Space Sci Rev DOI 10.1007/s11214-010-9635-1 Evolution of Icy Satellites G. Schubert · H. Hussmann · V. L a i n e y · D.L. Matson · W.B. McKinnon · F. Sohl · C. Sotin · G. Tobie · D. Turrini · T. Van Hoolst Received: 10 July 2009 / Accepted: 8 February 2010 © The Author(s) 2010 Abstract Evolutionary scenarios for the major satellites of Jupiter, Saturn, Neptune, and Pluto-Charon are discussed. In the Jovian system the challenge is to understand how the G. Schubert () Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA e-mail: [email protected] H. Hussmann · F. Sohl German Aerospace Center (DLR), Institute of Planetary Research, 12489 Berlin, Germany H. Hussmann e-mail: [email protected] F. Sohl e-mail: [email protected] V. Lainey IMCCE-Observatoire de Paris, UMR 8028 du CNRS, 77 Avenue Denfert-Rochereau, 75014 Paris, France e-mail: [email protected] D.L. Matson JPL 183-335, 4800 Oak Grove Drive, Pasadena, CA 91109, USA e-mail: [email protected] W.B. McKinnon Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, Saint Louis, MO 63130, USA e-mail: [email protected] C. Sotin JPL/Caltech, 4800 Oak Grove Drive, Pasadena, CA 91109, USA e-mail: [email protected] G. Tobie University of Nante, Nantes, France e-mail: [email protected] G. Schubert et al. present Laplace resonance of Io, Europa, and Ganymede was established and to determine whether the heat being radiated by Io is in balance with the present tidal dissipation in the moon. In the Saturnian system, Enceladus and Titan are the centers of attention. Tidal heating is the likely source of activity at the south pole of Enceladus, although the details of how the heating occurs are not understood. An evolutionary scenario based on accretion and internal differentiation is presented for Titan, whose present substantial orbital eccentricity is not associated with any dynamical resonance. The source and maintenance of methane in Titan’s present atmosphere remain uncertain. Though most attention on the Saturnian moons focuses on Titan and Enceladus, the mid-size satellites Iapetus, Rhea, Tethys, and the irregular satellite Phoebe also draw our interest. An evolutionary scenario for Iapetus is presented in which spin down from an early rapidly rotating state is called upon to explain the satellite’s present oblate shape. The prominent equatorial ridge on Iapetus is unexplained by the spin down scenario. A buckling instability provides another possible explanation for the oblateness and equatorial ridge of Iapetus. Rhea is the only medium-size Saturnian satellite for which there are gravity data at present. The interpretation of these data are uncertain, however, since it is not known if Rhea is in hydrostatic equilibrium. Pluto and Charon are representative of the icy dwarf planets of the Kuiper belt. Did they differentiate as they evolved, and do either of them have a subsurface liquid water ocean? New Horizons might provide some answers when it arrives at these bodies. Keywords Outer planet moons · Io · Europa · Enceladus · Dione · Titan · Iapetus · Rhea · Tethys · Phoebe · Pluto · Charon · Satellite evolution 1 Introduction The satellites of the outer planets provide important clues about the formation and evolution of the solar system as a whole. In this chapter we discuss the evolution of the major satellites of Jupiter, Saturn and Neptune. Pluto and Charon are also included since they represent the same population of objects as Neptune’s captured moon Triton. The outer planet moons are striking in their diversity and evolutionary paths. They have experienced physical processes that are unfamiliar in the bodies of the inner solar system, and, accordingly, they comprise a fascinating collection of objects for study. 2 Satellites in Resonance: Strong Thermal/Orbital-Dynamical Coupling Resonances can play an important role in the evolution of satellites. Orbital periods of satel- lites in resonance are commensurable and their mutual gravitational perturbations at con- junction (where perturbations are near maximum) occur periodically at the same orbital phase. Perturbations to the satellites’ orbital evolution are therefore significantly stronger D. Turrini INAF-IFSI, Via del Fosso del Cavaliere 100, 00133 Rome, Italy e-mail: [email protected] T. Van Hoolst Royal Observatory of Belgium, Avenue Circulaire 3, Uccle, 1180 Brussels, Belgium e-mail: [email protected] Evolution of Icy Satellites Fig. 1 Two examples of resonances in the outer solar system involving internal heating of satellites. Rota- tional energy of the primary planet and angular momentum are transferred to the innermost satellite due to tidal torques. Because of the resonance coupling, energy and angular momentum are distributed among the satellites locked in resonance. Part of the energy is dissipated as heat in the satellites’ interiors due to tidal flexing. This affects mainly the inner satellites