Context. Study the rotation of a celestial body is an efficient way to infer its interior structure, and then may give information of its origin and evolution. In this study, based on the latest shape model of Phobos from Mars Express (MEX) mission, the polyhedron approximation approach was used to simulate the gravity field of Phobos. Then, the gravity information was combined with the newest geophysical parameters such as GM and k(2) to construct the numerical model of Phobos' rotation. And with an appropriate angles transformation, we got the librational series respect to Martian mean equator of date.Aims. The purpose of this paper is to develop a numerical model of Phobos' rotational motion that includes the elastic properties of Phobos. The frequencies analysis of the librational angles calculated from the numerical integration results emphasize the relationship between geophysical properties and dynamics of Phobos. This work will also be useful for a future space mission dedicated to Phobos.Methods. Based on the latest shape model of Phobos from MEX mission, we firstly modeled the gravity field of Phobos, then the gravity coefficients were combined with some of the newest geophysical parameters to simulate the rotational motion of Phobos. To investigate how the elastic properties of Phobos affect its librational motion, we adopted various k(2) into our numerical integration. Then the analysis was performed by iterating a frequency analysis and linear least-squares fit of Phobos' physical librations. From this analysis, we identified the influence of k(2) on the largest librational amplitude and its phase.Results. We showed the first ten periods of the librational angles and found that they agree well with the previous numerical results which Phobos was treated as a perfectly rigid body. We also found that the maximum amplitudes of the three parameters of libration are also close to the results from a rigid model, which is mainly due to the inclination of Phobos and moments of inertia. The other amplitudes are slightly different, since the physics contained in our model is different to that of a previous study, specifically, the different low-degree gravity coefficients and ephemeris. The libration in longitude tau has the same quadratic term with previous numerical study, which is consistent with the secular acceleration of Phobos falling onto Mars. We investigated the influence of the tidal Love number k(2) on Phobos' rotation and found a detectable amplitude changes (0.0005 degrees) expected in the future space mission on tau, which provided a potential possibility to constrain the k(2) of Phobos by observing its rotation. We also studied the influence of Phobos' orbit accuracy on its libration and suggested a simultaneous integration of orbit and rotation in future work.
