Origin of the Saturn System

Chapter 3 Origin of the Saturn System Torrence V. Johnson and Paul R. Estrada Abstract Cassini mission results are providing new insights Historically, Saturn’s system was the next most readily into the origin of the Saturn system and giant planet satel- available after Jupiter’s for early telescopic observations. lite systems generally. The chapter discusses current mod- By the end of the seventeenth century, less than 100 years els for the formation of giant planets and their satellites and following Galileo’s first astronomical observations with the reviews major Cassini findings which help advance our un- telescope, the general structure of Saturn’s ring and satellite derstanding of the system’s formation and evolution to its system was known, encompassing its spectacular rings, gi- current state. ant Titan and four of the seven medium sized ‘icy satellites’ of the regular satellite system (satellites in nearly equato- rial, circular orbits). During the subsequent three centuries of telescopic observations many details of the ring struc- 3.1 Introduction ture were revealed and new satellites discovered, including Phoebe, the first of the irregular outer satellites to be discov- The results of the Cassini/Huygens mission must be inter- ered. The Pioneer 11 and Voyager 1 and 2 flybys in 1979, preted in the context of what we know about the formation and 1980/81 provided the first reconnaissance of the system and history of the Saturn system. These results in turn pro- by spacecraft, discovering and characterizing its magneto- vide many constraints on and clues to the conditions and pro- sphere and exploring the moons and rings from close range cesses which shaped the system. The purpose of this chapter for the first time, setting the stage for the Cassini/Huygens is not to provide a general tutorial or textbook on the the- mission. ory of giant planet and satellite formation. This is currently a In the past, the Jupiter and Saturn systems were frequently very active field of planetary and astrophysical research, with referred to as archetypical examples of ‘miniature solar sys- new concepts being developed continually in response to the tems’ and it was suggested that their ring and satellite sys- flood of new data from planetary missions and astrophysical tems were possibly formed in an analogous manner to the observations of star- and planet-forming regions from ground planets about the Sun. Current models of star and planetary and space-based telescopes. A full treatment of this topic system formation suggest a more complex picture, with a is beyond the scope of the current book, let alone a single range of processes and timescales leading to the formation of chapter. The reader is referred to a number of recent pub- the sun from interstellar material, an early gas and dust so- lications and reviews of the current state of research in this lar nebula, and finally planetary formation. Cassini/Huygens field (Canup and Ward 2009; Davis 2004; Estrada et al. 2009; observations have provided important new information and Reipurth et al. 2007). What we hope to achieve in this chapter constraints on many characteristics related to the system’s is to review the key concepts and current issues in planetary origin, including the composition of Saturn, the composition formation and discuss Cassini/Huygens results that relate to of the satellites and rings, the current dynamical state of the the problem of the origin of the system. satellites (spin, orbital eccentricity, resonances), and satellites’ internal structure and geological history. In this chapter we will first review briefly current mod- T.V. Johnson els for the formation of gas giant planets such as Jupiter and Jet Propulsion Laboratory, California Institute of Technology, MS 301-345E, 4800 Oak Grove Dr., Pasadena, CA 91109 Saturn and the formation of their satellite systems. The sec- e-mail: [email protected] ond portion of the chapter discusses Cassini results related to P.R. Estrada the question of origins, emphasizing the chemical conditions Carl Sagan Center, SETI Institute, 515 N. Whisman Rd., Mountain for condensation of material in the outer solar system, and View, CA 94043 new information about the densities and structures of Sat- e-mail: [email protected] urn’s satellites. M.K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, 55 DOI 10.1007/978-1-4020-9217-6_3, c Springer Science+Business Media B.V. 2009 56 T.V. Johnson and P.R. Estrada 3.2 Planet and Satellite Formation theoretical analysis. Isotopic dating of terrestrial materials, meteorites, lunar samples and Martian meteorites provides 3.2.1 Big Bang to the Solar Nebula constraints on the formation times for meteorite parent bodies and the terrestrial planets. These data can be correlated using both long-lived (e.g., the U-Pb system) and short-lived The events and conditions leading eventually to planetary chronometers (e.g., 26Al=27Al and 53Mn=54Mn) and show formation in our solar system can be traced