Phoebe's Differentiated Interior from Refined Shape Analysis

Context. Phoebe is an irregular satellite of Saturn and its origin, either between the orbits of the giant planets or from the Kuiper Belt, is still uncertain. The extent of dierentiation of its interior can potentially help inform its formation location because it is mainly determined by heat from 26-aluminum. The internal structure is reflected in the shape, assuming the body is relaxed to hydrostatic equilibrium. Although previous data analysis indicates Phoebe is close to hydrostatic equilibrium, heavily cratered surface makes it dicult to tease out the low-order shape characteristics. Aims. This paper aims to extract Phoebe’s global shape from the observations returned by the Cassini mission for comparison with uniform and stratified interior models under the assumption of hydrostatic equilibrium. Methods. The global shape is derived from fitting spherical harmonics and keeping only the low degree harmonics that represent the shape under the heavily crater surface. The hydrostatic theoretical model for shape interpretation is based on the Clairaut equation developed to order 3 (although 2 is sucient in this case). Results.We show that Phoebe is dierentiated with a mantle density between 1900 and 2400 kg/m3. The presence of a porous surface layer further restricts the fit with the observed shape. This result confirms the earlier suggestion that Phoebe accreted with sucient 26-aluminium to drive at least partial dierentiation, favoring an origin with C-type asteroids