Pioneer Saturn has already greatly expanded our knowledge of Saturn, its rings and moons. We now know that Saturn, in many ways, represents an intermediate case between Jupiter, the largest planet in the solar system, and Earth. The composition of Saturn’s interior is essentially the same as Jupiter’s, differing only in the size and extent of the various internal layers. Measurements for Saturn are consistent with a central core of molten heavy elements (probably mostly iron) which is the approximate size of the entire Earth, but about three times more massive. Surrounding the central core is an outer core of highly compressed hot, liquefied volatiles such as methane, ammonia, and water. This outer core is equivalent to approximately nine Earth masses. These core regions, however, represent a very small fraction of the planet, which is composed primarily of the very lightest gases, hydrogen and helium, and is almost 100 times the mass of the Earth. Because of the high pressure in Saturn’s interior, the hydrogen is transformed to its liquid metallic state. Above this metallic hydrogen shell are liquid molecular hydrogen and Saturn’s gaseous atmosphere and clouds, which make up the rest of the planet.

Electrical currents set up within the metallic hydrogen shell produce Saturn’s magnetic field, which was measured by Pioneer. In spite of Saturn’s large size, the magnetic field at the cloud tops is only slightly weaker than the field at the Earth’s surface. Saturn is unique in that its magnetic axis is nearly aligned with its rotation axis, unlike Earth and Jupiter.

Saturn’s magnetic field is also much more regular in shape than the fields of the other planets. At large distances from Saturn, the magnetic field is deformed by the inward pressure of the solar wind. Near the noon meridian (close to the inbound Pioneer trajectory), the solar wind causes a compression of the field; in the dawn meridian (close to the outbound trajectory), the field is swept back and presumably forms a long magnetic tail. In both cases, Pioneer Saturn crossed the outer boundary of the magnetic field several times as the field moved in and out, responding to changing solar wind pressure. Pioneer also observed inward and outward boundaries.

The magnetic envelope surrounding Saturn is intermediate in size and energetic particle population between those of the Earth and Jupiter, the only two other planets known to be strongly magnetized. The three other planets investigated thus far (Venus, Mars, and Mercury) and Earth’s moon have little or no magnetism. Virtually all our knowledge of Saturn’s magnetic environment has been obtained by Pioneer. The spacecraft found rings of particulate material and several small moons near the rings, which strongly affect Saturn’s trapped radiation. These features provide important diagnostic capabilities. A unique finding is the nearly total absence of radiation belt particles at distances closer to the planet than the outer edge of the visible rings.

The inner region of the thick magnetic envelope of Saturn, called the magnetosphere, contains trapped high-energy electrons and protons, with some evidence for heavier nuclei. The overall form of the magnetosphere is simple and compact, more similar to that of Earth than of Jupiter. The unique measuring capabilities of the Pioneer Saturn radiation detectors led directly to the discovery of a diffuse new ring of particulate matter in the region from about 10 to 15 planetary radii (1 Saturn radius = 60,000 km) from Saturn. This ring has been tentatively designated as the G-ring. The G-ring clearly causes particle absorption near the equatorial region. Moreover, Pioneer discovered another region inside 7 or 8 planetary radii in which the radiation belt particles were subjected to a strong loss or absorption, presumably caused by the presence of an extensive cloud of plasma corotating with the planet.

Inside about 10 planetary radii, the trapped radiation shows a high degree of axial symmetry around Saturn and is consistent with the centered dipole magnetic field observed by Pioneer. Saturn’s rings annihilate all trapped radiation at the outer edge of the A-ring, leaving a shielded region close to the planet in which the radiation intensity is the lowest so far encountered in this mission. This shielding prevents the further buildup of electron intensities at lower altitudes, which otherwise would have been present and would have made Saturn a strong radio source observable from Earth.

Pioneer found that several of Saturn’s moons absorb trapped particles from the radiation belts, producing prominent dips in the intensity. The effectiveness of absorption at the moons Tethys and Enceladus is particularly astonishing, and supports the idea that radiation belt particles are drifting inward slowly across the moons’ orbits.

A precipitous decrease in particle intensity, lasting only for about 12 seconds, was observed over a wide range of energies for both protons and electrons at a distance near 2.53 Saturn radii, 23 minutes after Pioneer crossed the Saturn ring plane inbound. At about the same time, an anomaly was also observed in the magnetic field measurements. These phenomena have been tentatively interpreted as indicating the presence of a nearby massive body absorbing the trapped radiation and perturbing Saturn’s magnetic field. The estimated radius of this object lies in the range of 100 to 300 km, based on the effectiveness with which it absorbed the high-energy radiation. The total radiation dose received at Saturn was equivalent to only 2 minutes in the Jovian radiation belts because Saturn’s radiation belts were so much weaker.

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.