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Saturn's Icy Moons: A Model for Their Origin and Bulk Chemical Composition

Introduction: I report the preliminary results of a new model for the formation of Saturn’s family of mid-sized icy moons, namely Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus. It is proposed that these 6 moons condensed from a concentric family of orbiting gas rings that were shed equatorially by the contracting proto-Saturnian (hereafter p-Sat) cloud. Gas ring shedding is a central feature of the Modern Laplacian Theory of Solar system origin (hereafter MLT) [1,2]. In the case of the p-Sat cloud, the rings are shed at orbital radii Rn (n = 0,1,2,..) that lie close to the present mean orbital distances of the satellites, except for Iapetus. It is suggested that this moon initially formed close to the present mean distance of Titan, namely RTi = 20.27RSat, where RSat = 60,268 km is Saturn’s equatorial radius. Iapetus was then displaced dynamically to its present orbit as a result of Titan’s capture by the p-Sat cloud. Now Titan’s mass exceeds that of the next largest moon Rhea by a factor of ~ 58. It is also ~14 times larger than the expected mass of condensate mcond ≈ 9.4 × 10 g which can form from a p-Sat gas ring – see below. On this basis, it has been argued that Titan cannot be a native moon of Saturn [3,4]. Instead, it originally condensed as a secondary growing embryo within the gas ring that was shed by the protosolar cloud (hereafter PSC) at Saturn’s orbit. It was then later captured through the action of gas drag at the edge of the p-Sat cloud, when the radius of that cloud was ~20 RSat. If the above picture is correct, then Titan’s capture was secured long before the p-Sat cloud had shrunk to the present orbit of Rhea (8.75RSat). Perhaps Titan gravitationally stymied the formation of moons from the gas rings that were shed at radii ~11.5RSat and ~11.5RSat, in much the same way that Jupiter thwarted the growth of a single planet at the asteroidal belt. The Modern Laplacian Theory: According to the MLT, the planetary system and regular satellite systems of the gas giant planets condensed from concentric families of orbiting gas rings. These rings are shed by the parent cloud as a means for disposing of excess spin angular momentum during gravitational contraction. The mean orbital radii Rn and masses mn (n = 0,1,2,..) of the rings satisfy the equations