close to the primary, in the first case Io and, to a lesser extent, Europa, and in the second example Enceladus (sizes and distances not to scale) compared with the non-resonant (stochastic) case (see Greenberg 1982 and Peale 1986 for a general description). The main implication for the evolution of satellites is that the orbital eccentricities are forced and maintained as long as a resonance is stable, i.e., on geological timescales if the coupling is strong. Because the global tidal heating rate (due to tidal inter- action with the primary) depends on the eccentricity squared, long-term internal heat pro- duction is strongly linked to the occurrence of resonances. In some cases when the primary is close—for instance, Io and Jupiter—this type of tidal heating can significantly exceed the heat production due to radiogenic heating. In addition to the high eccentricities associated with stable resonant equilibrium configurations, orbital eccentricities can vary considerably when the satellites pass through resonances or when oscillatory states (strongly varying orbital and thermal states) occur. Oscillatory behavior results from the disequilibrium of ec- centricity forcing in a resonance and involves eccentricity damping due to tidal dissipation in the satellite. Mean motion resonances and tidal heating play an important role in the Jupiter system, mainly for Io and Europa, and in the Saturn system in the case of Enceladus (Fig. 1). Both examples are discussed below. G. Schubert et al. 2.1 Io, Europa, Ganymede 2.1.1 The Laplace Resonance The three inner Galilean satellites are locked in various resonances. For the thermal-orbital evolution the 2:1 Io–Europa mean-motion resonance and the 2:1 Europa–Ganymede mean- motion resonance are the most important ones. Conjunctions of Io and Europa are locked to Io’s perijove (resonance angle librating about 0°) and to Europa’s apojove (resonance angle librating about 180°). Conjunctions of Europa and Ganymede are locked to Europa’s peri- jove (resonance angle librating about 0°) but to neither apsis of Ganymede (resonance angle circulating through 360°). The combination of the two 2:1 resonances yields the libration of the Laplace angle l1 − 3l2 + 2l3 about 180°. The li (i = 1, 2, 3) are the mean longitudes of Io, Europa, and Ganymede, respectively. This implies that whenever Europa and Ganymede are in conjunction, Io is on the opposite side of Jupiter. The Laplace configuration is stable and, after differentiating the mean longitudes with respect to time dli /dt = ni , is usually expressed by n1 − 3n2 + 2n3 = 0(1) where the ni are the mean motions of Io, Europa and Ganymede, respectively. This three- body coupling is called the Laplace resonance, named after Pierre Simon de Laplace, who first demonstrated the stability of the orbital commensurabilities on theoretical grounds. Detailed reviews of the dynamics of the Galilean satellite system are given by Greenberg (1982) and Peale (1986). The forced eccentricities associated with the above mentioned 2:1 resonances are 0.0041, 0.0101 and 0.0006 for Io, Europa and Ganymede, respectively. In the case of Europa the ec- centricity is forced by both the 2:1 resonance with its inner neighbor Io and the 2:1 resonance with its outer neighbor Ganymede. Whereas the free eccentricities are negligible for Io and Europa (order of 10−5), the free eccentricity of 0.0015 is the major contribution to the ec- centricity of Ganymede. The free eccentricity is the remnant of the initial eccentricity after satellite formation (or after an unusual event, e.g., a major impact or a former resonance passage), which decreases with time due to tidal dissipation in the satellite. However, Show- man and Malhotra (1997) have shown that an impactor capable of creating Ganymede’s free eccentricity would have to have had a mass 102 to 103 times greater than the mass of the impactor that formed Gilgamesh, the largest impact basin on Ganymede. The free ec- centricity is not associated with any resonance and can be regarded as the eccentricity that would persist if all the other satellites in the system were removed. Because of their forced eccentricities and their vicinity to Jupiter, Io and Europa are tidally heated on geological timescales up to the present. 2.1.2 Origin and Evolution of the Laplace Resonance At present it is unclear how the three-body resonance has formed. There are two conceivable scenarios: (a) a primordial origin by migration of the newly formed satellites due to inter- actions with the circumjovian disk (Peale and Lee 2002) and (b) a subsequent formation of the resonances by differential expansion of the orbits due to tidal torques from Jupiter (Yoder 1979; Yoder and Peale 1981). The latter is the ‘classical scenario’ in which Io spirals outwards more rapidly than Europa because Io is closer to Jupiter and experiences larger tidal torques.