| Summary: | The understanding of outer planet satellites has improved considerably, when the Voyager spacecraft flew by Jupiter, Saturn, Uranus, and Neptune, followed by the advent of the Galileo spacecraft in the Jovian system (1995-2003) and the Cassini spacecraft in the Saturnian system (since 2004). We will be discussing geophysical constraints on the interior structure and composition of selected Jovian and Saturnian icy satellites and investigate conditions under which potentially habitable liquid water reservoirs could be maintained. Active volcanism on Io and tectonic features on Europa emphasize the importance of tidal heating in the Jupiter system, maintained by gravitational interaction due to the resonant orbits of Io, Europa, and Ganymede (Laplace resonance). Ganymede is strongly differentiated and possesses a self-sustained, dipolar magnetic field. Magnetic induction signals were observed at Europa, Callisto, and possibly Ganymede, suggesting the presence of globe-encircling, briny reservoirs of liquid water below the surface. Saturn's largest moon Titan is intermediate between Ganymede and Callisto in respect to its radius, mean density and degree of internal differentiation. Titan is likely to harbour a cold, extended internal liquid reservoir, similar to the large icy satellites of Jupiter, but more enriched in ammonia. Enceladus' south pole hot spot and associated water-vapour plumes may be caused by localized tidal heating above a liquid water reservoir in contact with silicate rock below. Interior models of icy bodies would certainly benefit from future geophysical exploration, which would include gravitational and magnetic field sounding from low-altitude orbit and close flyby, combined with altimetry data and in-situ monitoring of tidally-induced surface distortion and time-variable magnetic fields. Additional important constraints are provided by the surface geology, spectral properties, and chemistry of each individual satellite.
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