What Use Callisto?
W.B. McKinnon (Washington Univ., Saint Louis) Callistos 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 en
