Geophysical Constraints on Phobos's Interior Structure

Introduction. Phobos is the innermost satellite of Mars and is locked in a 1:1 spin-orbit resonance. Its response to Martian tides is one of the principal sources of information about the interior of the satellite since the tidal response of a body is determined to first order by its internal properties (e.g., rigidity and density). In this study, we focus on the tidal bulge height as a possible means of determining the interior structure of Phobos. We compute the gravitational potential and tidal deformation of the satellite for a series of possible models of its interior structure with a focus on locating regions of maximum displacement so as to configure a potential orbiter-satellite system for optimal return. For this purpose, we solve, on the one hand, the full three-dimensional (3-D) elastostatic problem on the deformed body to determine the displacement field and, on the other hand, Poisson’s equation for the gravitational potential. Our approach relies on the higher-order spectral-element method [1,2] and a recently developed extension to the waveform modelling package Salvus [3]. We investigate the possibility of distinguishing between first-order models in the form of homogeneous, icerock mixtures, and layered models using currently available observations that include the degree-2 gravitational coefficients and the libration in longitude in addition to the aforementioned tidal displacement field.