ORIGIN AND THERMAL EVOLUTION OF ICY SATELLITES

The paper reviews the problem of formation and evolution of the so-cal led ''regular satellites'' of the giant planets, and it consists of tw o parts: the first describes the possible origin of the satellites, th e second studies their evolution, attempting to stress the relations o f the present status of the satellites with their evolutionary history . The formation of regular satellite systems around giant planets is p robably related to the formation of the central planet. Some character istics of regular satellite systems are quite similar, and suggest a c ommon origin in a disk present around the central body. This disk can originate through different mechanisms which we will describe, paying attention to the so-called ''accretion disk'' model, in which the sate llite-forming material is captured. The disk phase links the formation of the primary body with the formation of satellites. The subsequent stages of the disk's evolution can lead first to the formation of inte rmediate size bodies, and through the collisional evolution of these b odies, to the birth of satellite ''embryos'' able to gravitationally c apture smaller bodies. Given the scenario in which icy satellites may be formed by homogeneous accretion of planetesimals made of a mixtures of ice and silicates, if no melting occurs during accretion, the sate llites have a homogeneous ice-rock composition. For the smaller satell ites this homogeneous structure should not be substantially modified; only sporadic local events, such as large impacts, can modify the surf ace structure of the smaller satellites. For the larger satellites, if some degree of melting appears during accretion, a differentiation of the silicate part occurs, the amount of differentiation and hence the core size depending on the fraction of gravitational potential energy retained during the accumulation process. Melting and differentiation soon after the accretion, for the larger satellites, could also depen d on the convective evolution in presence of phase transitions and gen erate an intermediate rock layer, considerably denser than the underly ing, still homogeneous core, and unstable to overturning on a geologic time scale. Moreover the liquid water mantle could be a transient fea ture because the mantle would freeze over several hundred million year s. For these large bodies the stable configuration is expected to be o ne consisting of a silicate core and a mantle of mixed rock and ice.