Titan was imaged in the spirit of exploration and discovery. With this image, which is now a part of recorded history, our visual information on Titan is materially increased, but is still only roughly equal to Galileo’s information on Saturn itself in 1610 at the time that he prepared his first sketch. Better images of Titan will be obtained by the Voyager spacecraft.] SCIENCE HIGHLIGHTS 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.