[iii] The exploration of the outer solar system by the two U.S. Voyager spacecraft marks an important turning point in understanding the universe. Prior to the Voyager and Pioneer missions, the planetary exploration program concentrated on the terrestrial bodies of the inner solar system: Mercury, the Moon, and Mars. The Moon and Mercury, now geologically dead, display ancient battered crusts developed under the torrential bombardment of debris remaining from the accretion of the solar system. Both display evidence of fleeting episodes of early volcanism as they cooled down. Mars, to a large degree, exhibits the same basic pattern. Owing to its tenuous carbon dioxide atmosphere and moderate amount of water, Mars shows evidence of additional processes: sand dunes and wind scour, and features that appear to be related to glacial action and brief fluvial episodes. We were trained by these experiences to expect the planets of the solar system (and, by implication, of the universe) to belong to a fairly narrow spectrum in terms of the types of geologic processes and histories they could have. Armed with this perspective, we journeyed with the Voyager spacecraft to the environs of Jupiter and Saturn, inspecting many things along the way, including the miniature planetary systems consisting of the Jovian and Saturnian arrays of moons. The Voyager spacecraft arrived first at Jupiter, where we examined Io, Europa, Ganymede, and Callisto. Two of them we knew to be roughly the size and density of our own Moon; the other two we knew to be the size of Mercury, but to consist roughly of half water or water ice. In this cold setting, we expected lifeless, plain, simple, geologic styles and histories that would interest only the most esoteric planetary geologists. To our surprise we discovered an intensely volcanically active moon laced with sulfur and sulfur dioxide driven volcanoes (Io); a moon with a complex, intricate, interwoven network of stretch marks and cycloidal ridges (Europa); and a moon with a bizarre jumble of forms driven by a process of ice-raft tectonics that resembles in some aspects Earth's own process of continental drift (Ganymede). The two spacecraft flew on to Saturn. Here the moons were even smaller. The bodies we would examine geologically have diameters that range from less than half the diameter of our Moon down to only a few hundred kilometers. Evidence indicated that these objects could not have substantial geologic variability or complex geologic histories. How could an object the size of Enceladus, with only 10 millionths the mass of Earth, in a deep freeze at only 90 K ( - 300° F), composed of water ice frozen to the consistency of the hardest rocks, display substantial activity? The maps presented in this atlas portray the diversity that we found. Examination of three of the smallest classical Saturnian moons, Mimas, Enceladus, and Hyperion, reveals a diversity many times broader than the narrow terrestrial view we had held prior to the Voyager missions. Mimas is close to what we expected&emdash;a frozen, pockmarked ball of cold rubble. Hyperion (not yet mapped in detail and hence not included in the atlas) is about the same size as Mimas but shows a highly irregular shape indicating that it is a fragment torn from a much larger, rigid object. On Enceladus, the greatest surprise, regions of cratered plains suggest great antiquity, yet other regions are free of craters, at least to the limit of resolution. Our best models indicate that this tiny body must have been active in the geologically recent past and is probably, at least from time to time, [iv] geologically active today. The lesson we learned is that wherever we look, we will find energy sources that will drive the geologic engines of planets: some radiogenic, some accretional heat, some gravitational, and, probably, some chemical and some electromagnetic. We have also learned that no matter how cold the ambient environment, materials are available to serve as lubricants to mobilize the interiors of the planets. On the Saturnian satellites, the likely lubricants are methane and ammonia. As you peruse the materials in this atlas, remember how limited our expectations of the Saturnian moons were compared to what we found; imagine how diverse planetary objects throughout the universe must truly be.