Planetary Chronology Workshop 2006 6023.pdf IAPETUS AND ENCELADUS: FIRST CONTRIBUTIONS OF HIGH-PRECISION GEOPHYSICAL MODELING TO SOLAR SYSTEM CHRONOMETRY AND CHRONOLOGY. D. L. Matson1, J. C. Castillo1, T. V. Johnson1, and J. Lunine2,3, 1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, 91109 Pasadena, CA, USA ([email protected]), 2Lunar & Planetary Lab, University of Ari- zona, 1629 E. University Blvd, 85721, Tucson, AZ, USA / INAF, Istituto di Fisica dello Spazio Interplanetario Via del Fosso del Cavaliere 100, 00133, Rome ITALY. Introduction: As a result of the Cassini Imaging Absolute dates indicate that the Earth-Moon system and Radio Science data, the Saturnian satellite densi- formed after Iapetus and the Saturnian system. The ties are now available to high accuracy (Jacobson et al. fact that Iapetus formed before the Moon may give us 2005). The weighted average density for the medium- a different prospective on dating surface by impacts. sized, regular Saturnian satellites is ~1230 kg/m3, (4) An important aspect of this new chronol- about 30% lower than the average (uncompressed) ogy concept is the use of CAIs time of formation as a density for the Galilean satellites. This new data, com- reference for the formation of the Solar System. While bined with updated values for parameters involved in CAIs’ absolute age has been determined with an un- thermal modeling, result in interiors much colder than certainty of (+/-0.000 6 By), solids older than CAIs previously considered in the series of geophysical have been identified [9]. Determining the chronology models produced after the visit of the Voyager mission of the formation the first solids is crucial to future ap- to the Saturnian system. In order to drive endogenic plication of Iapetus’ age to the Solar system. (tectonic) activity and dynamical evolution, Castillo et al. [1] and Matson et al. [2] have proposed short-lived Conclusion: Previous studies using 26Al as a fine- radiogenic species as a potential heat source. scale radiochronometer treated meteorites only. Future Fine-Scale Radiochronometry: This opens the models must unify the different components of the door to fine-scale radiochronometry in planetary sci- Solar system chronology on a framework of absolute ences, because the main short-lived contributors are dates. We will be able to see hitherto for hidden rela- 26Al (half-life = 0.716 My) and 60Fe (half-life = 1.5 tionships between nucleosynthesis, first solids and My). Modeling of Iapetus’ spin rate evolution [1], planetesimal formation, CAIs and chondrules, to the Enceladus’ thermal state [2], but also constraints on formation of the different giant planet systems (in Mimas and Tethys orbital evolution and Rhea’s inter- agreement in dynamical studies), and the formation of nal structure, either agree or are consistent with a for- the inner Solar System. Implications also regard the mation time within 3 My after the creation of the Cal- formation and evolution of extrasolar systems. cium-Aluminum Inclusions (CAIs). The latter are our reference time scale as their occurrences in chondrites Acknowledgements: We thank S. Krot and G. have been dated precisely as 4.5672 +/- 0.0006 By [3]. Huss for very helpful discussions. Implications: Such a result opens new frontiers in This work was carried out by the Jet Propulsion planetary sciences, some of them will be discussed Laboratory, California Institute of Technology, under during the presentation: NASA contract. (1) Finding the evidence for 26Al in the early Saturnian system provides strong constraint on the References: [1] Castillo J. et al. (2005) BAAS 37, wide distribution of this radionuclide in the early Solar 39.04. [2] Matson D. L. et al. (2006) LPS XXXVII, System. This has direct implications on the origin of 2219. [3] Amelin Y. et al. (2002) Science 297, 1678- 26 Al, as well as the formation of other short-lived ra- 1681. [4] Young E. D. et al. (2005) Science 308, 223- 60 dionuclides, especially Fe. 227. [5] Mostefaoui S. et al. (2005) ApJ 625, 271-277. (2) Inferring times of formation from the heat [6] Shukolyukov A. and Lugmair G. (1993) Science required in thermal models is dependent on the initial 259, 1138-1142. [7] Giese et al. BAAS 37 (2005). [8] abundance of the major short-lived radionuclides. For Castillo et al. manuscript in preparation. [9] Shu- a long time, a canonical abundance has been recog- 26 kolyukov A. and Lugmair G. (2004) GCA 68, 2875- nized for Al. However, recently a supercanonical 2888. value has been proposed [4]. Also, there is no agreed canonical value for 60Fe, whose initial abundance has been updated toward higher values [5] since its identi- fication in meteorites [6]. (3) It is now possible to link relative and ab- solute dates. Both data are available for Iapetus [7, 8]. .