back to the Big that the formation of chondrules and the parent bodies of ig- Bang, about 13.7 Ga, based on current measurements of cos- neous meteorites (Eucrites) began within a few million years mological constants (Spergel et al. 2007). Galactic formation following CAI condensation (McKeegan and Davies 2007). began within a few Gyr or less. Current estimates for the age The age of the Earth and Moon can be inferred from studies of our Milky Way Galaxy are 10–13 Ga with some stars of the oldest rocks on these objects and isotopic data con- in associated globular clusters dating back even earlier. The straining the time of core formation on the Earth. There are material out of which our solar system formed was the result still significant uncertainties in these interpretations which of billions of years of stellar evolution and processing, yield- are beyond the scope of this chapter to explore. A recent re- ing the mix of elements in what is usually called ‘cosmic’ or view of relevant studies up to 2006 has the accretion of the ‘solar’ abundance. Earth and core formation complete by 4:46 Ga, with the It is generally agreed that the Sun and its associated Moon forming from a giant impact during the late stages of circum-stellar disk formed from the collapse of a large Earth’s accretion 4:52 Ga (Halliday 2007). interstellar molecular cloud, very probably in association A dated event potentially related to the final stages of with a massive-star-forming region similar to that seen in the planet formation is the Late Heavy Bombardment. This is Orion nebula (Boss 2007). Two basic mechanisms for the based on a strong clustering of lunar impact breccia ages collapse have been proposed, gravitational instability and at 3:9 Ga and the lack of breccias with significantly older collapse triggered by a shockwave from a supernova or other ages. These data have been interpreted either as the ‘tail-off’ nearby stellar event. Both mechanisms appear to be viable, of accretion or as a real ‘spike’ in the impact cratering rate in but have different timescales and implications for the early the inner solar system at 3.9 Ga (see Warren 2007). Recent solar nebula, with gravitational collapse taking 10 Myr studies of the dynamical evolution of the early solar system while triggered collapse would be more rapid, 1 Myr. The have provided support for the ‘spike’ or ‘lunar cataclysm’ presence of short lived radioactive isotopes in the early solar explanation (see discussion of migration and the ‘Nice’ system materials has favored the supernova shock-triggered model below). model (e.g., 26Al, and particularly 60Fe, which can only be made in a supernova), with other shock sources, such as outflow from massive AGB stars, regarded as less probable 3.2.2.2 The Outer Solar System based on stellar evolution models (Boss 2007; McKeegan and Davies 2007). Presolar grains and the daughter products For the outer solar system, we do not yet have sample- of the now extinct short lived isotopes found in primitive derived dates for the giant planets or their satellites, and so meteorites are the only remaining physical evidence of this the formation times must be constrained by astrophysical early phase of the solar system’s formation. observations of other star systems and theoretical studies of As the Sun formed from the collapsing molecular cloud, giant planet formation. Two major advances in astrophysical surrounded first by an envelope of gas and then an ac- research have recently led to a greatly improved understand- cretion disk, the earliest condensable materials appeared. ing of planetary formation around stars similar to our Sun: These materials, preserved in refractory fragments in primi- the explosive rate of discovery of extrasolar planets in the tive meteorites, are known as Calcium Aluminum Inclusions last decade and the study of protoplanetary disks, dust disks (CAIs), and provide our first direct ties to the timeline and and debris disks around other stars with powerful ground conditions which led to planet formation. CAIs have been and space-based telescopes (such as the Keck telescope, radiometrically dated using lead isotopes, yielding an age of Hubble Space Telescope and the Spitzer mission). Two 4:567:2 ˙ 0:6 Ga (Amelin et al. 2002). important results from this rapidly growing area of research can be summarized as: 1. Planetary formation is frequently associated with star formation and 2. Giant, gas-rich planets 3.2.2 The Solar Nebula to Planets form very quickly – in less than 10 million years for most systems and in many cases within a few million years. In 3.2.2.1 The Inner Solar System particular, evidence for gas loss in the disks around young stars suggest that giant planets (which must form before How rapidly the planets formed from the gas and dust of the most of the gas is removed from a planetary disk) form in early solar nebula is the subject of much current research and less than 107 year (Meyer et al.

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