34th Meeting of the AAS Division on Dynamical Astronomy, May 2003
10 Satellites and Rings
Oral, Tuesday, May 6, 2003, 3:25-5:10pm,

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[10.03] On the Problem of Phoebe's Family

M. Cuk, J. A. Burns (Cornell U.), D. Nesvorny (SWRI), V. Carruba (Cornell U.)

With their large sizes, Himalia and Phoebe are distinguished among the irregular satellites of Jupiter and Saturn, respectively. Their orbital semimajor axes and eccentricities are similar as well; however, Phoebe follows an almost-ecliptical retrograde orbit, quite unlike that of prograde Himalia. Moreover, Himalia has a well-defined dynamical family, whereas Phoebe does not appear to, although several recently discovered irregular satellites of Saturn (2000 S1, S7, S9 and S12) have inclinations like Phoebe's (Gladman et al. 2001). These latter moons have significantly larger orbits than Phoebe's, with several not even crossing Phoebe's path, making a common origin problematic.

To ascertain whether any of the irregulars with orbits close to Phoebe's might be family members, we have run Monte-Carlo simulations that follow the loss and reaccretion of Phoebe-cratering ejecta. None of our runs could produce the presently observed distribution of small retrograde satellites of Saturn. This disagrees with the suggestion of Gladman et al. as to the nature of these small Saturnians. If Phoebe had ever been disrupted, these simulations imply that observable fragments should remain on close-by orbits that would not yet have been re-accreted owing to their long synodic periods with Phoebe. What might account for such different histories of similar-sized satellites? Possibly Phoebe was not disrupted whereas proto-Himalia was because orbital velocities are half as much in the former's neighborhood (cf. Nesvorný et al. 2003). This is especially relevant since satellite-satellite collisions are much more likely than those with heliocentric impactors (Nesvorný et al. 2003). Phoebe's population of craters, to be imaged during Cassini's close flyby on June 12, 2004, may be distinctive because of its collisional history.

The author(s) of this abstract have provided an email address for comments about the abstract: cuk@astro.cornell.edu

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.