Saturn's Icy Moon Rhea: A Prediction for its Bulk Chemical Composition and Physical Structure at the Time of the Cassini Spacecraft First Flyby

Abstract

I report a model for the formation of Saturn's family of mid-sized icy moons to coincide with the first flypast of Rhea by the Cassini spacecraft on 2005 November 26. It is proposed that the moons had condensed from a concentric family of orbiting gas rings that were shed some 4.6610 9 yr ago by the proto-Saturnian (hereafter p-Sat) cloud. The p-Sat cloud is made up of gas and residual grains of the gas ring that was shed by the proto-Solar cloud (hereafter PSC) at Saturn's orbit. The bulk of the condensate within this proto-Solar ring accumulates to form Saturn's central core of mass~10-20 M È (M È = Earth mass). The process of formation of Saturn's solid core thus provides an opportunity for the p-Sat cloud to become depleted in rock and water ice relative to the usual solar abundances of these materials. Nitrogen, which exists as uncondensing N 2 in the PSC and as NH 3 in the p-Sat cloud, retains its solar abundance relative to H 2 . If the depletion factor of solids relative to gas is z dep = 0.25, as suggested by the low mass of Rhea relative to solar abundance expectations, the mass-percent ratio of NH 3 to H 2 O in the dense p-Sat cloud is 36:64. Numerical and structural models for Rhea are constructed on the basis of a 'cosmogonic' bulk chemical composition of hydrated rock (mass fraction 0.385), H 2 O ice (0.395), and NH 3 ice (0.220). It is difficult to construct a chemically differentiated model of Rhea whose mean density matches the observed value r Rhea = 1.23 ± 0.02 g cm -3 for reasonable bounds of the controlling parameters. Chemically homogeneous models can, however, be constrained to match the observed Rhea density provided that the mass fraction of NH 3 is permitted to exceed the cosmogonic value by a factor z NH 3 = 1.20-1.35. A large proportion of NH 3 in the ice mass inhibits the formation of the dense crystalline phase II of H 2 O ice at high pressure. This may explain the lack of compressional features on the surface of the satellite that are expected as a result of ice II formation in the cooling core. The favoured model of Rhea is chemically uniform and has mass proportions of rock (0.369), H 2 O ice (0.378), and NH 3 ice (0.253). The enhancement factor of NH 3 lies within the measured uncertainties of the solar abundance of nitrogen. The satellite is very cold and nearly isodense. The predicted axial moment-of-inertia coefficient is [C/MR 2 ] Rhea = 0.399 ± 0.004.

Interiors and Evolution of Icy Satellites

“…This would most likely also rule out the existence of a deep subsurface ocean that would have been conceivable if the satellite were differentiated and contained at least small amounts of ammonia (Hussmann et al, 2006). It was also suggested that a high abundance of ammonia ice in Rhea's interior may have prevented the formation of the ice-II phase in the central regions (Prentice, 2006). The formation of an ice-II core would be associated with a global contraction of the satellite that would have left its marks at the satellite's surface.…”

Interiors and Evolution of Icy Satellites

“…Prior to the Cassini flyby in Nov 2005, it was unclear if Rhea is differentiated and hence various possibilities had to be taken into account (Castillo-Rogez, 2006). It was also suggested that a high abundance of ammonia ice in Rhea's interior may have prevented the formation of the ice-II phase in the central regions (Prentice, 2006). The C 22 /J 2 ratio is consistent with hydrostatic equilibrium within the given error bounds (Iess et al, 2006).…”

Early Thermal History of Rhea: The Role of Serpentinization and Liquid State Convection

“…The ammonia bearing compounds are absent on the surface but radar indicates an increase in ammonia with depth. Anyway, the total content of ammonia in Rhea is probably low (Ostro et al 2006, Prentice 2006.…”