Phoebe’s differentiated interior from refined shape analysis

N Rambaux,J C Castillo-Rogez

doi:10.1051/0004-6361/202039189

Abstract

Context.Phoebe is an irregular satellite of Saturn, and its origin, from either between the orbits of the giant planets or the Kuiper Belt, is still uncertain. The extent of differentiation 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, its heavily cratered surface makes it difficult to tease out its 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 underneath the heavily cratered surface. The hydrostatic theoretical model for shape interpretation is based on the Clairaut equation developed to the third order (although the second order is sufficient in this case).Results.We show that Phoebe is differentiated with a mantle density between 1900 and 2400 kg m−3. 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 sufficient 26-aluminium to drive at least partial differentiation, favoring an origin with C-type asteroids.

Highlights

Phoebe is the largest of Saturn’s irregular moons and presents unique features, such as a dark regolith surface related to hydrated silicates (Clark et al 2005) and a retrograde orbit, which indicate that Phoebe could be a captured body
The origin of Phoebe is debated; it could be a trans-Neptunian object (TNO) (Johnson & Lunine 2005) or a C-type body based on the study of its spectrum (Clark et al 2005) as well as on comparisons between geophysical properties expected for TNOs and C-type asteroids (Castillo-Rogez et al 2019)
The values computed from the Clairaut equation to the first order and the third order are compared to the observed shape derived by Castillo-Rogez et al (2012) and that derived in the present paper

Summary

Introduction

Phoebe is the largest of Saturn’s irregular moons and presents unique features, such as a dark regolith surface related to hydrated silicates (Clark et al 2005) and a retrograde orbit, which indicate that Phoebe could be a captured body. The origin of Phoebe is debated; it could be a trans-Neptunian object (TNO) (Johnson & Lunine 2005) or a C-type body based on the study of its spectrum (Clark et al 2005) as well as on comparisons between geophysical properties expected for TNOs and C-type asteroids (Castillo-Rogez et al 2019). Phoebe is too far away for Saturn’s tidal friction to drive spin-orbit synchronization. Phoebe has a rotation period of about 9.2735 h (Bauer et al 2004), while its orbital period is 551 days

Objectives

Results

Conclusion