37th DPS Meeting, 4-9 September 2005
Session 39 Icy Satellites II
Oral, Wednesday, September 7, 2005, 2:15-4:00pm, Law LG19

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[39.04] A Geophysical Study Of Iapetus: The Need For And Consequences Of Al26

J. C. Castillo, D. L. Matson (JPL/CalTech), C. Sotin (Lab. de Planetologie et Geodynamique, Nantes, France), T. V. Johnson (JPL/CalTech), J. I. Lunine (LPL), P. C. Thomas (Cornell U.)

We have carried out a detailed geophysical study of Iapetus. We define a timeline for the early history of Iapetus by modeling its thermal and tidal evolution. The tidal despinning Iapetus until it gets locked with its orbital motion period of 79.33-d requires the satellite’s interior be warm enough, i.e., highly dissipative, during its thermal evolution. We infer from Iapetus' density (1090+/-32kg/m3, i.e., a rock mass fraction of about ~20% (Jacobson et al., DDA Meeting 2005)) that this requirement is not met if thermal evolution is driven only by long-lived radiogenic isotopes, as described in the literature. Introduction of short-lived radiogenic isotopes (e.g., Al26) such as found in meteoritic Calcium Aluminum Inclusions (CAI) appears to be the only means for Iapetus to complete its tidal evolution over the age of the Solar System.

Besides, this model provides a context for forming the equatorial bulge observed by Cassini Imaging (Denk et al., LPSC 2005). This shape corresponds to the frozen expression of a 17-h rotation period hydrostatic state. After depletion of short-lived radiogenic species, the reduced power production results in rapid cooling the satellite and thickening its lithosphere. The global relaxation time of the satellite increases dramatically when the rotation period is about 17 h, which allows the preservation of Iapetus' ellipsoidal shape until present.

These results imply that Iapetus formed in less than 1.5 My after the formation of CAIs, and provide a constraint on the formation timescale of the Saturnian system. We will discuss further consequences of this conclusion on our understanding of the Saturnian system's evolution.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C.

The author(s) of this abstract have provided an email address for comments about the abstract: Julie.C.Castillo@jpl.nasa.gov

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