  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 Orbiter spacecraft on
26 November 2005. It is proposed that these moons had condensed from a
concentric family of orbiting gas rings that were cast off some 4600 Myr ago by
the contracting proto-Saturnian cloud. Numerical and structural models for Rhea
are 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 large
proportion of NH3 in the ice mass inhibits the formation of the dense
crystalline phase II of H2O ice at the satellite's centre. This may explain the
absence of compressional features on the surface. The favoured model of Rhea
has a chemically uniform interior and is very cold. The satellite is nearly
isodense and the predicted value of the axial moment-of-inertia factor is
C/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 Orbiter
will discover indirect evidence for NH3 through the sublimative escape of this
ice from the outer layers, especially near the equatorial zones. Wasting of NH3
would weaken the residual soil, so making the edges of craters soft and prone
to landslides. It will be exciting to learn what Cassini discovers.
