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

摘要

I report a model for the formation of Saturn's family of mid-sized icy moonsto coincide with the first flypast of Rhea by the Cassini Orbiter spacecraft on26 November 2005. It is proposed that these moons had condensed from aconcentric family of orbiting gas rings that were cast off some 4600 Myr ago bythe contracting proto-Saturnian cloud. Numerical and structural models for Rheaare constructed on the basis of a computed bulk chemical mix of hydrated rock(mass fraction 0.385), H2O ice (0.395), and NH3 ice (0.220). The largeproportion of NH3 in the ice mass inhibits the formation of the densecrystalline phase II of H2O ice at the satellite's centre. This may explain theabsence of compressional features on the surface. The favoured model of Rheahas a chemically uniform interior and is very cold. The satellite is nearlyisodense and the predicted value of the axial moment-of-inertia factor isC/MR^2 = 0.399 +/- 0.004. NH3 is unstable at Saturn's distance from the Sun,except near the polar regions of the satellite. Perhaps the Cassini Orbiterwill discover indirect evidence for NH3 through the sublimative escape of thisice from the outer layers, especially near the equatorial zones. Wasting of NH3would weaken the residual soil, so making the edges of craters soft and proneto landslides. It will be exciting to learn what Cassini discovers.