Weak, long-wavelength (kilometric) radio busts had been detected from Saturn, so it was presumed to have a magnetic field. However, there was debate over whether this would be sufficiently strong to ward off the solar wind and form a magnetosphere of trapped radiation. Those researchers who thought it could, expected Pioneer 11 to find the bow shock somewhere between 50 and 20 radii from the planet. On 30 August, as the spacecraft crossed the 50-radii line, it was still in the solar wind. The first indication of a change was early on 31 August, at 24 radii. The wind was gusty, however, and an hour and a half later the magnetosphere was compressed and the spacecraft found itself once more in the solar environment. Another 12 hours passed before it finally crossed the magnetopause, by which time it had closed in to a mere 17 radii. The Plasma Wave Spectrometer sensed the electrical waves washing back and forth. These waves occur at frequencies that can be heard by the human ear, so
Pioneer 11's route through the Saturnian system, contrived so that it would test the route through the ring plane that would later permit a Voyager spacecraft to attempt the Grand Tour of the outer Solar System.
this data is readily transformed to audio to enable the researchers to listen to what the spacecraft's 'electronic ears' heard as it flew through the highly dynamic electromagnetic environment.7,8
On 31 August, as Pioneer 11 closed to within 1 million kilometres of Saturn, it built up a spin-scan image of the ring system off to one side of the globe, which caused an immediate sensation. Not only did the silhouetted view show considerable structure in the ring system, and the presence of a very thin ring (later designated the 'F' ring) 3,000 kilometres beyond the 'A' ring, but also what appeared to be a new moonlet 35,000 kilometres farther out. The 'F' ring was so narrow that the spin-scan imaging system could not resolve it all the way around; it was evident only as a short section of arc at the cusp of the ansa where the line of sight passed through a longer column of the material. The 'B' ring, which is the brightest part of the system on the sunlit face, was so dense that from the spacecraft's perspective it appeared dark. The 'A' ring was not so opaque, and the fact that some parts of it passed more light indicated that there was some structure within it.9 The 'C' ring was bright because the fine material it contained forward-scattered sunlight efficiently. The fact that Cassini's Division was bright from this perspective indicated that it contained a significant amount of fine material - contrary to the belief that it was swept completely clear. The imagery revealed considerably more structure than had been conjectured. Instead of the expected smooth sheets of icy particles, each of the 'classical' rings was found to comprise narrow ringlets. In 1943 B.F. Lyot, using the 24-inch refractor at the Pic du Midi Observatory with excellent seeing high in the French Pyrenees, had observed 11 divisions and hints of finer structure as variations in brightness on the 'B' ring, and while his drawings had been received sceptically by his contemporaries, he had been proved spectacularly correct.
The major gaps in the ring system could be accounted for by the presence of the satellites beyond, in particular Mimas, but they could not explain the fine structure. After J.C. Maxwell had proved that the rings were composed of a myriad of discrete particles, each of which pursued its own orbit around Saturn, it had been presumed that the transient fine structure that was reported from time to time was a 'density wave' effect inducing a fluctuation that propagates through the system. However, while this could potentially explain everything, in fact it explained nothing specific, so the fine structure in the ring system was a delightful mystery for a new generation of theoreticians.
As Pioneer 11 closed in, its Ultraviolet Photometer detected a bright hydrogen glow from the planet's illuminated hemisphere, a fainter glow from a large region of space tracing out Titan's orbit, and a dim glow from the 'B' ring, suggesting that the ring material is slowly degassing.
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