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W.B. McKinnon (Washington Univ., Saint Louis)
Callisto’s unique state of partial differentiation can be used to constrain satellite accretion models. It is well appreciated that the satellite must have accreted slowly enough not to have triggered an accretional melt-down. It must also not have accreted so early as to be melted by short-lived radiogenic heating due to 26Al and 60Fe. Structural models for Callisto indicate a hydrated rock mass fraction very close to 0.5. With an initial (t = 0 at CAI condensation) 26Al/27Al = 5 x 10-5 and a contribution from 60Fe, a primordial, undifferentiated Callisto can warm from 100 to 251 K and just melt ~all its ice if it accreted at t = 2.5 Myr. This is a hard lower limit to the formation time of Callisto as it ignores accretional heating. It may also constrain the formation time of Jupiter. A softer lower limit is obtained by only requiring Callisto not reach the ice minimum melting temperature at depth, after which otherwise the gravitational energy released by unmixing would trigger runaway differentiation (as described by Friedson and Stevenson 1983), or t > 3.5 Myr. The sizes of the satellitesimals that built Callisto must also have not been so large that they buried their accretional heat deeply enough that the heat could not have reached the surface on the accretional time scale. Large, multi-100-km satellitesimals would have been especially adept at triggering differentiation, as the dense rock or ice-rock slurry created at impact would sink rapidly to the satellite’s center, even if the center was cold. Callisto’s true utility to solar system science lies beneath.
The author(s) of this abstract have provided an email address for comments about the abstract: mckinnon@wustl.edu
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.