact that this moonlet was located 800 kilometres beyond the 'A' ring prompted the realisation that it was this moonlet, rather than a resonance with one of the larger moons beyond, that was responsible for the ring's sharp outer edge.

As the Sun was behind the spacecraft as it penetrated the Saturnian system, the moons, irrespective of where they were in their orbits, were almost fully illuminated and only albedo variations were apparent. Topographic relief would not become evident until the spacecraft was so deep within the system that it could view a moon 'from the side', when relief would be apparent along its terminator. Surface coverage was therefore controlled by a combination of the spacecraft's trajectory through the system and the positions of the moons in their orbits. To maximise the coverage, the spacecraft was programmed to take photographs throughout the two days that it would spend within the system, capturing the moons under various illuminations and resolutions as they pursued their orbits. An early long-range image of Iapetus showed a dichotomy that confirmed the existence of a well-defined boundary to its strangely dark hemisphere.

Voyager 2 was serving as an interplanetary monitoring platform, reporting upon the state of the solar wind en route to Saturn. The wind had eased considerably since Pioneer 11's visit the previous year, and it had been predicted that Voyager 1 would meet the bow shock at 40 planetary radii. However, when Voyager 2 reported a gust on 8 November, this range was reduced to between 30 and 35 radii. By 9 November, Voyager 1 was 5 million kilometres from Saturn. The large-scale albedo variations on Titan were now unambiguous. The northern hemisphere was slightly darker than the southern hemisphere, and there was a faint dark equatorial band and a dark 'hood' over the north pole, but the disk was otherwise remarkably bland. ''No fine structure of any kind has been seen on Titan,'' B.A. Smith told the journalists who were eager to see the moon's surface. ''We'll just have to wait, and watch daily to see if anything shows up.'' The Saturnian system was inclining its north pole slightly sunward, so perhaps the gross asymmetry was a seasonal variation in either the production of the haze or in the dynamical processes that keep it aloft. A significant discovery had already been made, however. The Ultraviolet Spectrometer had noted strong hydrogen emission from a diffuse toroidal disk about 4 planetary radii thick, ranging between 8 and 25 radii, corresponding to a distance midway between the orbits of Rhea and Dione out to slightly beyond Titan's orbit. This was significant because it meant that if the hydrogen was from Titan, then some process had to be replenishing the supply, otherwise it would have long since diffused throughout the system.

If the geologists were frustrated that Titan's surface remained hidden, they were fascinated by the longshots of some of the inner moons. Studies of their orbits had established that they had low densities, implying that they were mostly water, and spectroscopy had indicated icy surfaces, but almost nothing was known of their physical state. In gross terms, moving outwards from Saturn, the icy moons could be classified in pairs by the size of their diameters: Mimas and Enceladus (400 to 500 kilometres), Tethys and Dione (about 1,000 kilometres) and Rhea and Iapetus (about 1,500 kilometres). An early view of Rhea's trailing hemisphere at a resolution of 100 kilometres per pixel suggested the presence of bright fuzzy spots and streaks with a filamentary structure. Intriguingly, there were similar patterns on Dione. Because the illumination was face-on these were surface albedo variations. Might they be similar to the sulci on Ganymede? Even though all of these moons were much smaller than Ganymede, such patterns raised the prospect of the moons having once been geologically active, and if so, this was big news. With the solar wind taking a turn for the worse, the prediction for the bow shock was refined to between 29 and 22 radii - the latter value being what it had been the previous year. On 10 November, an early image of the trailing hemisphere of Tethys showed a circular feature 200

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