Libration and obliquity of Mercury from the BepiColombo radio science and camera experiments

(eng) Mercury's internal structure is the most puzzling among the terrestrial planets and the BepiColombo space mission will play an important role in advancing our understanding of the structure, formation, and evolution of Mercury. Mercury possesses a slow rotation period of 58,6 days and revolves around the Sun in 88 days, revealing a 3:2 spin-orbit resonance. The planet exhibits an equatorial asymmetry from which a solar torque arises, which induces a periodic variation of the planet's rotation, or libration, with a period of 88 days. In the presence of a liquid core, the amplitude of the forced libration will be greatly enhanced compared to the amplitude in the case of a completely solidified planetary interior because a liquid core does not participate in the libration and the moment of inertia of the mantle is about half of the total moment of inertia of Mercury. Insight into Mercury’s deep interior will be obtained from observations from the “rotation experiment”, which will involve observations of the 88-day forced libration, the obliquity, and the gravity field of Mercury. The librations will be determined from the comparison of target surface positions at different times using an onboard camera. We have simulated repeated photographic measurements of selected target positions on the surface of Mercury in order to estimate the accuracy of the reconstruction of the orientation and rotational motion of the planet, as a function of the orbit of the spacecraft, different surface illuminations, the number of different targets considered and their locations on the surface of the planet. We show that the obliquity and libration can be retrieved through the rotation experiment to a precision level meeting or exceeding the minimum BepiColombo mission requirements.