Radio Doppler data from a single flyby of Rhea by the Cassini spacecraft have been used to infer the mass and quadrupole gravitational moments for this Saturnian satellite. The flyby occurred on 26 November 2005 at a closest approach distance q of 1266.13 km and at an inclination of 10.266° to the Rhea equator. The relatively small inclination angle implies a large uncertainty in any independent determination of the zonal gravitational harmonic J2 , although the sectoral harmonic C22 is measurable. Further, by imposing the hydrostatic constraint J2 = 10/3 C22 on the data inversion, the resulting value for C22 can be used to derive an axial moment of inertia by the theory of figures, hence a useful constraint on the interior structure is obtained. Surprisingly, there are three published gravitational fields for Rhea, and they do not agree. Our results (GRL, 34, L02202) are consistent with an undifferentiated interior with about 25% rock-metal and 75% water ice by mass. Results by Iess et al. (Icarus, 190, 175) are consistent with hydrostatic equilibrium, but a significantly smaller value of C22 implies a satellite that is "almost" undifferentiated, but not totally differentiated. Mackenzie et al. (GRL, in press) report a significantly larger value of J2 which is consistent with a nonhydrostatic Rhea. When these three determinations and their error bars are plotted on a C22 versus J2 diagram, the range of axes needed to show all results are 700 to 975 for J2 and 220 to 275 for C22 , in units of 10-6 . The diagram suggests that a single flyby has returned rather useless results, revealing little or nothing about Rhea's internal structure. To the contrary, we conclude that with a careful analysis of the data from a single Doppler pass, over an observation interval from 5058 s before closest approach to 4223 s after closest approach, a smaller and more useful region of the C22 versus J2 diagram can be identified. We find no convincing evidence from the data that Rhea is nonhydrostatic. Under the assumption of hydrostatic equilibrium, the new results are consistent with our previous conclusion that Rhea is homogeneous, with some compression of the water-ice component and a transition from ice I to ice II at depth.
37th COSPAR Scientific Assembly
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