Icy Satellites: Geological Evolution and Surface Processes

Abstract

The sizes of the Saturnian icy satellites range from ~ 1;500 km in diameter (Rhea) to ~20km (Calypso), and even smaller ‘rocks’ of only a kilometer in diameter are common in the system. All these bodies exhibit remarkable, unique features and unexpected diversity. In this chapter, we will mostly focus on the ‘medium-sized icy objects’ Mimas, Tethys, Dione, Rhea, Iapetus, Phoebe and Hyperion, and consider small objects only where appropriate, whereas Titan and Enceladus will be described in separate chapters. Mimas and Tethys show impact craters caused by bodies that were almost large enough to break them apart. Iapetus is unique in the Saturnian system because of its extreme global brightness dichotomy. Tectonic activity varies widely — from inactive Mimas through extensional terrains on Rhea and Dione to the current cryovolcanic eruptions on Enceladus — and is not necessarily correlated with predicted tidal stresses. Likely sources of stress include impacts, despinning, reorientation and volume changes. Accretion of dark material originating from outside the Saturnian system may explain the surface contamination that prevails in the whole satellite system, while coating by Saturn's E-ring particles brightens the inner satellites. So far, among the surprising Cassini discoveries are the volcanic activity on Enceladus, the sponge-like appearance of Hyperion and the equatorial ridge on Iapetus — unique features in the solar system. The bright-ray system on Rhea was caused by a relatively recent medium impact which formed a ~40km crater at 12°S latitude, 112°W longitude, while the wispy streaks on Dione and Rhea are of tectonic origin. Compositional mapping shows that the dark material on Iapetus is composed of organics, CO2 mixed with H2O ice, and metallic iron, and also exhibits possible signatures of ammonia, bound water, H2 or OH-bearing minerals, and a number of as-yet unidentified substances. The spatial pattern, Rayleigh scattering effect, and spectral properties argue that the dark material on Iapetus is only a thin coating on its surface. Radar data indicate that the thickness of the dark layers can be no more than a few decimeters; this is also consistent with the discovery of small bright-ray and bright-floor craters within the dark terrain. Moreover, several spectral features of the dark material match those seen on Phoebe, Iapetus, Hyperion, Dione and Epimetheus as well as in the F-ring and the Cassini Division, implying that throughout the Saturnian system. All dark material appears to have a high content of metallic iron and a small content of nano-phase hematite. However, the complete composition of the dark material is still unresolved, and additional laboratory work is required. As previously concluded for Phoebe, the dark material appears to have originated external to the Saturnian system. The icy satellites of Saturn offer an unrivalled natural laboratory for understanding the geological diversity of different-sized icy satellites and their interactions within a complex planetary system.