In addition to images of Saturn, the brightness, color, and polarization of the reflected light were also measured by the imaging photopolarimeter on Pioneer Saturn. These measurements are used to study the cloud layers of Saturn and Titan and to model the vertical structure of the atmospheres of these two bodies. In the scans that made the images, the banded structures of Saturn and of the rings were obtained in fine detail. These are essential in studying the atmosphere, rings, and moons. A new Saturnian ring, which has been tentatively designated the F-ring, was discovered in the images. It is narrower than 500 km in width, but is important because it forms an outside barrier to the bright A- and B-rings. The gap between the F- and A-rings has been designated the Pioneer Division by the Pioneer team. A small moon, which either was previously unknown or had been previously discovered from Earth but lost again, was found in the Pioneer Saturn images. After its initial discovery, this new moon continued on its 17-hour orbit around the planet and passed near Pioneer as the spacecraft entered the ring system. It is quite conceivable that this moon is the same one that perturbed the radiation belt particles and produced the anomaly in the magnetic field measurements.
Infrared observations obtained during the Saturn flyby revealed the temperatures in the atmospheres of Saturn and the rings and in the atmosphere of Titan. It was found that Saturn has a temperature of about 100 K (about 280°F below zero) and, according to these observations, has an internal heat source of enough strength that the planet emits approximately 2.5 times as much energy as it absorbs from the sun. The equatorial yellowish band observable in many of the images was found to be several degrees colder than the planet at other latitudes and is probably a zone of high clouds resembling similar zones on Jupiter. As expected, the rings were extremely cold, 65 to 75 K (about 330°F below zero), at the time of encounter. The temperature differences between the illuminated and unilluminated sides of the rings, and the rate of cooling as the ring particles go into Saturn’s shadow, suggest that the ring particles are at least several centimeters in diameter and the rings themselves are many particle diameters thick. The very minor perturbation to Pioneer’s trajectory, as it passed under the visible rings, indicates that the rings probably consist of ices.
As Pioneer passed through Saturn’s ring system, very sensitive meteoroid detectors observed the impact of five particles on the spacecraft, particles that were about 10 micrometers (0.0005 inch) in diameter. Two impacts occurred while the spacecraft was above the rings and three while the spacecraft was below the rings. No impacts were detected going through the ring plane, but the Pioneer instrument cannot detect individual impacts that occur less than 77 minutes apart. This characteristic would have prevented detection of ring particles because of the impacts detected just before both ring plane crossings. It is uncertain whether the micrometeoroids detected by Pioneer Saturn were stray ring particles deflected out of Saturn’s ring plane or whether they were particles from interplanetary space drawn inward toward Saturn by its strong gravitational field.
Close to the point of closest approach to Saturn, the spacecraft’s radio transmissions were affected by Saturn’s ionosphere. The manner in which the radio signals were absorbed indicates that Saturn has an extensive ionosphere composed of ionized atomic hydrogen with a temperature of about 1250 K in its upper regions. This high temperature requires an extensive energy source other than the sun. This phenomenon was also observed at Jupiter.
Pioneer measured ultraviolet glow throughout the Saturnian system. This ultraviolet glow is due to the scattering of the light from the sun by atomic hydrogen. The observations of ultraviolet emission from an extensive cloud of hydrogen gas surrounding Saturn’s visible rings are especially interesting. The rings themselves are presumably the source of this hydrogen. On the planet’s disk, the ultraviolet observations show significant latitude variations, suggesting the possibility of aurora near Saturn’s polar regions. A similar extensive cloud of hydrogen was also seen partially surrounding Titan’s orbit.
These very preliminary findings by Pioneer Saturn represent only a small fraction of what will ultimately be learned about Saturn and its environment as the spacecraft data are analyzed in greater detail over the weeks and months ahead.
Schematic of the solar wind interaction with Saturn’s magnetosphere. The solar wind arrives from the direction of the sun, is deflected at Saturn’s bow shock, and flows around Saturn in the magnetosheath (orange region), as indicated by the arrows. The sizes of the magnetosheath and radiation belts change in response to the external solar-wind pressure, becoming smaller when the external pressure is larger and vice versa.
Diagram of Saturn’s inner trapped radiation belts. The energetic particle fluxes generally become more intense closer to Saturn. Decreases in particle flux at the locations of Saturn’s moons are due to the sweeping up of the energetic particles caused by particles striking the moons and being absorbed by them. Also, there are decreases in particle flux at the outer edge of the rings, where the energetic particles are also absorbed, so that a region free of trapped radiation is created from the outer edge of the rings to Saturn.