Signature of Phobos interior in its gravity field and libration

The origin of the Martian moons is still controversial. Although recent work has proposed that the two Martian moons, Phobos and Deimos, have formed from an early giant impact on Mars (Rosenblatt et al., Nat. Geo., July 2016), a scenario based on an asteroid capture can still not be excluded (Murchie et al., in Asteroids IV, Univ. Press of Ariz., 2015). More data are thus required to further investigate the formation of the Martian moons and of Phobos in particular. Among these data, those constraining the interior structure may help to decipher the formation of Phobos. The conditions of formation (e.g. temperature, pressure and size of disk particle) and the subsequent history of Phobos (e.g. disruption, impacts etc.) affects its interior properties. In fact, because of its small size and mass, Phobos could have a very puzzling interior structure. In extreme case of large porosity, that is very likely inside the Martian moon, the gravity field, rotation and interior structure could be significantly affected. Therefore, probing the interior structure of Phobos by precisely determining its gravity field and libration should allow us to ascertain whether large scale heterogeneity exist inside the satellite and perhaps trace back its origin. In the present study, we predict the Phobos gravity field and libration amplitude for different plausible interior structure models proposed in the literature and we discuss the ability of a radio-science experiment, onboard an orbiter, to distinguish between them.