37th DPS Meeting, 4-9 September 2005
Session 47 Icy Satellites
Poster, Wednesday, September 7, 2005, 6:00-7:15pm, Music Lecture Room 5

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[47.12] Saturn's mid-sized moons: a Model for their Origin, Bulk Chemical Composition and Physical Structure

A. J. R. Prentice (Monash University, Australia)

I report the results of a revised model for the formation of Saturn's family of icy moons following Cassini's first encounter with Enceladus (URL: www.lpi.usra.edu/meetings/lpsc2005/pdf/2378.pdf). It is proposed that the mid-sized moons condensed from a concentric family of gas rings that were shed by the contracting proto-Saturnian cloud (hereafter p-SatC) [Earth, Moon & Planets 30 209 1984; 87 11 2001]. The rings are shed at orbital radii Rn (n = 0,1,2, ...) that lie close to the present orbital distances, putting aside both orbital expansion due to steady loss of p-SatC mass during contraction and migration due to tides since formation. Iapetus is an exception. This moon initially condensed adjacent (but prior) to Rhea at Rn ~12 RS, where RS = 60,268 km. It was then displaced outwards its present orbit as a result of Titan's capture by the p-SatC. Titan is not a native moon of Saturn. Instead it formed as a planetesimal in the same solar orbit as Saturn's solid core and later became captured through the action of collisional drag and tidal forces (URL: www.aas.org/publications/baas/v36n4/dps2004/576.htm). Most likely, Titan destroyed two pre-existing native of moons of Saturn that once existed at 17 RS and 24 RS. Hyperion is the shattered remnant of one such moon. Much of those lost moons is now buried in Titan's upper mantle and are the source of Titan's N2/CH4 atmosphere. Hyperion consists of rock (mass fraction 0.333), water ice (0.370), ammonia ice (0.240) and clathrated methane (0.057). The model for the thermal evolution of the p-SatC predicts that Enceladus and all moons of smaller orbit formed from LIQUID water and rock. Today, these moons are fully frozen with central rocky cores. The rock fractions of Mimas and Enceladus are 0.277 and 0.574, respectively. Tethys and Dione condensed as solids and are uniform mixtures of rock (0.086, 0.494) and water ice, plus small amounts (0.001, 0.00005) of ammonia as mono-hydrate. Rhea contains pure ammonia ice (0.220), in addition to water ice (0.395) and rock (0.385).

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.