Continuing Ring Studies

In April 2005 Cassini began to use a succession of Titan encounters to steepen the inclination of its orbit. Revolution 7 began with apoapsis on 23 April at a distance of 40.6 planetary radii. On 28 April Cassini made a main engine burn of 131.6 seconds for a delta-V of 20.5 metres per second in order to revise the altitude of the fly-by of Enceladus in July down to 175 kilometres. There was a non-targeted fly-by of Titan on 4 May at a range of 864,305 kilometres. Six frames over a 16-minute interval on 1 May revealed S/2005S1 orbiting in the Keeler Gap, which is 250 kilometres inside the outer edge of the 'A' ring.249 The moonlet's existence had been inferred from its gravitational effects -notably the July 2004 sighting of 'spikes' and 'wisps' in the outer edge of the gap. Once it had been spotted, it was traced back to imagery taken on 13 April and its orbit was confirmed to be very near the exact centre of the gap. It was about 7 kilometres in size. Named Daphnis, this was only the second moonlet to be discovered within the rings; the first being Pan in the Encke Division.

Cassini's tour was designed to provide occultations in order to probe the structure of the rings. In addition to stellar occultations that occurred when the spacecraft's instruments viewed star light filtering through the rings, the primary mission included 20 radio occultations, at which times the spacecraft flew behind the rings as seen from Earth. The first radio occultation occurred on 3 May 2005.250,251 The Deep Space Network monitored a strong carrier signal issued by the spacecraft. To exploit the fact that variously sized particles would attenuate different frequencies to differing extents, S-Band, X-Band and Ka-Band transmissions were made simultaneously. The results showed that particles less than about 5 centimetres in size were abundant in the outer part of the 'A' ring and in the 'C' ring, but scarce in the inner part of the 'A' ring and across the 'B' ring. This correlated with the compositional variations noted while Cassini was north of the ring plane immediately after the Saturn Orbit Insertion manoeuvre. The results also showed that the inner and outer parts of the 'B' ring had ringlets that were hundreds of kilometres wide and varied greatly in the amount of material that they contained, and also that the 5,000-kilometre-wide core of the 'B' ring contained several bands of material that were almost 4 times as densely packed as in the 'A' ring and 20 times as in the 'C' ring. The 'B' ring contrasted with the smoother 'A' ring and with the 'C' ring, the outer part of which comprised many dense narrow sharp-edged ringlets. There were more than 40 density waves in the 'A' ring, many of which were in the outer region, near the moons that orbit just beyond. ''Some of these waves have been seen in other Cassini observations,'' pointed out Essam Marouf of San Jose State University, ''but not in this large number, and not with this exceptional clarity.''

Revolution 8 began with apoapsis at 40.6 planetary radii on 12 May 2005, but there were no encounters with Titan this time. On 24 May and 11 June Cassini's Visual and Infrared Mapping Spectrometer was able to monitor occultations of omicron Ceti, the long-period variable star called Mira, by the outer part of the ring system. The data showed an unexpected asymmetry - more star light passed through at points during the egress phase than when the star was equidistant from Saturn on ingress. This indicated that the ring material was arranged in narrow strips, radiating outward at a skewed angle. This was a case of the 'wakes' that form when the

An image taken on 2 May 2005 showing the 7-kilometre-sized moonlet designated S/2005S1, whose presence within the Keeler Gap has shaped the edges of the gap.

gravitational attraction of the particles in the rings competed with the tendency of the tidal forces of Saturn to draw them apart. ''When the wakes are viewed almost end-on, the 'A' ring appears at its most transparent," explained P.D. Nicholson of Cornell University, ''and when they are viewed from the side the ring becomes almost opaque.'' This was the most direct evidence so far of gravitational wakes.252 Owing to their narrow width of about 100 metres, it was impractical to image them optically. Although it had been hoped that Cassini would confirm this phenomenon, its position 1.6 million kilometres from the planet, with only 3.5 degrees between the ring plane and the line of sight to the star, made the effect pronounced. ''We weren't expecting it to be so obvious,'' Nicholson reflected.

By this time, having observed several stellar occultations, the Ultraviolet Imaging Spectrograph team knew that the 'A' ring contained clumps.253 The spacing between the clumps was greater than the widths of the clumps themselves, which ranged in size from 'automobiles' to 'haulage trailers' - too small to be resolved by Cassini's cameras. ''This is the first time we've been able to measure the size, orientation and spacing of these particle clumps,'' noted team leader L.W. Esposito. The individual clusters were largest near the middle of the 'A' ring, and smaller towards the edges. ''If we could get close enough, the clumps would appear as short flattened strands of spiral arms with very few particles between them,'' explained Joshua Colwell of the University of Colorado at Boulder. Loosely bound by the gravity of the material that they contained, these structures would be periodically torn apart by Saturn's gravitational tides. This lent support to the theory that the ring material is not simply degrading towards ever finer material, but is also being recycled by being reaccreted into small bodies.

In fact, an analysis of imagery taken shortly after Cassini passed south of the ring plane on 1 July 2004 had shown traces of gravitational disturbances in the 'A' ring indicating the presence of four small embedded moonlets, each of which was calculated to be of the order of 100 metres in size. The difference between the sizes of these objects and Pan (30 kilometres) and Daphnis (7 kilometres) was that they were too small to sweep a clear channel in the rings. They were able to be spotted only

An analysis of imagery taken by Cassini on 1 July 2004 after Saturn Orbit Insertion revealed the presence of intermediate-size moonlets in the body of the 'A' ring.

because their gravitational wakes were superimposed on a particularly smooth part of the 'A' ring. The fact that they were found on images which, between them, covered a tiny fraction of the system implied that more remained to be found.254 Furthermore, as Matthew Tiscareno of Cornell University said, ''These intermediate-sized particles tell us that Pan and Daphnis are probably just the largest members of the ring population, rather than interlopers from elsewhere.''

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